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U-value

The U-value collects a construction's total thermal resistance into a single number: the heat loss per square meter and degree of temperature difference. A low U-value means good insulation. Building codes often set requirements for the maximum U-value of walls, roofs and windows.

U = \dfrac{1}{R_{tot}A}

U-value from total resistance

\dot Q = UA\Delta T

heat loss from the U-value

Symbols

$U$	U-value	W/(m²·K)
$A$	area	m²

Example

A wall has $R_{tot}A = 3.5$ m²K/W. $U = 1/3.5 \approx 0.286$ W/(m²·K). With $A = 15$ m² and $\Delta T = 30$ K: $\dot Q \approx 128.6$ W.

The lower the U-value, the better the insulation – the opposite of the thermal conductivity

k

, where a higher value means worse insulation.

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Part of Heat Transfer: Conduction.