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Building Envelope Design Guide - Wall Systems

by Daniel J. Lemieux, AIA and Paul E. Totten, PE
Wiss, Janney, Elstner Associates, Inc.

Last updated: 10-08-2010

Introduction

The basic function of the envelope or enclosure of a building or structure is to protect the covered or otherwise conditioned interior spaces from the surrounding environment. This fundamental need for shelter is a concept that is as old as the recorded history of mankind. However, as our needs have evolved and technologies have advanced, the demand placed on designers to both understand, and integrate, a wide range of increasingly complex materials, components, and systems into the building enclosure has grown in equal proportion. This is particularly true when one considers the emerging threat of terrorism and the impact of that threat on the design and construction of the building enclosure. However, despite the recent emphasis on blast-resistant wall systems and hardening of the building enclosure (see the Blast Resistance section for additional information on this topic), uncontrolled rainwater penetration and moisture ingress remain two of the most common threats to the structural integrity and performance of the building enclosure.

This guide, and the additional resources referenced herein, is intended to facilitate a better understanding of the basic principles behind heat, air, and moisture transfer (including bulk rain water penetration and precipitation management) through the exterior walls of a building or structure. Specifically, it focuses on six (6) commonly specified exterior wall systems in the United States, and illustrates how proper selection, use and integration of the various materials, components and systems that comprise those wall systems is critical to the long-term durability and performance of the building enclosure.

The details associated with this section of the BEDG on the WBDG were developed by committee and are intended solely as a means to illustrate general design and construction concepts only. Appropriate use and application of the concepts illustrated in these details will vary based on performance considerations and environmental conditions unique to each project and, therefore, do not represent the final opinion or recommendation of the author of each section or the committee members responsible for the development of the WBDG.

Description

Selected Terms and Definitions

The following is a summary of selected terms and phrases used throughout this design guide. The definitions provided below are not intended for general design and construction application. Instead, they are intended only to provide a general understanding of these terms as they relate specifically to building envelope design and performance as discussed in this design guide:

Exterior Cladding: Generally defined as a protective layer or finish affixed to the exterior side of a building enclosure system.

The term "cladding" is often used as a general reference to a wide variety of naturally occurring and synthetic, or man-made, building envelope materials, components and systems. Typically, these elements are quarried, manufactured or otherwise developed and/or altered to render them suitable for use on the exterior of a building or structure, and are frequently derived from, or tailored to, the available resources, raw materials and climatic conditions unique to a particular geographic region or exposure. Exterior cladding is generally the first, though not necessarily the primary line of defense against bulk rainwater penetration.

"Wet" Zone: The section of an exterior wall system or assembly that is intended, by design, to be exposed to the short and long-term effects of bulk rainwater penetration and/or moisture ingress. Typically, it is the zone located on the outboard side of the innermost drainage plane in an exterior wall system or assembly.

"Dry" Zone: The section of an exterior wall system or assembly that is not intended to be exposed to the short and long-term effects of bulk rainwater penetration and/or moisture ingress.

Typically, this is the zone located on the inboard side of the innermost drainage plane in an exterior wall system or assembly. Materials in this zone often have a relatively low moisture tolerance, and very little, if any, storage capacity for moisture.

Drainage Plane: Any element exposed to weather or otherwise residing at the line between the "wet" and "dry" zones of an exterior wall system or assembly.

This plane is generally intended to be either waterproof and vapor-impermeable or water resistant and vapor-permeable depending upon wall type, material selection, and climate, and is designed to shed bulk rainwater and/or condensation downward and outward in a manner that will prevent uncontrolled water penetration into the conditioned spaces of a building or structure. In a barrier wall system, the exterior cladding also serves as the principal drainage plane and primary line of defense against bulk rainwater penetration. In cavity wall construction, however, the principal drainage plane and primary line of defense against bulk rainwater penetration is located inside the wall cavity, generally on the inboard side of the air space (either directly applied to the outboard surface of the exterior sheathing layer or, in the case of insulated cavity walls, on the outboard surface of the rigid or otherwise moisture-impervious insulation layer).

Air Barrier: Any element, or combination of elements, that is intended, by design, to control the movement of air across an exterior wall system or assembly.

In order to remain effective, the air barrier must: a) reside within the wall assembly; b) be continuous in three-dimensions from roof-to-wall-to-foundation, c) consist of materials and components that are, either individually or collectively, sufficient in stiffness and rigidity to resist air pressure differentials across the exterior wall assembly without permanent deformation or failure and; d) be durable and structural rigid to withstand the construction process.

The interior and exterior air pressures across an air barrier system that need to be examined include, but are not limited to, pressures caused by wind, stack effect, and mechanical systems.

Air barriers may be located at different locations within a wall system, and the placement of the air barrier needs to be indicated by the designer on the drawings. Please see the table and details within the WBDG that show various methods of forming an air barrier system. Some materials that are part of the air barrier may also have vapor retarder properties. The designer must carefully consider placement of the air barrier when the air barrier material(s) will act both as an air barrier and as a vapor retarder to determine if drying of the system will be inhibited by the location of this material within the assembly. Portions of the air barrier may require regular maintenance and an allowance should be made within the design to accommodate this maintenance.

Air Retarder: Any element that effectively resists or otherwise slows, either intentionally or unintentionally, the rate of airflow across an exterior wall system or assembly.

Depending upon its measurable level of resistance to airflow, the air retarder can, in certain instances, be incorporated into the overall air barrier for an exterior wall system or assembly. However, these products alone typically cannot prevent airflow across a wall assembly and, as such, should not be used in lieu of an effective air barrier.

Vapor Retarder: Any element that is intended to control or otherwise limit the flow of water in its vapor form (diffusive vapor flow, or vapor "drive") across an exterior wall system or assembly.

The selection and placement of a vapor retarder within an exterior wall system or assembly must be in compliance with applicable building codes and regulations, and should also be carefully considered with regard to material selection and the specific use, occupancy, location and climate of a building or structure. It is not uncommon, for example, to discover that individual elements of an exterior wall system or assembly will function inadvertently as "misplaced vapor barriers" in that assembly. Depending upon the predominant direction of diffusive vapor flow across the wall system, this can result in unplanned moisture in the "dry" zone of the wall assembly. This can lead to in-service (and often concealed) corrosion of mild steel anchors, fasteners, metal studs and related components and accessories, as well as moisture-related degradation of wood, gypsum wallboard and similar moisture-sensitive materials, and potential mold growth. A climate-specific, Hygrothermal or similar analysis of moisture transfer through a given exterior wall design is often desirable to further address this concern, particularly as it relates to proper material selection at each layer of the exterior wall assembly.

Insulating Element: Any element, or combination of elements, that is intended to control or otherwise regulate heat loss and heat gain across an exterior wall system or assembly.

In general, the insulating element in an exterior wall system or assembly should be selected and located in a manner that is designed to minimize the risk and effects of thermal bridging ("shorts") and subsequent condensation in the "dry" zone of the assembly. Insulating elements that are selected for placement in the "wet" zone of an exterior wall system or assembly (considered desirable in colder climates of the United States) should be carefully considered during the design phase, and must contain the physical properties and material characteristics necessary to remain fully functional and intact when in the presence of water. Selected rigid insulation products can, when properly detailed, also function as an internal drainage plane within a cavity wall system.

Structural Element: Any element, or combination of elements, in an exterior wall system or assembly that is intended to effectively resist both live (wind) and dead loads acting on a building or structure through the efficient and effective distribution of those loads to the underlying (or exposed) structural frame.

On projects where the design intent is to incorporate, expose, or otherwise express primary elements of the structural frame as part of the exterior wall system, it is critical that these elements be selected, detailed and specified in a manner that will allow for a fully integrated, thermally efficient, weather-tight building envelope. This is particularly true at interface conditions between exposed structural elements and adjacent "infill" sections of exterior walls, where a failure to properly coordinate drainage planes and maintain a continuous thermal break or line of thermal separation across these interfaces can lead to uncontrolled bulk rainwater penetration, condensation, and similar unwanted moisture at the dry side of the wall assembly.

Interface Condition: Interface conditions are generally considered to be the lines of separation, or transition, between individual façade elements in an exterior wall system or assembly. These conditions typically include interfaces between two or more differing wall types occurring within the same wall system (eg: a drained masonry cavity wall adjacent to a precast or EIFS barrier wall system), as well as the lines of separation, or transition, that exist at the perimeter of window openings, mechanical ventilation, electrical conduit, plumbing lines and similar exterior wall penetrations, as well as roof-to-wall, foundation-to-wall, and wall-to-exterior paving interfaces.

Each of these conditions must be considered carefully during the design and submittal review phases of a project in order to successfully coordinate construction tolerances, as well as to anticipate the impact of installation sequencing on the overall constructability and performance of these details. Large-scale, three-dimensional details of these conditions are often desirable, and may be necessary on larger, more complex projects in order to ensure continuity of drainage planes, separation between "wet" and "dry" zones of the exterior wall, air barrier continuity, and overall constructability of the wall system. Similarly, when preparing a performance specification that allows several different glazed aluminum window and/or curtain wall products during bidding, it is also desirable for the architect to select a particular manufacturer and design the interface details around that system. The selected product or system should then be acknowledged in the project specifications so that, in the event that a substitution is made or alternate is submitted for review and consideration, the contractor is required to include interface detailing in those submittals that is consistent with the detailing (and, therefore, the design intent) shown in the original construction documents. This approach ensures that the architect and contractor are properly focused on the importance of interface detailing during the design and bidding phases of the project, and substantially reduces the risk that refinements made during the shop drawing review process will result in added costs to the owner.

