The formula
R = thickness ÷ conductivity, and U = 1 ÷ R
Where it comes from
There are only two numbers and they are inverses. Resistance R measures how well it insulates: the higher, the better. Transmittance U measures how much gets through: the lower, the better. R comes from dividing the thickness in metres by the conductivity of the material, the lambda given on its data sheet; U is simply one divided by R. Ten centimetres of mineral wool at lambda 0.035 gives R = 2.86 and U = 0.35.
How to work it out by hand
- Convert the thickness to metres
- Look up the conductivity of the material on its data sheet
- Divide the thickness by the conductivity: that is R
- One divided by R is U
What is worth knowing
What this page works out is the insulation layer, not the whole wall. A real build-up also has brick, render, a cavity and the two surface resistances of the air on either side, and resistances of layers in series add together. That is why the U of a finished wall is always better than the figure here. And there is one detail that wrecks any calculation: thermal bridges. An uninsulated floor slab edge or column lets more heat through on its own than several square metres of well-insulated wall, and no thickness of mineral wool elsewhere makes up for it.