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Heat Transfer Coefficient Calculator

Estimate the overall heat transfer coefficient for a wall using the heat transfer coefficient calculator.

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U-value11.6667 W/(m²·K)the building-industry figure — lower is better insulated
Heat flow rate443.3333 W19 K across 0.042857 K/W of resistance
Conductive resistance0.002857 K/Wthickness ÷ (conductivity × area)
Thermal conductivity1.05 W/(m·K)Glass
Temperature difference19 K
Heat flux per square metre221.7 W/m²
R-value (SI)0.08571 m²·K/W0.4867 in US units (ft²·°F·h/BTU), which are about 5.68× the SI figure — the two are constantly confused
Thermal diffusivity5.0e-7 m²/sk ÷ (ρc) — how fast a temperature change propagates, as opposed to how much heat flows
Time for heat to penetrate the thickness72 sroughly L² ÷ α — 0.02 hours, and note it goes as the SQUARE of thickness
Volumetric heat capacity2.1e+6 J/(m³·K)the thermal mass per unit volume
Surface film resistance, both faces0.04 K/W93.33% of the total — on a thin, conductive pane the films dominate, which is why single glazing is not as bad as its conductivity suggests
Energy over 24 hours10.64 kWh
At twice the thickness415.625 Wconduction halves, but any surface films do not — so the total rarely halves
The same thickness of polystyrene145.1389 W31.8× less conductive than glass

The formula

Q̇ = kA ΔT ÷ L; R = L ÷ kA; Q̇ = ΔT ÷ R

Resistances in series, like a circuit

Conduction is an exact analogue of Ohm's law: temperature difference plays the part of voltage, heat flow of current, and thermal resistance of electrical resistance. Layers in a wall add their resistances in series exactly as resistors do, which is what makes multi-layer construction calculable.

Surface films matter more than people expect. Still air clinging to each face of a pane adds resistance of its own, and on a thin conductive material those films can dominate the total. That is why a 6 mm sheet of glass — whose glass alone offers almost no resistance — still slows heat noticeably, and why double glazing works chiefly by trapping a further air layer rather than by adding glass.

Two figures describe the same thing from opposite ends. U-value is conductance per unit area, so lower is better insulated; R-value is its reciprocal, so higher is better. Be careful which R: the US figure in ft²·°F·h/BTU is about 5.68 times the SI figure in m²·K/W, and quoting one as the other is a common and large error.