Flashing: Any element, or combination of elements, intended to collect, contain or otherwise divert bulk rainwater to the exterior side of a building enclosure system. The principal purpose of flashing is to prevent bulk rainwater penetration into the dry zone of an exterior wall system or assembly and, subsequently, the interior, conditioned spaces of a building or structure. These elements are typically located at interface conditions between primary façade elements of a building enclosure system.

In cavity wall construction, through-wall flashing is typically required, at a minimum, above all wall penetrations and at similar interruptions to the downward flow of rainwater inside the wall cavity. Typically, these materials should be designed in a manner that directs rainwater beyond the exterior wall surface, and should be assembled using materials that are UV stable, non-corrosive and impervious to the potentially negative effects of extreme temperatures and changes in moisture content. Flashing terminations must also be fitted with a self-supporting, fully sealed end-dam to prevent rainwater from entering the dry zone of an exterior wall system or assembly.

End-Dam: Any element, or combination of elements, designed to prevent rainwater collected by a through-wall flashing system from entering the dry zone of an exterior wall system or assembly at wall penetrations (such as window openings), building corners and similar terminations is a single line of flashing.

Moisture Management System: Any element, or combination of elements, intended to control or manage the safe distribution of moisture through an exterior wall system or assembly. A properly designed moisture management system typically addresses each of the following three (3) processes for a given geographic region or climate:

  • Precipitation and Bulk Water Movement
  • Air Flow
  • Diffusive Vapor Flow

The effects of precipitation and bulk water movement, which includes both frozen (snow/ice) and unfrozen (rainwater) precipitation, must be controlled and safely distributed at the building enclosure. Issues to consider when addressing this process include average annual rainfall for a given region and exposure, predominant wind direction and average wind speed during typical rain events, rainfall type and volume, and the rate of wetting that occurs at a given façade element or substrate within the "wet" zone of an exterior wall system or assembly.

Air and diffusive vapor flow are two processes that are particularly vulnerable to improper material selection at each layer of an exterior wall system or assembly. This is particularly true with regard to the proper location and placement of drainage planes, air barriers, and vapor retarders in a given wall assembly, and the behavior of those elements in a given geographic region or climate. Understanding the storage capacity and rate of drying unique to each material selected for use at each layer of an exterior wall system or assembly is critical to the long-term durability and performance of the overall building enclosure. Improper material selection at one layer of an exterior wall assembly can negatively influence the in-service durability and performance of each successive layer, thereby limiting or rendering ineffective the ability of the entire assembly to effectively resist bulk rainwater penetration and moisture ingress.

Computer modeling, together with input and assistance from a design professional specializing in building envelope design and performance, is often desirable to further evaluate these issues. In addition, a submittal requirement for the contractor to develop a comprehensive building enclosure quality assurance program is also desirable, particularly on larger or more complex projects, in order to ensure that each element of the exterior wall moisture management system is properly installed and tested for air and water penetration resistance, as well as thermal performance, prior to installation and acceptance by the design professional and architect/end-user.

Wetting: Wetting can occur as a result of direct or indirect exposure of a façade element, or elements, to bulk rainwater penetration, as well as due to diffusive or convective vapor flow across an exterior wall system or assembly that results in condensation inside the wall system. Once wetted, capillary transfer within, or between, layers of an exterior wall assembly can also occur, and can be further exacerbated by moisture loads inherent to an exterior wall product or material shortly after initial installation.

Drying: Drying can occur by two processes: evaporation and desorption. The following will generally influence the rate of drying of an element within an exterior wall system or assembly:

  • Orientation and exposure of the wetted material within an exterior wall system or assembly
  • Level of saturation of the wetted material
  • Indoor and outdoor ambient air temperature and relative humidity
  • Physical properties of the material itself
  • Individual vapor permeance of each layer of an exterior wall system or assembly
  • Overall vapor permeance of an exterior wall system or assembly
  • Rate and condition of ventilation air moving across the system

Each of these characteristics must be considered during the design phase of a project, and should be carefully evaluated with regard to the geographic region, climate, orientation, and exposure that is specific to each project.

Storage Capacity: Storage capacity is the ability of any material or element in an exterior wall system or assembly to safely absorb and "hold" moisture.

A variety of materials commonly used in exterior wall design and construction can safely absorb and store relatively significant levels of moisture. However, these materials (such as clay brick, concrete masonry and some natural stones) can also be susceptible to long-term, moisture-related deterioration and failure if exposed to repeated and prolonged wetting during the normal service life of a building or structure. In addition to the potentially negative effects of freeze-thaw cycling and efflorescence (the deposit of soluble salts at or near the exposed surface of masonry that often results in discoloration and spalling) that can occur during the drying process in masonry, this is also of particular concern when these materials are located in the "dry" zone of a wall system or assembly. In this location, repeated and prolonged wetting of these materials can contribute to in-service deterioration of adjacent materials through capillary action, as well as the potential for mold growth inside the concealed spaces of the wall system or assembly. Typically, the use of materials with relatively high storage capacities for moisture, particularly when used as a means for moisture management in an exterior wall system or assembly, should be located in the "wet" zone of the wall.

Diffusive Vapor Flow: The transfer of moisture in its gaseous state through the various layers of an exterior wall system or assembly.

The rate and predominant direction of diffusive vapor flow is directly related to, and influenced by interior and exterior ambient air temperatures and relative humidity, as well as differences between interior and exterior vapor pressures and the individual vapor permeability of each layer in a given exterior wall system or assembly. As noted previously, moisture-related problems due to improperly located vapor retarders within an exterior wall system or assembly are often the result of improperly inhibited or otherwise restricted diffusive vapor flow. This is particularly true in mixed-humid climates, where the predominant direction of diffusive vapor flow in a given year can be difficult to accurately predict.

Gravity Flow: The flow of moisture through an enclosure after a wetting event caused by the downward force of gravity on water in its liquid state.

Capillary Action: The absorption of moisture into small voids in a material until the void is full. Capillary action, or "wicking" of water into cellulose-based façade elements, such as wood and wood fiber-based products, is a common source of in-service, moisture-related deterioration of exterior wall systems and assemblies.

Wind-Driven Rain: The process by which rainwater is "driven", or forced, through an exterior wall system or assembly, due either to existing voids in the wall system itself, or to voids created by allowable, in-service deflection of the wall system under applied wind loads.

In the design of glazed aluminum windows, curtainwall, skylights and storefront framing systems, effective control and management of wind-driven rainwater are basic design concepts that have been well understood for over 40 years. The design of these systems, and the corresponding performance classes assigned to the various products available from this industry, are predicated on the performance of these systems when subjected to simulated wind-driven rain events during both laboratory and field mock-up testing. The test pressures used during these tests are influenced by building height and gust factors unique to each project, and are typically calculated using basic wind speeds assigned to each geographic region of the United States by both local and national building codes.

Advective Moisture Flow: The bulk movement of air as a mechanism for the transfer of moisture in its vapor state across an exterior wall system or assembly.

For example: In humid and mixed-humid climates, moisture-laden air that enters into an enclosure and comes in contact with elements below the dew point (due to the element being at a cooler temperature) can result in condensation within the enclosure and may lead to long-term moisture problems. An example of this is relatively humid, unconditioned exterior air entering into a steel stud wall during the summer cooling season, where it then comes into contact with steel stud surfaces that are sufficiently low in temperature for condensation to occur on the steel stud surface.

Convective Moisture Flow: The bulk movement of moisture involving the transfer mechanisms of molecular diffusion and advection.

For example: The drying rate of a rain-saturated brick veneer can be predicted through an analysis of this process, thereby further defining the potential impact that this and similar materials with a relatively high storage capacity for moisture can have on inward-acting diffusive vapor flow across an exterior wall system or assembly.

Conductive Heat Flow: The flow of heat through direct molecular contact, either through a single material or through multiple materials. For solid materials (and thus building materials), this is the method by which most heat flow or transfer typically occurs. For example, materials used in an exterior wall assembly that are considered to be highly conductive to heat flow can result in significant heat loss or gain through the assembly if not thermally protected, or separated, within the enclosure.

Convective Heat Flow: The flow of heat by molecules (either liquid or gas) via a change in their heat content. This method of heat flow can happen between fluids and solid elements, or strictly within fluids.

Radiation: The transfer of heat by electromagnetic waves through a gas (or vacuum), and requires a line of sight between the source and the contact surface. Since all objects above absolute zero radiate heat, the net transfer of heat is the condition that must be considered. Radiation is a relatively common mechanism for some methods of heating and cooling (radiant heating or cooling, for example), and is also a concept that must be understood to properly employ shading devices for passive heating and cooling of exterior wall systems and assemblies.

