U-value

It is a measure of a heat loss. Units W/m2K which shows how many watts are lost per square meter of material when the temperature between inside and outside are different even 1K.

To find the U-value, you need to know thickness (m) of the materials and thermal conductivity (lambda).

Firstly, calculate the R-value, thermal resistance, (m2K/W)  by thickness/thermal conductivity:

R = l/lambda

Secondly, you need to add all R-values together, for example, if you are calculating wall U-value, find a sum of brick, cavity, insulation, block R-values.

Rtotal = R1 + R2 +...

Finally, to find U-value, thermal transmittance, (W/m2K), you have to divide 1 from the total R-value.

U = 1/Rtotal


Below is an example of U-value calculation according to my sketch of cavity wall section.