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U-value

The U-value tells how much heat leaks through one square metre of wall for each degree of difference between inside and outside. A low U-value means good insulation. It is calculated from the sum of the thermal resistances of all the layers.

R = \frac{d}{\lambda}

thermal resistance of one layer

U = \frac{1}{R_{si} + \sum R_i + R_{se}}

U-value of the whole wall

Symbols

$U$	thermal transmittance	W/m²K
$d$	thickness	m
$\lambda$	thermal conductivity	W/mK
$R_{si},\ R_{se}$	inside and outside surface resistance	m²K/W

Example

200 mm insulation ( $\lambda = 0.036$ ) gives $R = 5.56$ . With $R_{si} + R_{se} = 0.17$ :

$U = 1/5.73 \approx 0.17$ W/m²K.

Resistances add up, U-values do not.

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