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.
Worked example
A 12-inch round supply duct, 30 feet long, carries 55 °F cooled supply air through a 130 °F attic with R-6 flex duct insulation:
- Surface area: π × (12/12 ft) × 30 ft = 94.2 ft²
- Temperature difference: 130 − 55 = 75 °F
- Heat gain: 94.2 × 75 / 6 = 1,178 BTU/h
That is heat loading your air conditioner without cooling any room. Upgrading to R-8 insulation cuts the gain to 94.2 × 75 / 8 = 883 BTU/h, a saving of nearly 300 BTU/h for one duct run.
Effect of R-value on heat transfer
The relationship is straightforward — doubling the R-value halves the heat transfer rate:
| R-value | Heat gain (same duct and ΔT as above) |
|---|---|
| R-4.2 (old flex) | 1,682 BTU/h |
| R-6 (current min in many codes) | 1,178 BTU/h |
| R-8 (better practice) | 883 BTU/h |
| R-12 | 589 BTU/h |
Common energy codes now require R-6 minimum for ducts in unconditioned attics and R-8 or higher in climate extremes. Older homes frequently have R-4.2 or less.
The role of air leakage
This tool calculates conductive loss only — heat flowing through the duct wall. A real duct system also loses energy through air leakage at seams, connections, and fittings. In leaky older homes, duct leakage can equal or exceed conduction losses. The two problems call for different fixes:
- Conduction: Increase insulation R-value or move ducts into conditioned space.
- Leakage: Seal all joints and connections with mastic (not duct tape, which fails over time) and test with a duct blaster at 25 Pa to confirm less than 4% leakage to outside for ENERGY STAR.
Addressing leakage is often higher priority than adding insulation because it also wastes conditioned air volume — air you paid to heat or cool that never reaches the room.
When to use this calculator
- Auditing an existing system: Identify which attic duct runs contribute the most heat gain in summer and prioritize re-insulation or re-routing.
- Design sanity checks: Estimate duct losses to verify they are a reasonable fraction of the room load before finalizing a Manual J calculation.
- Comparing insulation upgrades: Model the BTU/h improvement from R-6 to R-8 to justify the added cost of better flex duct or rigid insulation jacketing.