Infiltration: The introduction of unconditioned air into the conditioned, interior spaces of a building or structure due to voids in the enclosure system.

Exfiltration: The loss of conditioned air from a building or structure due to voids in the enclosure system, and/or the introduction of conditioned air into unconditioned spaces of an exterior wall assembly.

Mixing: The process by which unconditioned air and conditioned meet, and mix, within an exterior wall assembly. Depending upon the air temperature and relative humidity in the cavity space at the time that mixing occurs, condensation may form on the colder elements located in the dry zone of the wall assembly (such as metal studs during the summer cooling season).

Change of State: A change of state, or phase change, from liquid to gas, or from liquid to solid, which results in a gain or loss of energy. The movement of energy can become latent heat. Changes of state occur at constant temperature.

A basic understanding of these terms and phrases is required in order to properly understand and interpret the information conveyed in this guide by the design professional.

Basic Exterior Wall Types

Exterior wall types commonly associated with above-grade, commercial building enclosure design and construction in the United States can generally be classified as follows: as a cavity wall, a barrier wall, or a mass wall. Following is a summary of the characteristics of each wall type:

Cavity Wall

A cavity wall (also referred to as "screen" or "drained" wall systems) is considered by many to be the preferred method of construction in most climatic and rainfall zones in the United States. This is due primarily to the pressure-equalization that can be achieved, and the redundancy offered by this type of wall assembly to resist uncontrolled, bulk rainwater penetration. A term commonly used to describe clay brick and/or concrete masonry wall systems installed over a largely open, unobstructed air space/drainage cavity, this term is now used more generically to define any wall system or assembly that relies upon a partially or fully concealed air space and drainage plane to resist bulk rainwater penetration and, depending upon the design, to improve the overall thermal performance at the building enclosure. Drained cavity walls typically include the following general characteristics:

  • An exterior cladding element that is intended to either shed or absorb the majority of bulk rainwater penetration before it enters the concealed spaces of the wall assembly (the initial, though not primary, line of defense against rainwater penetration in this type of wall assembly).
  • A drainage cavity, or air space, that is intended to collect and control rainwater that passes through the exterior cladding element and re-direct that water to the building exterior. The cavity may be ventilated for pressure equalization, either mechanically or passively, to facilitate this process by preventing negative pressure that may draw rainwater across the cavity into the "dry" sections of the wall assembly via anchors, wall ties, and similar penetrations).
  • An internal drainage plane that is intended to function as the primary line of defense against uncontrolled rainwater penetration. This layer serves functionally as the dividing line between the "wet" and "dry" sections, or "zones," of the exterior wall assembly. This layer can be created using a variety of both dry sheet-good or wet, trowel-applied products depending upon the climate in which the building is to be located and the desired level of vapor permeability necessary to prevent condensation and potential mold growth on the dry side of the exterior wall assembly.
  • An insulating layer, which can be located either inboard or outboard of the internal drainage plane depending upon the geographic region and climate in which the building or structure is to be located.

Although a drained cavity wall offers many advantages over the other three (3) types of exterior wall systems discussed in this section, it should be noted that an improperly designed and/or executed cavity wall system can be very disruptive and costly to properly and effectively repair after construction is complete. Corrosion of mild steel wall ties, structural connections and related wall elements, together with interior mold growth, often remain concealed from view in this type of wall system, and can continue for a period of years before manifesting themselves in a location that can be readily observed and remediated. Furthermore, because the primary drainage plane and many of the most critical interface details are often concealed inside the wet zone in this type of wall system, direct intervention and repair of these elements can be highly invasive and disruptive to an occupied building, and will often negatively impact the overall appearance of the building. To mitigate these concerns, a comprehensive building envelope quality assurance program similar to the program discussed later in this section is often considered extremely desirable with this type of wall system in order to ensure that critical cavity wall elements are properly designed and effectively installed at the time of original construction.

In pressure-equalized, "rainscreen" cavity wall systems, the primary drainage plane, and principal air barrier are located in the same plane between the wet and dry zones of the wall assembly. In colder climates, the insulation is also placed outboard of the innermost (primary) drainage plane in this type of wall assembly, inside the wet zone (drainage cavity) of the wall. This approach, which dates back to the 1960's in North America, can be extremely effective in resisting uncontrolled, bulk rainwater penetration. However, the principal advantage of this system, which is to prevent a negative air pressure differential from occurring across the exterior wall assembly (a condition that can "draw" rainwater through the enclosure and into the building), can also be extremely difficult to effectively achieve in the field. This is due primarily to the relatively complex detailing often required at exterior wall penetrations through the concealed air barrier and primary drainage plane, and the correspondingly high level of workmanship required to effectively seal those conditions to prevent the flow of unconditioned air inward across the exterior wall assembly.

Barrier Wall

As the name implies, this term is commonly used to describe any exterior wall system of assembly that relies principally upon the weather-tight integrity of the outermost exterior wall surfaces and construction joints to resist bulk rainwater penetration and/or moisture ingress. This type of wall system is commonly associated with precast concrete spandrel panels, certain types of composite and solid metal plate exterior cladding systems, and early generation exterior insulation and finish systems (EIFS). Although often considered a more cost-effective and, therefore, desirable alternative to either cavity or mass walls assemblies, barrier walls are cause for some concern in that they: a) offer only a single line of defense against bulk rainwater penetration; b) often include relatively complex interface details that require a level of workmanship in the field that is beyond the capabilities of the individual trades, and; c) require a relatively high degree of routine maintenance to remain effective in the long term, resulting in increased long-term maintenance costs. In short, this system can arguably be considered a "zero tolerance" wall system, whereby any defect in design, installation, or workmanship can result in immediate and direct rainwater penetration into the dry zone of the exterior wall system or assembly and, more critically, the conditioned spaces of a building or structure.

Mass Wall

Unlike a cavity wall system, where the wall is constructed with a wall cavity and through-wall flashing to collect and redirect bulk rainwater to the building exterior, mass walls rely principally upon a combination of wall thickness, storage capacity, and (in masonry construction) bond intimacy between masonry units and mortar to effectively resist bulk rainwater penetration. For economic reasons, mass walls are less common in design and construction today. However, when constructing an addition, or incorporating a portion of an existing building into a new building or structure, the design and behavior of mass walls relative to storage capacity and both heat and moisture transfer must be understood by the design professional. In addition to bulk rainwater penetration and moisture ingress that is often difficult to track (and therefore effectively isolate and repair) in this type of wall construction, the potentially negative effects of drying must also be considered when designing around or otherwise restoring this type of wall system. Evaporative drying across this type of wall assembly, either to the interior or exterior, can contribute to efflorescence (soluble salts deposited at or near the wall surface, which can lead to visible discoloration and spalling), deterioration of interior portland cement plaster finishes (a relatively common finish in mass walls constructed in the early 20th century), and organic/microbial growth on either the interior or exterior exposed wall surfaces.

Cavity wall diagramBarrier wall diagram

Left: Figure 1. Cavity Wall Diagram and Right: Figure 2. Barrier Wall Diagram

Mass wall diagram

Figure 3. Mass Wall Diagram

Basic elements of the exterior wall

Figure 4. Basic Elements of the Exterior Wall
1. Exterior Cladding (Natural or Synthetic)
2. Drainage Plane(s)
3. Air Barrier System(s)
4. Vapor Retarder(s)
5. Insulating Element(s)
6. Structural Elements

Wall system functions

Figure 5. Wall System Functions

Moisture transfer diagram ('Hot-humid' climate shown)

Figure 6. Moisture Transfer Diagram ("Hot-humid" climate shown)

Rain protection of mass walls, and understanding the rate the wall will get wet, the amount of moisture it is capable of storing, and the drying rate become important design considerations, as exceeding the safe storage capacity for long periods of time may create long term moisture problems for the surrounding substrates that might come in contact with the mass wall. In some cases, if adjoining or renovating an existing building while constructing an addition or a new building, it may be appropriate to retrofit a mass wall and turn it into a drained wall, especially if interior finishes have been experiencing moisture related problems caused by water and/or moisture infiltration through the system.

Common Elements of an Exterior Wall

Each of the above wall types, or combination thereof, generally consists of the following basic elements, or layers:

  • Exterior Cladding (Natural or Synthetic)
  • Drainage Plane(s)
  • Air Barrier System(s)
  • Vapor Retarder(s)
  • Insulating Element(s)
  • Structural Elements

Several of these layers may, at the discretion of the design professional, serve multiple purposes. For example, in barrier wall design and construction, the exterior cladding material may be designed to function both as the primary drainage plane and principal air barrier for a building or structure. Similarly, in cavity wall construction, rigid insulation placed inside the exterior wall cavity, if properly detailed, may also function as the air barrier and a drainage plane for a given exterior wall system or assembly.

