Why does duct location matter so much?

A duct in conditioned space loses heat back into the conditioned area, so it is not truly lost. A duct in a 130 degree attic or a 40 degree crawlspace sees a huge temperature difference, so it gains or loses real energy that never reaches the rooms.

What R-value should attic ducts have?

Modern energy codes typically require R-6 to R-8 for ducts in unconditioned attics, and R-8 or higher in extreme climates. Older flex duct may be only R-4.2. Higher R-value lowers the heat transfer in direct proportion.

Does this include air leakage losses?

No. This tool calculates only conductive loss through the duct walls. Air leakage from poorly sealed joints can equal or exceed conduction in leaky systems. Seal joints with mastic and test with a duct blaster to address leakage separately.

How do I use the result in a load calculation?

Add the duct gain or loss to the room or system load it serves. ACCA Manual J handles this with a duct-loss percentage, but a direct conductive estimate like this is useful for spot-checking a long run through a hot attic or cold crawlspace.

Ductwork Heat Gain / Loss Calculator

Q: What is the duct heat loss formula?

Conductive heat transfer is the surface area divided by the insulation R-value, multiplied by the temperature difference between the duct air and the surrounding space. Q in BTU/h equals A in square feet times delta-T in Fahrenheit divided by R.

Ducts running through a blazing attic or a freezing crawlspace bleed real energy before the air ever reaches the room. This calculator estimates the conductive heat gain or loss through the duct walls from the surface area, insulation R-value, and the temperature difference between the duct air and the surrounding unconditioned space.

How it works

Conductive heat transfer through an insulated surface is governed by the simple steady-state relation:

Q (BTU/h) = A (ft²) × ΔT (°F) / R

A   = duct surface area (entered, or π × diameter × length for round duct)
ΔT  = | duct air temp − surrounding space temp |
R   = duct insulation R-value (h·ft²·°F / BTU)

If the surrounding space is hotter than the duct air, the duct gains heat (a penalty on cooling); if the space is colder, the duct loses heat (a penalty on heating). The result is the conductive component only.

Example and notes

A round 12-inch supply duct, 30 feet long, has a surface area of about π × 1.0 × 30 = 94 square feet. Carrying 55 degree cooled air through a 130 degree attic with R-6 insulation gives Q = 94 × 75 / 6 = about 1,175 BTU/h of heat gain — heat that loads your air conditioner without cooling any room. Doubling the insulation to R-8 cuts it to about 880 BTU/h. Remember this ignores air leakage; sealing joints with mastic is just as important as adding insulation.