General Principles and Commitments
Achieve a minimum of daylight factor of 2 percent (excluding all direct sunlight penetration) in 75 percent of all space occupied for critical visual tasks. Provide automatic dimming controls or accessible manual lighting controls, and appropriate glare control.
Requirement: "Achieve a minimum of daylight factor of 2 percent"
Light levels in a building space are typically measured in footcandles or lux. Light from electric lights is fairly constant so exact levels of lighting can be obtained through the selection and layout of light fixtures. When it comes to daylighting the light source is the sky vault which is outside the building so it becomes necessary to install glazed openings (windows and skylights) to admit adequate daylight to meet space needs. Designing for exact levels of light from daylight is difficult since the light source is constantly changing depending on time of day and weather conditions. Instead of calculating exact levels of light (footcandles or lux) in a given space from daylight a daylight factor was created to measure relative light levels. The daylight factor is best defined as:
DF = Iin/Iout x 100%
DF = daylight factor
Iin = illumiance due to daylight on the indoor working plane
Iout = illumiance outdoors on a unobstructed horizontal plane
There are three possible ways that daylight can reach the indoor working plane. They are:
- Visable light directly from the sky vault (define)
- Light reflected from exterior surfaces.
- Light entering the space and reflecting from interior surfaces.
All three of these components need to be accounted for to determine the daylight factor.
A daylight factor of 2 is a typical level one would want to achieve for an office space. It assumes that 2% of the total light that is outside the building ends up on the working plane or desktop. Assuming an average outdoor illumiance of 2500 footcandles and a daylight factor of 2% the indoor illumiance on your desktop would be 50 footcandles (2% x 2500 footcandles = 50 footcandles). Other typical daylight factors for various spaces are:
Discussion Groups – 14
Residential Living Room – 1
Residential Kitchen – 2
Office - detail work – 4
Office - drafting – 6
Office - corridors – 0.5
Schools - classrooms – 2
Schools - art rooms – 4
Hospitals - wards – 1
Hospitals - waiting rooms – 2
Sports facilities – 2
Warehouse - bulk storage – 0.5
Warehouse - medium size storage – 1
Warehouse - small item storage – 2
Calculating a daylight factor based on your building design includes window/skylight sizes, overhangs/lightshelves, glass types, and exterior/interior reflectances can become very complex. A simple rule of thumb to approximate the daylight factor for daylit spaces using vertical windows is:
DF = 0.1 x PG
DF = daylight factor
PG = Percentage of glass to floor area
Assuming a 1000 sf office space has 200 sf of windows. The daylight factor would be:
DF = 0.1 x (200/1000)
DF = .02 or 2%
For more detailed calculation methods which take into account the three major components (light from sky vault, light reflected from exterior surfaces and light reflected from interior surfaces) that make up the daylight factor refer to the references in this guidance.
Requirement: "(excluding all direct sunlight penetration)"
Traditional daylighting excludes direct beam sunlight which typically introduces unwanted heat, glare and extreme contrast resulting in increased energy use and uncomfortable conditions. The sun is constantly changing positions (altitude and azimuth) during the day and throughout the seasons. The MOU guidance recognizes that excluding "all" direct sunlight penetration may not be possible or desirable. If the building design incorporates both passive solar heating and daylighting excluding the direct beam would not be desirable since this is the main source of heat. The intention of "all" would be in an office/classroom/business type environment where passive solar heating (direct beam sunlight) would not be part of the design and would lead to poor daylighting. Direct sunlight should not be used when calculating the daylight factor.
Excluding direct beam sunlight starts with good building orientation, placing the long axis of the building facing due north and south. Windows facing north will receive the greatest amount of daylight and can dispense with overhangs and fins for most latitudes (recognizing that some direct beam penetration will be possible during summer sunrise and sunset - see Figure 1).
The second best daylighting is from south facing windows which can be protected from direct beam sunlight using fixed overhangs. The least desirable daylighting is from east and west facing windows because of the difficulty of eliminating direct beam sunlight during mornings (East side) and afternoons (West side). The only way to completely omit direct beam sunlight during this time is movable shading devices such as blinds and roller shades which will also block the daylighting. Figure 2 provides solar angles for various latitudes which can be used to size fixed south facing overhangs.
Keep in mind that these solar angles allow for both daylighting and passive solar heating so it will be necessary to divide the window wall into two areas, an upper glass area for continuous daylighting protected by a overhang that prevents both summer and winter direct beampenetration, and a lower area which provides both daylighting and view with light colored blinds or roller shades to control winter direct beam penetration. A more optimum design would make use of light shelves to bounce light further back into the space (see Figure 3).
Requirement: "in 75 percent of all space occupied for critical visual tasks."
Critical visual tasks can best be defined as office / classroom type work which involves reading printed text, entering data into computers, writing and drawing. The best way to daylight 75% of critical visual task area is to provide accessibility to exterior windows. This is best accomplished by elongating the building with most of the space facing north and south (see Figure 4). If elongating the building and/or going to multiple floors to provide maximum window area for the majority of the space is not possible then clerestory windows, atriums and/or skylights should be considered. Note that providing daylight from above through the use of clerestories and atriums will be more costly than simple vertical windows. Care should also be given to carefully designing skylights to prevent unwanted solar heat gain.
Requirement: "Provide automatic dimming controls or accessible manual lighting controls,"
For continuous occupied space (offices / classrooms) automatic controls that constantly adjust the electric lighting in response to the available daylight should be used. The best type makes use of dimmable ballasts controlled by photosensors operating T5 or T8 fluorescent lamps. Past studies have shown that on/off controls tend to annoy those working in the space and occupants may not bother to use manual lighting controls to turn off lights when enough daylight is available. If manual controls do have to be used they should be clearly marked, in an accessible location and the occupants trained to use them. For transition spaces such as corridors, restrooms, conference rooms, warehouses or any other spaces where people are constantly moving in and out simple on/off controls connected to photo sensors would be the best solution.
Requirement: " and appropriate glare control."
The best way to control glare is to control the arrangement of people in the daylit space. The major forms of glare are direct and indirect. Occupants in a daylit space should always be placed so they are sitting perpendicular to the window wall (See Figure 5). Facing directly toward the window wall will result in direct glare and facing directly away from the window wall will result in reflected glare off of computer screens. Placement of partitions in open office spaces is also very important in allowing maximum penetration of daylight with minimum glare (see Figure 6).
For additional information on daylight design refer to the references in this guidance.
Model Contract and Specification Language
- Basics of Daylighting in a Green Environment—Offers an overview of daylighting principles as well as daylighting design strategies and technologies. The course also discusses common daylighting mistakes and provides links to additional resources including free daylighting software.