Decisions such as these are typically made during the Schematic Design phase of a project, when basic programmatic requirements such as building use, orientation, environmental exposure, and overall response to the surrounding climate (micro and macro) are first given consideration by the design team. These decisions are then further refined during the Design Development and Construction Document phases of a project, when a more detailed and specific response to issues and concerns related to air/moisture transfer through the building enclosure, material selection, coordination of drainage planes, interface detailing, and the long-term durability and performance of exterior wall systems and assemblies is required. This process can be defined as a building enclosure assessment process. Throughout this process, it is both useful and advantageous for the design professional to consider the building enclosure as a series of layers that, with proper material selection and the effective coordination of drainage planes at interface conditions, should result in a fully integrated, thermally-efficient and weather-tight building enclosure. The ability of the material to meet the design intent at each layer, whether it is intended to be the drainage plane product, a vapor retarder, an air barrier or part of the rainwater management system, must be analyzed. Careful review of the material properties at each layer, including the air and vapour permeance, resistance to bulk water entry (depending on where the material is located within the assembly), structural rigidity, and bulk-water storage capacity needs to be completed. Once completed, the building enclosure system as a whole need to be critically examined to understand how the materials and components are interacting.

The durability, and the limitations and appropriateness of the materials used for the building enclosure, especially for critical layers such as the drainage plane and air barrier, also require close examination during the building enclosure assessment process. This review is critical as the use of inappropriate or less durable building enclosure products may result in unplanned premature failure of the enclosure and higher long-term maintenance costs that are greater than initial costs of the superior material or component.

Interface Conditions1

Uncontrolled rainwater penetration and moisture ingress are two of the most common threats to the structural integrity and performance of the building envelope. Together, they represent up to 80% of all construction-related claims in the United States.2 Of these failures, there is a growing body of evidence based on our experience, that suggests that errors and omissions in the design and installation of the façade interfaces, rather than in the façade materials, components and systems themselves, are the primary (and most frequently overlooked) sources of uncontrolled rainwater penetration through the building envelope. To better understand the factors influencing this trend, one must first begin with an examination of the design phase of a project.

1Lemieux, Daniel J. and Driscoll, Martina T., "Breaking the Skin", International Conference for Building Envelope Science and Technology (ICBEST) 2004
2Bomberg, M.T. and Brown, W.C. (1993), "Building Envelope and Environmental Control: Part 1-Heat, Air and Moisture Interactions" Construction Canada 35 (1), 15-18.

Often, the proper integration of related components of a building façade is doomed from the start. All too frequently, the demands of both time and budget combine to create an environment where architects are forced to develop a series of largely generic building envelope details in a short period of time in order to get a project out to bid. While this approach may be successful in bringing the design phase in "on time and under budget," it often proves short-sighted and costly for an owner when an incomplete set of construction documents is the result. Although it is difficult for an architect who has prepared a performance specification to fully detail façade interface conditions during the design phase of a project, due in large part to fact that the actual products and product profiles to be supplied for a job are not yet known, it is critical that the construction documents carry enough information to convey both the importance of a fully-integrated building façade and the design intent of the architect with regard to how those materials and systems (and their often conflicting levels of detail and specified performance) will be reconciled at the interfaces.

A failure on the part of the architect to provide this information prior to bidding effectively shifts the design responsibility for these conditions from the architect to the general contractor and/or the individual subcontractors responsible for each portion of the work. This approach, while perhaps expedient in that it can reduce the initial design cost and limit the liability and exposure of the architect, has the practical effect of elevating the shop drawing submittal process from a simple review for "overall design intent" to an exercise in additional (and often complex) detailing that is beyond the capacity of the contractors involved and, more importantly, beyond the scope of the originally bid work and the contractual responsibilities of the individual subcontractors.

"Value Engineering"

The impact of these and related decisions on the initial cost of a building or structure also must be considered, particularly during the owner/contractor "value engineering" (VE) process. Efforts by the contractor or owner/developer to reduce the initial cost of a building or structure by applying the principles of value engineering to the design and construction of the building enclosure must be carefully weighed by the Architect/Engineer-of-Record against the best long-term interests of the owner/end-user based on the intended service life of the building or structure. This is particularly true with regard to laboratory and field performance testing, as well as the selection of through-wall flashing materials and related moisture management systems and accessories. In too many instances, pre-construction laboratory mock-up and field quality assurance tests typically required by the design professional to verify the constructability and performance of the building enclosure are considered cost-prohibitive on a project and, therefore, eliminated during the VE process. Similarly, through-wall flashing materials selected by the design professional, in part, for long-term durability and performance (such as stainless steel or lead-coated copper flashings and drips) are often substituted for lower performing materials that, despite their lower initial cost, are significantly more vulnerable to in-service degradation and failure. The cost associated with successfully addressing uncontrolled rainwater penetration and moisture ingress arising out of decisions made during the VE process can be significant, and the work highly disruptive and invasive. Again, the desired outcome of the VE process must be carefully weighed by the design professional against the long-term interests of the owner/end-user, and the intended service life of the building or structure.

Fundamentals

Basic Functions

The envelope, or "enclosure," of a building or structure serves a variety of basic functions. As noted by Burnett and Straube3 and others, the enclosure is a separation between the interior and exterior environment that experiences a variety of loadings, including, but not limited to, structural loadings, both static and dynamic, and heat, air, and moisture loads. The enclosure must then control these loads and support these loads. This includes both short-term and long-term loadings. The enclosure can also be used to carry or distribute some services within the building. In addition, the enclosure will have several aesthetic attributes, which can be summarized as finishes. We have attached a brief description of the intent of design for three of these loadings: heat, air, and moisture.

3Dr. Eric Burnett, Dr. John Straube, Various Technical Papers and Publications

Burnett and Straube have defined four general building enclosure function categories. All enclosure elements have to provide the following functions:

  • Support
  • Control
  • Finish (aesthetics)
  • Distribution of Services (where required)

Support

Enclosures, including exterior wall systems, must be capable of withstanding all internal and external forces applied to them. The majority of these forces are structural loading. The loads include both static and dynamic loading including, but not limited to, dead load, live loads, wind loads, earthquake loads and possible blast loads. These loads have to be properly supported, resisted, and transferred.

Control

Enclosures, including wall systems, have to be able to control mass, energy, and particulate flows both within and across the system. These include, but are not limited to, heat, air, moisture, smoke, odor, fire, blast, birds, and insects.

Finish

The finish function at both the exterior and interior is the aesthetics of the finished surface, the visual, textural, and other aspects the designer wishes to convey with the visible elements of the system. Of the elements of an enclosure, wall systems typically have the largest consideration for finish.

Distribution

This function relates to the distribution of services through a building, both within a single element, and also through multiple elements.

Design Considerations

It is important to have a basic understanding of the mechanisms that can affect air and moisture transfer and how material selection, design, and construction can impact the proper drying of a building enclosure in any climate. In particular, architects, engineers, contractors, building scientists, owners and others involved in the construction and maintenance of the building enclosure must understand the wetting and drying process, the safe storage capacity for moisture of the materials specified, and the manner in which those materials are likely to behave in a given climate. They must understand how poor design and/or construction with limited regard to the wetting/drying/storage process can have a potentially devastating impact on the long-term durability and performance of the building enclosure.

In general, moisture moves from higher temperatures to lower temperatures, from a higher pressure to a lower pressure (whether air or vapor pressure), and from areas of higher moisture content to areas of lower moisture content. However, these driving forces do not always act in the same direction, and can be affected by interior and exterior air pressure differences, moisture loads, and material vapor pressure differences. Moisture transferred by these mechanisms may occur through capillary action, gravity forces, diffusion, or through voids in a substrate. In a humid or mixed-humid climate during warmer months, moisture may also be deposited within the building enclosure when warm moisture-laden exterior air transferred through voids in the exterior substrate is allowed to come in contact with and mix with cool conditioned air that is leaking from the interior.

The location and source of the moisture must be properly considered and implemented during the design and construction process to ensure a durable building. External moisture sources, such as rainwater, irrigation systems, planters, and moisture-laden air, as well as interior moisture sources, such as plumbing leaks, sprinkler failures, or improperly exhausted bathrooms, kitchens, laundry rooms, steam rooms, or swimming pools are common sources of moisture that must be properly controlled. A total system approach needs to be considered so that sections of the design do not drastically alter the assumptions of the enclosure designer, or mechanical system designer.

An effective building enclosure should also include a continuous and defined air barrier that is rigid enough to survive wind loading and air pressures across it, durable enough to remain intact throughout construction, and installed in such a way that it is continuous between building elements. The air barrier should be thought of in three-dimensional terms rather than a two-dimensional drafting detail. It is not desirable in a mixed-humid climate to have an air barrier material that also has vapor retarder properties since depending on where the air barrier is placed in the assembly it has the potential to hamper drying to the interior in warmer months and to the exterior in colder months. In other climates (cold, hot-humid) it is not desirable to have the air barrier material have vapor retarder properties if it is to be placed on the cold side of the wall, since this can create a moisture problem during drying.

If a vapor retarder is required, it should only be installed on the warm side of the wall. However, the warm side of a wall or other building element is defined differently in cold, warm, and mixed climates. Based on research by several practitioners, and recommendations, a vapor retarder may not be required in some mixed-humid climates at wall elements where the building needs to dry to the interior in the summer and to the exterior in the winter. However, design of a system without a vapor retarder needs to be examined and closely coordinated with the design of the mechanical system.

