Heat Gain Calculator
Estimate conductive heat gain through a wall, roof, or floor from U-value, area, and ΔT.
Estimates for planning only — verify results against local energy codes and a qualified mechanical or building engineer. This tool calculates the conductive (fabric) component only; total cooling load also includes solar gains through glazing, internal gains from occupants and equipment, and infiltration.
How it works
- 1Enter the surface U-value (W/m²·K) for the wall, roof, or floor.
- 2Enter the surface area and the outside-to-inside temperature difference.
- 3The calculator applies Q = U·A·ΔT and shows the conductive gain in W, kW, and BTU/h.
Use cases
- Checking whether a wall build-up meets a target peak cooling load.
- Comparing the gain reduction from better insulation or glazing.
- A quick sanity-check during cooling-load design review.
Frequently asked questions
What does the U-value represent?
The U-value (W/m²·K) is the overall heat-transfer coefficient, combining conduction through each layer and surface films. Lower is better — from ~0.15 (insulated roof) to ~5 (single glazing).
Why is this only part of the cooling load?
It is the conductive (fabric) component only. Total cooling load also includes solar gain through glazing, internal gains from people and equipment, and infiltration.
Worked example: 30 m² roof, U = 0.25, ΔT = 15 °C?
Q = 0.25 × 30 × 15 = 112.5 W (≈ 384 BTU/h) of steady-state conductive gain, before solar and thermal-mass effects.
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