Mechanical Insulation Design Guide - Design Objectives

by the National Mechanical Insulation Committee (NMIC)

Last updated: 06-19-2014

Temperature Drop Calculator for Air Ducts

Calculates the Temperature Change of Air Flowing in a Duct
Reference: 2009 ASHRAE HoF, Chapter 4, Equation 48, (pg 4.21)

This calculator estimates the temperature drop (or rise) of air flowing in a duct.


(Input information may generally be be obtained from design documents or field measurements)

  • Line 1. Enter the temperature of the air entering the duct
  • Line 2. Enter the ambient temperature (the average temperature of the air surrounding the duct)
  • Line 3. Enter the flow rate of the air carried by the duct in cubic feet per minute
  • Line 4. Enter the length of the duct run in feet
  • Line 5. Enter the perimeter of the duct in inches
  • Line 6. Enter the R-value of the selected duct insulation. Available R-values will vary depending on insulation products selected.


  • One dimensional, steady state heat transfer
  • Specific heat of air assumed constant at 0.24 Btu/(lbm·°F)
  • U-factor of duct walls assumed constant
  • Air film resistances (inner + outer) assumed constant at 0.85 h·ft2·°F/Btu
  • The impact of duct leakage is neglected

Note: NA = Insulation Size Not Available. Some sizes and thicknesses may not be commercially available.

Input Information
1. Entering air temperature, °F
2. Ambient temperature, °F
3. Flow rate of air, cfm
4. Length of run, ft
5. Outside perimeter of duct, in
6. R-value of duct insulation, (h ft² °F)/Btu