A moisture problem generally occurs when the building element susceptible to damage (such as rot, corrosion, and microbial growth) is exposed to air, and allowed to remain "wet" at a level that is above its safe storage capacity for moisture for an extended period of time. This can occur when a building element is exposed to direct and repeated rainwater penetration, or is otherwise inhibited in some way from effectively "drying" due to improper design and/or construction. If left untreated, these materials, which very often contain organic, cellulose or cellulose-based products, will then create a condition inside the wall assembly that is conducive to mold growth and other moisture related damage.

Performance Issues

Thermal Performance


Heat Transfer

There are four methods by which heat can move through any substance. These are conduction, convection, radiation, and heat transfer through a change of state. Designers need to understand heat transfer in order to determine allowances for thermal movements (both contraction and expansion) of each element and between elements, in order to calculate the energy efficiency of the design and determine potential energy use for the structure, examine durability and to examine the risk of moisture related problems, based on temperature and mainly the risk for freezing or condensation within the system. As defined by Straube [4] and others, and expanded on here, the following are definitions that can be applied to heat transfer.

Conductive Heat Flow: The flow of heat through direct molecular contact, either through a single material or through multiple materials. For solid materials (and thus building materials), this is the method by which most heat flow or transfer typically occurs. For example, materials used in an exterior wall assembly that are considered to be highly conductive to heat flow can result in significant heat loss or gain through the assembly if not thermally protected, or separated, within the enclosure.

Convective Heat Flow: The flow of heat by molecules (either liquid or gas) via a change in their heat content. This method of heat flow can happen between fluids and solid elements, or strictly within fluids.

Radiation: The transfer of heat by electromagnetic waves through a gas (or vacuum), and requires a line of sight between the source and the contact surface. Since all objects above absolute zero radiate heat, the net transfer of heat is the condition that must be considered. Radiation is a relatively common mechanism for some methods of heating and cooling (radiant heating or cooling, for example), and is also a concept that must be understood to properly employ shading devices for passive heating and cooling of exterior wall systems and assemblies.

Change of State: A change of state, or phase change, from liquid to gas, or from liquid to solid, which results in a gain or loss of energy. The movement of energy can become latent heat. Changes of state occur at constant temperature.

When designing, it is important to have a working knowledge of these concepts, as well as how various HVAC systems are used not only to heat and cool a space, but also to distribute fresh air and to pressurize a building. Although there are several commercially available software programs that can be used to calculate the energy efficiency of a building enclosure system, these software programs are somewhat limited in that they cannot accurately determine the net effect of air transfer through voids in the enclosure system on heat loss or gain across an exterior wall system or assembly.

Moisture Protection


Air Transfer

Air can transfer both heat and moisture through an enclosure system. Voids in the air barrier, and at interfaces in the enclosure, can allow a significant amount of conditioned air to exit the building and, conversely, a significant amount of unconditioned exterior air to enter the building. Because air is a much larger transfer mechanism for moisture than vapor flow, the following processes must be adequately controlled as part of the building enclosure moisture management system:

Infiltration: The introduction of unconditioned air into the conditioned, interior spaces of a building or structure due to voids in the enclosure system.

Exfiltration: The loss of conditioned air from a building or structure due to voids in the enclosure system, and/or the introduction of conditioned air into unconditioned spaces of an exterior wall assembly.

Mixing: The process by which unconditioned air and conditioned meet, and mix, within an exterior wall assembly. Depending upon the air temperature and relative humidity in the cavity space at the time that mixing occurs, condensation may form on the colder elements located in the dry zone of the wall assembly (such as metal studs during the summer cooling season).

Moisture Transfer

Moisture transfer can occur through a building through multiple mechanisms. Moisture related problems are perhaps the largest set of problems buildings experience within the United States. The design community needs to have a better understanding and employ better design practices to reduce the number of moisture related problems that buildings within the United States experience. Elements within the wet zone of the building (outbound of the final drainage plane or barrier) are allowed to become wet. The susceptibility of the elements within the wet zone to moisture related problems needs to be carefully examined as well as the susceptibility of inadvertent wetting of elements within the dry zone. Several proprietary software programs are now commercially available and can be used to examine the wetting, storage of moisture, and drying of the enclosure based on actual rainfall and climate specific data. Designers need to ensure that these three processes are understood, and that the enclosure will properly balance the wetting, storage capabilities, and drying of the enclosure.

Wetting: Wetting can occur as a result of direct or indirect exposure of a façade element, or elements, to bulk rainwater penetration, as well as due to diffusive or convective vapor flow across an exterior wall system or assembly that results in condensation inside the wall system. Once wetted, capillary transfer within, or between, layers of an exterior wall assembly can also occur, and can be further exacerbated by moisture loads inherent to an exterior wall product or material shortly after initial installation.

Drying: Drying can occur by two processes: evaporation and desorption. The following will generally influence the rate of drying of an element within an exterior wall system or assembly:

  • Orientation and exposure of the wetted material within an exterior wall system or assembly
  • Level of saturation of the wetted material
  • Indoor and outdoor ambient air temperature and relative humidity
  • Physical properties of the material itself
  • Individual vapor permeance of each layer of an exterior wall system or assembly
  • Overall vapor permeance of an exterior wall system or assembly
  • Rate and condition of ventilation air moving across the system

Each of these characteristics must be considered during the design phase of a project, and should be carefully evaluated with regard to the geographic region, climate, orientation, and exposure that is specific to each project.

Storage Capacity: Storage capacity is the ability of any material or element in an exterior wall system or assembly to safely absorb and "hold" moisture.

A variety of materials commonly used in exterior wall design and construction can safely absorb and store relatively significant levels of moisture. However, these materials (such as clay brick, concrete masonry and some natural stones) can also be susceptible to long-term, moisture-related deterioration and failure if exposed to repeated and prolonged wetting during the normal service life of a building or structure. In addition to the potentially negative effects of freeze-thaw cycling and efflorescence (the deposit of soluble salts at or near the exposed surface of masonry that often results in discoloration and spalling) that can occur during the drying process in masonry, this is also of particular concern when these materials are located in the "dry" zone of a wall system or assembly. In this location, repeated and prolonged wetting of these materials can contribute to in-service deterioration of adjacent materials through capillary action, as well as the potential for mold growth inside the concealed spaces of the wall system or assembly. Typically, the use of materials with relatively high storage capacities for moisture, particularly when used as a means for moisture management in an exterior wall system or assembly, should be located in the wet zone of the wall.

Diffusive Vapor Flow: The transfer of moisture in its gaseous state through the various layers of an exterior wall system or assembly.

The rate and predominant direction of diffusive vapor flow is directly related to, and influenced by interior and exterior ambient air temperatures and relative humidity, as well as differences between interior and exterior vapor pressures and the individual vapor permeability of each layer in a given exterior wall system or assembly. As noted previously, moisture-related problems due to improperly located vapor retarders within an exterior wall system or assembly are often the result of improperly inhibited or otherwise restricted diffusive vapor flow. This is particularly true in mixed-humid climates, where the predominant direction of diffusive vapor flow in a given year can be difficult to accurately predict.

Gravity Flow: The flow of moisture through an enclosure after a wetting event caused by the downward force of gravity on water in its liquid state.

Capillary Action: The absorption of moisture into small voids in a material until the void is full. "Wicking" of water into cellulose-based façade elements, such as wood and wood fiber-based products, is a common source of in-service, moisture-related deterioration of exterior wall systems and assemblies.

Advective Moisture Flow: The bulk movement of air as a mechanism for the transfer of moisture in its vapor state across an exterior wall system or assembly.

For example: In humid and mixed-humid climates, moisture-laden air that enters into an enclosure and comes in contact with elements below the dew point (due to the element being at a cooler temperature) can result in condensation within the enclosure and may lead to long-term moisture problems. An example of this is relatively humid, unconditioned exterior air entering into a steel stud wall during the summer cooling season, where it then comes into contact with steel stud surfaces that are sufficiently low in temperature for condensation to occur on the steel stud surface. Moisture-related problems due to diffusive vapor flow can also occur with a misplaced vapor retarder.

Convective Moisture Flow: The bulk movement of moisture involving the transfer mechanisms of molecular diffusion and advection.

For example: The drying rate of a rain-saturated brick veneer can be predicted through an analysis of this process, thereby further defining the potential impact that this and similar materials with a relatively high storage capacity for moisture can have on inward-acting diffusive vapor flow across an exterior wall system or assembly.

Wind-Driven Rain: The process by which rainwater is "driven", or forced, through an exterior wall system or assembly, due either to existing voids in the wall system itself, or to voids created by allowable, in-service deflection of the wall system under applied wind loads.

In the design of glazed aluminum windows, curtainwall, skylights, and storefront framing systems, effective control and management of wind-driven rainwater are basic design concepts that have been well understood for over 40 years. The design of these systems, and the corresponding performance classes assigned to the various window and door products available from this industry, are predicated on the performance of these systems when subjected to simulated wind-driven rain events during both laboratory and field mock-up testing. The test pressures used during these tests are influenced by building height and gust factors unique to each project, and are typically calculated using basic wind speeds assigned to each geographic region of the United States by both local and national building codes.

