Heat Load Calculator (U-value method)
Heat loss = U × area × ΔT for each surface, summed.
Result
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How to use this calculator
- Enter wall area and U-value (or R-value's inverse).
- Enter window area and U-value.
- Enter ΔT (indoor − outdoor).
About this calculator
Heat loss through walls and windows: each surface loses heat at U × area × ΔT. Lower U-value = better insulation. R-value is the inverse (R = 1/U). Modern walls: R-19 (U≈0.05). Modern double-pane low-e windows: R-3 (U≈0.30). Single-pane windows: R-1 (U≈1.0) — they leak heat ~3× faster.
Frequently asked
U-value vs R-value?+
U = heat-flow rate per ft² per °F. R = 1/U. R-30 wall has U-value ≈ 0.033.
How much loss through windows?+
Often 25-35% of total heat loss in older homes despite being only 10-15% of envelope.
Should I include ceiling and floor?+
Yes for full Manual J. This calculator simplifies to walls + windows; add ceiling/floor for completeness.
How to find U-value?+
Window manufacturer label (NFRC). Wall: from R-value of insulation + framing.
Air leakage?+
Significant in old homes. Add 30-50% to envelope-only calculation. Modern tight homes: <10%.
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