Duct Heat Gain/Loss Through Unconditioned Space
Conductive duct heat gain/loss Q = U A dT (U = 1/R) and the resulting supply-air temperature change dT_air = Q/(1.08 CFM). 1,000 CFM of 55 F air in R-4 duct, 100 ft^2, a 120 F attic warms 1.5 F; R-8 halves it. Steady-state conduction, no leakage. A design aid.
Formula and source
U = 1/R_duct; Q = U A dT (Btu/h); dT_air = Q / (1.08 x CFM).
The conductive duct heat-gain/loss relation from the ASHRAE Handbook - Fundamentals duct-design method, by name.
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