Fire Safety

Please see each individual wall type section for specific information on fire safety.

Acoustics

Please see each individual wall type section for specific information on acoustics.

Productivity

It is widely understood, and has been shown in a variety of different studies, that human comfort is directly related to productivity and performance. This is particularly true in the workplace, and has been a core principle of good design practice with regard to commercial space planning and the design of the building enclosure for years. The amount of daylight allowed into interior spaces, and the individual control of thermal comfort in office spaces, are two things that sustainable design tries to capture to improve worker productivity. The thermal efficiency of the enclosure becomes critical to allowing individual temperature control of an occupant's space, as a highly inefficient enclosure will allow more heat gain and loss and may cause unintended thermal cycling. Additionally, air infiltration at fenestration elements, window condensation problem, and direct water ingress will all provide occupants with distractions that will pull there attention away from the task at hand.

The indoor air quality (IAQ) can also be affected by a moisture related failure in the building enclosure. Microbial growth may occur within the exterior wall assembly or at any other building element may create a condition where "free" uncontained mold is able to make it to occupied space and potentially affect the occupant's heath and well being. Please see the Mold section for more information.

It is therefore imperative that the enclosure, the boundary that is meant to separate the interior environment from the exterior environment, be carefully designed and constructed so that a thermal, air, or moisture related failure does not occur that can eventually lead to affecting occupant comfort levels and thus negatively impact worker productivity.

Material/Finish Durability

All dissimilar metals and metal accessories that reside inside the "wet" zone of exterior wall assemblies require separation with a non-metallic, UV-stable isolator to prevent galvanic-induced corrosion of the less noble metal. Similarly, galvanic potential caused by rainwater run-off between dissimilar metals should also be considered during design and detailing of exterior wall assemblies.

Please see each individual wall type section for specific information on material finishes and durability.

Maintainability

Please see each individual wall type section for specific information on maintainability.

Quality Assurance4

The following is an outline of a proposed building envelope quality assurance (commissioning) program currently being developed by committee for potential use and incorporation into NIBS Guideline No. 3. The information presented is in narrative form. However, it should be understood that, in order to be effective, this process must be expanded to include sub-tasks and milestones intended to address the individual quality assurance requirements of each façade material, component or system, as well as the goals and requirements unique to each project.

Also see other WBDG Commissioning pages: Building Commissioning, Determine Project Performance Requirements, Document Compliance and Acceptance, Plan the Commissioning Process

4Lemieux, Daniel J. and Totten, Paul E., "The Importance of Building Envelope Commissioning for Sustainable Structures", Performance of Exterior Envelopes of Whole Buildings IX, International Conference

Preliminary Design Meeting

A preliminary design meeting should be held between the building enclosure quality assurance specialist (hereinafter referred to as commissioning agent) and the owner, owner's representatives, architect of record, mechanical engineer, contractor (if already determined and hired) the mechanical commissioning authority, other consultants (including the LEED consultant if sustainable design is required), and other applicable members of the design team to discuss the project and the commissioning agent's involvement as a sub-consultant to the mechanical commissioning authority, and to better understand what objectives the owner wishes to achieve. Any drawings and specifications already prepared should be made available to the commissioning agent prior to this meeting. From this meeting, the commissioning agent should be able to develop a list of building enclosure elements that may potentially affect the building enclosure objectives (high thermal efficiency) the team is intending to capture, and what elements may be improved to maximize other potential objectives that team may be able to pursue, based on the owners needs. The commissioning agent will also identify what systems of the building enclosure may likely require a more thorough review and what types of performance testing should be included in the contract documents to verify air and water penetration resistance of the fenestration, wall, and roofing systems, and if appropriate, portions of the foundation system. The above building enclosure quality assurance process is supplementary to the integrated design process regarding the total building design and development.

Schematic Design and Coordination

The architect of record, mechanical engineer and, if appropriate based on the scale and complexity of the project, the commissioning agent should meet to discuss the design intent, and complete design charettes to determine potential design options and building enclosure systems for the design, the owner's requirements, and develop an initial basis of design. As part of this process, the design team should identify and review the overall design intent of the building enclosure systems (barrier versus rain screen versus cavity wall), material selection choices, interface complexities, budget restraints, and specific site specific climate concerns that the enclosure design will need to address. The design team should consider whole building integration and the interaction of the building enclosure with the mechanical systems. This set of charettes may be done separately from initial charettes with the architect of record and other consultants (such as a LEED consultant) to aid the team in determining initial project decisions and direction. The architect will then need to prepare initial schematic designs and 3-D models of the intended structure and site-specific building orientation.

Design Development

The architect of record and other members of the design team, together with the commissioning agent, should review and research materials for the various systems and assemblies now identified as potential systems that will satisfy the owner requirements. In developing the design, computer modeling for a variety of different attributes and elements that will affect the system should be completed. Various system and sections should be evaluated and compared. The design team should model sections and different combinations of materials and assemblies for vapor drive, moisture storage capacity, environmental impact, geographic implications, siting, exposure, microclimate/macroclimate, solar/shading, thermal efficiency and rainwater resistance, especially wind driven rain. Energy modeling and moisture analysis modeling should both be completed. Computer modeling for heat and moisture transfer should be completed on wall, roof, window and similar building envelope components and assemblies. Moisture transfer modeling will aid the designer in determining if the envelope they have picked may experience long term or short-term moisture problems. Various programs are available to complete the moisture-modeling task, with the better programs being based on long-term historical climate data and region specific wind driven rain data. Energy modeling is useful in refining the thermal design characteristics that can in turn be used to optimize the mechanical system design. From this, a refined set of design documents should be developed, including initial material specifications.

Depending on the project stage for the design team, the commissioning agent should peer-review one initial set of the Design Development drawings and outline specifications pertaining to the building enclosure and provide a written summary of issues and concerns noted during this review. The commissioning agent should provide additional information to the mechanical commissioning authority to be included in the draft commissioning plan and specifications pertaining to the building enclosure systems and subsystems.

Preliminary Construction Documents

The commissioning agent should review relevant portions of the construction documents and project specifications at roughly 50 percent completion, and provide a written summary of issues and concerns noted during this review related to heat, air, moisture transfer and bulk rain water penetration resistance. The commissioning agent can then perform a value engineering review, if requested, to indicating areas where cost tradeoffs can be reasonably applied to reduce the overall cost of the project without significant sacrifice to the long-term durability and performance of the building enclosure. During this review, the commissioning agent should pay particular attention to the requirements in the specifications for samples, technical data, mock-ups, performance testing, and the details at interface conditions as shown on the drawings. The objective of this review will be to identify issues and concerns that may compromise the water tight integrity and moisture and thermal performance of the building enclosure.

During this portion of the commissioning process, the design team may need assistance in developing conceptual details and may require additional information for the building enclosure systems. If any details, plans, or specification sections are modified by the architect of record in response to this review, the commissioning agent should review those changes prior to issuance of the construction documents for bidding. If elements recommended for change by the commissioning agent have not been accepted by the architect of record, they should notify the commissioning agent as to why and the commissioning agent will then need to provide written documentation to the owner, indicating what risks may be associated with not implementing those changes.

One or more meetings will need to be completed with the design team, the owner and owner's representatives, the mechanical commissioning agent, and other appropriate consultants to discuss design options and costs, systems and elements that should not be eliminated, and the inherent risk in doing so. "Value engineering" decisions will be critical during these meetings. Interface conditions will need to be discussed. The following items should not be removed from the project as a means to save up front dollars. These include interface flashings, preconstruction mock-ups, and any and all areas of enclosure redundancy that are critical to the durability of the structure. It has been our experience that with many of the up-front dollars saved by say, eliminating flashings, the dollars for repair are much more than twice the cost.

Final Construction Documents

After the comments developed during the "50%" peer review have been evaluated and applied by the design team, the architect of record and other members of the design team can proceed with finishing the construction documents, including all drawings and technical specifications. The final assemblies and systems identified will have to again be checked to ensure that they will satisfy the owner requirements. Additional computer modeling including energy modeling and moisture analysis modeling will likely need to completed, depending on changes being made to the documents based on the initial review comments by the commissioning agent. The modeling should be useful in refining the decisions made during the design development phase. The impact of preliminary cost estimates may lead the team to look at a variety of alternatives. The design team will need to explore different and timely/more cost-effective means to achieve the same end, but without sacrificing core objectives of the commissioning process. The design team should emphasize long-term durability and performance of materials; components and systems that comprise the building envelope and consider the impact of each design decisions on the "whole building design." Mechanical systems should be concurrently examined with the building envelope decisions.

If the process is managed effectively by the architect, this step should be relatively easy in that proper materials are already in the budget for the project and adequate detailing is the only remaining hurdle. The commissioning process will help to ensure a more complete set of CDs as the design team will likely be more thorough in selecting materials not only as a renewable resource, but for long-term durability and performance. The interfaces and detailing completed should be at a much higher level than noncommissioned projects, partly based on the initial commissioning review.

