U-value (thermal transmittance) is the rate of heat transfer through a structure per unit area per degree of temperature difference. Lower U = better insulation. Units: W/(m²·K). U = 1/R_total.

How is wall heat loss calculated?

Q = U × A × ΔT, where Q is heat flow (W), U is thermal transmittance (W/m²K), A is wall area (m²), and ΔT is temperature difference between inside and outside.

R-value is thermal resistance: R = thickness/conductivity for each layer. Total R = R_si + ΣR_layers + R_se (including surface film resistances). U = 1/R_total. Higher R = better insulation.

What are typical wall U-values?

Uninsulated brick wall: U ≈ 2.0 W/m²K. Insulated cavity wall: U ≈ 0.3-0.5 W/m²K. Well-insulated wall (modern code): U ≈ 0.15-0.25 W/m²K. Passive house: U < 0.15 W/m²K.

What are surface film resistances?

R_si (internal surface) ≈ 0.13 m²K/W and R_se (external surface) ≈ 0.04 m²K/W account for the thin boundary layer of still air at each surface. They are included in the total thermal resistance calculation.

Free Engineering Tool

Heat Loss Through Wall Calculator

Calculate heat loss Q = U × A × ΔT through composite walls. Add material layers, set temperatures, and get U-value and heat flow.

Q = U·A·ΔTMulti-Layer#176

Results

Heat Loss Q

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U-Value

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Total R-Value

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Heat Flux q

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Composite Wall Heat Transfer

R_si = 0.13 m²K/W (internal surface), R_se = 0.04 m²K/W (external surface).

Heat Loss Through Wall Calculator | U-Value | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Composite Wall Heat Transfer