Peer Review Prior to Bidding

The commissioning agent will review one set of the final architectural drawings and applicable specification sections for the building enclosure and provide comments on remaining concerns on the enclosure design paying particular attention to the details and interfaces as shown on the drawings. The commissioning agent will again also review and comment on the requirements in the specifications for samples, technical data, mock-ups, and performance testing. The objective of this review is to identify details, requirements, or methods which may compromise the water tight integrity and thermal performance of the building in order to call these identified items to the attention of the architect of record and the owner for their action. Issues such as constructability and material compatibility need to be examined by the commissioning agent and specific guidance should be supplied via written documentation to the owner and the architect of record at each phase of review. Materials that appear to be poor choices based on durability and a potential for rapid replacement need to be identified, especially on sustainable projects, where product replacement requiring new resources in a short time frame should be examined versus the use of a more durable product that may not be as environmentally appropriate on first selection.

If any details, plans, or specifications are modified by the architect of record, the commissioning agent will need to review these prior to issuance of the design documents. If elements recommended for change have not been completed, the commissioning agent should provide written documentation to the owner, indicating what risks may be associated with not implementing these items.

The peer review process is an important step in assisting the architect of record in making good decisions on the selection of materials, and method of integrating those materials into a durable building envelope.

Bid Review

The commissioning agent should be available to provide assistance to the owner and the architect of record in reviewing submittals, if included with the bid, for any proposed product substitutions for building enclosure elements, to determine the risk or equivalence of the proposed substitution.

"Value Engineering"

Efforts by the contractor or owner/developer to reduce the initial cost of a building or structure by applying the principles of value engineering to the design and construction of the building enclosure must be carefully weighed by the Architect/Engineer-of-Record against the best long-term interests of the owner/end-user based on the intended service life of the building or structure. This is particularly true with regard to laboratory and field performance testing, as well as the selection of through-wall flashing materials and related moisture management systems and accessories. In too many instances, pre-construction laboratory mock-up and field quality assurance tests typically required by the design professional to verify the constructability and performance of the building enclosure are considered cost-prohibitive on a project and, therefore, eliminated during the VE process. Similarly, through-wall flashing materials selected by the design professional, in part, for long-term durability and performance (such as stainless steel or lead-coated copper flashings and drips) are often substituted for lower performing materials that, despite their lower initial cost, are significantly more vulnerable to in-service degradation and failure. The cost associated with successfully addressing uncontrolled rainwater penetration and moisture ingress arising out of decisions made during the VE process can be significant, and the work highly disruptive and invasive. Again, the desired outcome of the VE process must be carefully weighed by the design professional against the long-term interests of the owner/end-user, and the intended service life of the building or structure.

Shop Drawing and Submittal Review

Concurrent with the architect of record and engineer of record review of shop drawings, the commissioning agent will need to review shop drawings prior to release and fabrication for building envelope requirements and provide written comments to the owner and architect of record.

Depending upon the scale and complexity of the project, the commissioning agent should be retained to assist the architect/engineer of record with his/her review of contractor submittals pertaining to the building envelope to verify their conformance with the contract documents and owner requirements. Regardless of whether review of all of the submittals is needed, contractor submittals for any building enclosure products the contractor would like to use as a substitute will also need to be examined by the commissioning agent, paying particular attention to the comparability of the substituted product, its possible durability, compatibility with adjacent materials, and whether it not it has any material properties, such as a lower vapor permeance, that increase the potential for a moisture-related failure.

Building Envelope Pre-Construction Coordination Meeting

The commissioning agent will need to participate in one kick-off meeting prior to beginning construction with the various members of the design and construction teams, including, but not limited to, the owner, owner's representatives, architect of record, mechanical engineer, general contractor, and all subcontractors that will be involved in the construction of the building envelope, including, but not limited to, the roofing, wall system, flashing, sealant, fenestration, concrete, steel, HVAC, electrical, interior framing and drywall contractors and the mechanical commissioning authority and other applicable members of the design and construction team. This meeting will be to discuss construction sequencing and the coordination of trades and the reporting that will be completed during construction of the building envelope and related other elements.

Building Envelope Pre-Installation Meetings with Individual Trades

The commissioning agent will also need to participate in each of the pre-installation meetings to review and discuss critical aspects of the construction, as well as to re-emphasize the importance of coordination among the trades to ensure the successful integration and weather-tight installation of the various materials, components, and systems that comprise the building enclosure.

Laboratory and Field-Constructed Mock-Ups and Performance Testing

Depending upon the size, scale, and complexity of the project, both laboratory and field-constructed mock-ups to verify constructability and performance should be given serious consideration by the design team. This is particularly true with regard to verification of structural performance, as well as air and water penetration resistance and the thermal efficiency of the building enclosure. Pre-construction mock-ups should be designed to include primary façade elements and interface conditions that are: a) unique to the design of the building or structure, b) representative of the entire building enclosure, and; c) reflective of actual job conditions that will be encountered in the field. Contractor personnel used to construct the mock-ups should be the same personnel who will be responsible for the actual work in the field, and should be made fully aware of changes, modifications or refinements in the design and/or installation of the building enclosure resulting from the mock-up and performance testing process.

A certified, independent testing laboratory qualified to perform each of the tests included in the project specifications should be retained to perform all laboratory and field testing. The design professional, commissioning agent, and representatives of the owner/developer, general contractor (or construction management firm) and appropriate subcontractors should be present during all tests, and should document, in writing and with supporting photographs and sketches, any changes in the building enclosure design arising out of the mock-up and performance testing program. Individual test methods, standards and required test pressures will vary based on the material, component or system being tested and the climate, location and exposure unique to each project. Test pressures should be carefully coordinated by the design professional in the project specifications to ensure that the specified test pressures for each system are uniform and consistent across the entire wall system or assembly, and meet or exceed the performance levels required by local, state, and national building codes.

On-Site Construction Observation and Quality Assurance Services

The commissioning agent will need to review the contractor's quality assurance inspection plan. To ensure that the findings and recommendations accepted by owner and the architect of record are properly executed and tested in the field, the commissioning agent will have to provide periodic, on-site review of the work in progress during the course of construction at the building envelope. A written record of deficiencies should be maintained by the commissioning agent and forwarded to the architect/engineer of record, owner, and general contractor in a timely manner throughout the construction process, together with a written record of how and when each deficiency was corrected in the field.

During this process, it is anticipated that the commissioning agent will participate in weekly or bi-weekly Building Envelope Quality Assurance Meetings chaired by the general contractor, with the appropriate subcontractors in attendance, to review and discuss issues and concerns noted by the commissioning agent during the previous week and what action will be taken to address those concerns. Our experience suggests that a minimum on-site presence of two (2) days per week will be required for most projects.

Project Close-Out

A final walk-through and "punch-list" survey of all deficiencies remaining on the project should be completed by the commissioning agent, architect/engineer of record and the owner. The commissioning agent may complete additional testing and commissioning of the envelope at this stage.

Building Envelope Maintenance Guide and Training

The commissioning agent should prepare a Building Envelope Maintenance Guide necessary to properly train on-site engineering and maintenance staff on the proper maintenance of the building envelope. This guide should also include a collection of all necessary closeout documents related to the building envelope, such as warranties and information on the materials utilized, including the appropriate catalog information, supplier, manufacturer, and ordering/contact information. The owner and staff should also be given guidance as to what elements will typically require outside contractor assistance for maintenance.

Applications

See current editions of ANSI/ASHRAE/IESNA Standard 90.1 and the ICC International Energy Conservation Code for information on climate zones. It is recommended that you review all details and systems relative to the climate that your project is situated in.

Above Grade Wall Selection Guide

PDF icon

The diagrams included in the following document below show two basic wall types, drainage cavity wall systems and barrier wall systems. The diagrams can be downloaded or viewed online Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title. Download Adobe Reader.

Drainage Cavity Wall Systems and Barrier Wall Systems  PDF

Details

Contained within this guide are a series of details that cover the following interface conditions; conditions that based on our experience are poorly or not appropriately detailed, or not detailed at all in many of the design packages we have peer reviewed as well as poorly constructed. Additionally, the majority of building enclosure failures we have seen within the wall system can be related back to an omission or other error related to these details. Each wall system section contains some of the fourteen details listed below with elements specific to that wall system. The details, graphics and related information shown above are intended to illustrate basic design concepts and principles only and should be considered collectively with the appropriate narrative sections of the Whole Building Design Guide (WBDG). The information contained herein is not intended for actual construction, and is subject to revision based on changes and/or refinements in local, state and national building codes, emerging building envelope technologies, and advancements in the research and understanding of building envelope failure and failure mechanisms. The actual design and configuration of these and similar details will vary based upon applicable local, state and national building code requirements, climatic considerations, and economic constraints unique to each project. Full compliance with the manufacturer's recommendations and recognized industry standards for each building envelope material, component and system specified for this and similar exterior wall assemblies is recommended, and should be reflected in the appropriate sections of the project specifications.

The details are for the following elements and interfaces:

  • Head and jamb flashing
  • Sill and jamb flashing
  • Interface between interior and exterior wall
  • Vertical expansion joint
  • Typical round penetration
  • Typical square penetration
  • Through wall flashing
  • Inside corner detail
  • Outside corner detail
  • Wall to balcony transition
  • Horizontal interface between barrier and drained wall
  • Vertical interface between barrier and drained wall

The details included in this guide are available in CADD format. CADD utilizes a series of layers to define elements. As noted earlier in this section (General Overview of Exterior above Grade Wall Systems), any building enclosure assembly can be thought of as a series of layers. The details for this guide have each building enclosure element defined as its own "layer" (the exception is on the stone details, where a second layer is utilized for each element on the step-by-step details to allow the element being installed in a particular step to be more prominent). By turning layers on or off within the CADD details, the user of this guide can see more clearly the intent of each element.

The details are not climate specific. As such, a series of tables have been included in this guide with recommendations for wall systems based on climate zones as defined by EEBA's Builder Guides. The details, the tables, and the information in this guide should be used in its entirety. If, for example, the climate zone being designed for required an exterior side vapor retarder, the designer should review the layers they have indicated on the project specific design and locate all layers that would have a permeance that would define it as a vapor retarder. The design team should review each layer utilizing a hygrothermal analysis (and common sense) to determine if the wall system they intend to utilize has a vapor retarder at the intended location and whether a misplaced vapor retarder is placed at a location other than that intended in the design. Conceptually, a master detail could be set-up by the design team during the Schematic Design phase with text identifying each product and its properties (air permeance, vapor permeance, thermal resistance, structural rigidity or load bearing capacity, etc.) and the design intent at each layer (drainage plane, air barrier, vapor retarder, thermal element, structural element etc.). The standard CADD layering terminology could then be renamed to clearly label the products and each layer and its intent. The building enclosure assessment could be completed on this initial section or series of sections for materials being considered for use on the project by reviewing what the intent of each layer is, whether it meets the intent, and whether any of the materials other properties create a potential for a moisture-related failure, by, for example, being a misplaced vapor retarder.

It is recommended that a set of details be included in the construction documents illustrating a fully integrated and effective air barrier system layer, drainage plane layer (including interfaces with flashings), and insulating element layer. Overall isometric or similar 3-dimensional details that clearly indicate each layer and interface condition may also be required. Details specific to a certain element that may not be clearly shown on an overall detail (such as a flashing splice) should be called out on the overall detail and a separate detail included indicating the installation of this element. Details should be included for each unique interface, transition, or penetration on the project.

Air Barrier System Details

Once the air barrier layer and products have been chosen and verified, a series of sections and plan views of the building should be developed showing air barrier continuity. This continuity could be traced out on the drawings. This is necessary to determine how the various materials will need to overlap and be sealed at openings and penetrations, at transitions between various wall types, at the foundation, and at the roof. A series of air barrier details are required to ensure proper construction of the air barrier system and to ensure its continuity. If the details are developed using a system in CADD that allows each layer to be turned off and on, showing the full view of the air barrier when it is needed and a partial view on an overall detail, the designer may spend less time in completing their set, as one set of details could be used to develop each detail set (i.e.—overall, drainage plane and flashing, air barrier and thermal barrier). The air barrier details should include 2-Dimensional and 3-dimensional isometrics clearly showing the location of the air barrier elements with respect to other elements and how the various materials for the air barrier system will be integrated. At a minimum, the following details should be developed:

  1. At the interface between wall types, whether between a barrier or cavity wall or two types of cavity wall
  2. At expansion joint locations
  3. At penetrations (roof, below grade or wall)
  4. At louver locations
  5. At door and fenestration (window, curtainwall, skylight, etc.) openings
  6. At the roof-to wall transition for each type roof or wall type on the project
  7. At the wall-to-below-grade transition
  8. At inside and outside corners
  9. If the sheathing or exterior side rigid insulation product has an air permeance value that qualifies it as a material to be used for an air barrier (see Tables) and is intended to be used as such, joint sealing and fastener sealing methods
  10. Roof membrane joints
  11. At sun-shade or other similar penetrations
  12. At all other unique, project-specific conditions requiring additional information to complete the air barrier system installation

Drainage Plane and Flashing Details

Once the drainage product(s) have been chosen and verified, a series of sections and plan views of the building should be developed showing drainage plane continuity. This continuity could be traced out on the drawings. This is necessary to determine how the various materials will need to overlap and be sealed at openings and penetrations and coordinated with flashings, at transitions between various wall types, at the foundation, and at the roof. A series of drainage plane details are required to ensure proper construction of the drainage plane as part of the moisture management system and to ensure its continuity and proper integration with flashing elements. The drainage plane details should include 2-Dimensional and 3-Dimensional isometrics clearly showing the location of the drainage plane material with respect to other elements and how the various materials for the precipitation and bulk water management system will be integrated. At a minimum, the following details should be developed:

  1. At all through-wall, window, door or other wall fenestration, and all penetration flashings
  2. At all roof flashing locations, particularly hatches, mechanical equipment curbs and skylights
  3. At the interface between wall types, whether between a barrier or cavity wall or two types of cavity wall
  4. At expansion joint locations
  5. At penetrations (roof, below grade or wall)
  6. At louver locations
  7. At door and fenestration (window, curtainwall, skylight, etc.) openings, including head, jamb, and sill flashings
  8. At the roof-to wall transtion for each type roof or wall type on the project
  9. At roof, plaza, or other drain locations
  10. At the wall-to-below-grade transition
  11. At inside corners
  12. At outside corners
  13. At sheathing and/or back-up wall joints
  14. Roof membrane joints
  15. At sun-shade or other similar penetrations
  16. At all other unique, project-specific requiring additional information to complete the drainage plane and flashing installation.

Insulating Element Details

Once the insulating element layer and products have been chosen and verified, a series of sections and plan views of the building should be developed showing the insulating element(s) continuity. This continuity could be traced out on the drawings. This is necessary to determine how the various materials will need to overlap and be coordinated at openings and penetrations, at transitions between various wall types, at the foundation, and at the roof. A series of insulating element details are required to ensure proper construction of this system and to ensure its continuity. The insulating element details should include 2-Dimensional and 3-Dimensional isometrics clearly showing the location of the insulating element(s) with respect to other elements and how the various materials for the insulating element will be integrated. At a minimum, the following details should be developed:

  1. At the interface between wall types, whether between a barrier or cavity wall or two types of cavity wall
  2. At the roof-to wall transtion for each type roof or wall type on the project, particularly continuity at the top of parapets, where ice dams may form in colder climates if interior heated conditioned air is allowed to enter the parapet and come in contact with the parapet cap with no insulating element below the cap
  3. At the wall-to-below-grade transition
  4. At expansion joint locations
  5. At penetrations (roof, below grade or wall)
  6. At louver locations
  7. At door and fenestration (window, curtainwall, skylight, etc.) openings
  8. At inside corners
  9. At outside corners
  10. At connections
  11. At shelf-angles
  12. At sun-shade or other similar penetrations
  13. At all other unique, project-specific requiring additional information to complete the insulating element installation

Emerging Issues

Hybrid Exterior Wall Systems and Emerging Technologies

In recent years, technological advancements in the design and manufacture of building enclosure materials, components and systems, together with an increasingly refined understanding of air/moisture transfer and the behavior of wind-driven rain on the building enclosure have lead to the development of several hybrid and sustainable exterior wall systems, several of which include:

  • Trombe Walls
  • Dynamic Buffer Zone and Mechanically Ventilated Walls
  • Double-Skinned Façades
  • Integrated Photovoltaics
  • Passively Ventilated Wall Systems
  • Radiant Heating and Cooling using Thermal Mass
  • Passive Heating and Cooling
  • LEED and Green Buildings

These and similar hybrid wall systems are often derived, at least in part, from the concepts embodied in one or all of the above-referenced Basic Exterior Wall Types. They typically include design features and individual building elements that are intended to improve or otherwise enhance the long-term durability and performance of the building enclosure, and are often adapted in response to issues and concerns that are unique to a particular geographic area and/or climatic region in which a building or structure is to be designed and built. Use and integration of these and similar hybrid wall systems should be given careful consideration by the Architect/Engineer-of-Record during the schematic design phase of a project. They should be discussed in detail with the owner/end-user, with particular emphasis placed on the overall design and installation complexity of the proposed exterior wall system(s), as well as the impact of those systems on the short and long-term cost, maintenance burden, durability, and performance of the building enclosure.

Relevant Codes and Standards

Code and standards applicable to each wall type are included in each of the individual sections for each wall type.

Additional Resources

WBDG

Design Objectives

Functional / Operational—Ensure Appropriate Product/Systems Integration, Functional / Operational—Meet Performance Objectives

Products and Systems

Section 07 92 00: Joint Sealants, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications (UFGS), DRAFT Federal Guide for Green Construction Specifications, MasterSpec®

Previous Building Envelope Federal Design Guidelines

Relevant Professional Associations

Relevant Research Organizations

Design Guides

Standards and Codes Links

Government Links

Air Barrier Associations and Information

Green Building and Sustainable Design Resources

Historic Preservation Resources

Fire Safety Organizations

Other Organizations