Wall U-Value Calculator
R-values add. U-values never do.
Work out Wall U-Value. R-values add. U-values never do. Names the misconception directly.
U-value of the build-up
0.351 W/m²K
Total R = 2.849 m²K/W · 77 W through 10 m²
R-values add across layers; U-values do not. That is the whole content of a build-up calculation and the thing most often got wrong — you cannot average U-values, or add them, or take the worst one. The share column shows where the resistance actually lives, and it is usually lopsided. A hundred millimetres of mineral wool typically provides over 80% of a wall's resistance while being a fraction of its thickness, which is why insulating is so much more effective than building thicker. A real U-value calculation also includes surface resistances on both faces, and any air gaps, and it accounts for thermal bridging through studs and mortar joints. Those corrections push a real wall's U-value above what the layers alone suggest, sometimes by a third.
How the Wall U-Value Calculator works
The U-value of a layered build-up, with each layer's share of the total resistance. R-values add across layers and U-values do not — you cannot average them, add them, or take the worst one.
Also known as: u value of my wall build up · add r values of layers · insulation thickness needed for u value · wall construction thermal calculator
Frequently asked questions
How do I calculate the U-value of a wall?
Add the R-values of every layer, then take the reciprocal. Each layer's R is its thickness in metres divided by its conductivity.
Why can't I just add U-values?
Because resistance is what adds, and U is the reciprocal of resistance. Adding U-values would say that stacking two identical layers makes a wall twice as bad, which is exactly backwards.
Which layer matters most?
Whichever has the highest R-value, and it is usually the insulation by a wide margin. A hundred millimetres of mineral wool typically provides over 80% of a wall's resistance while being a fraction of its thickness.
What is thermal bridging?
Heat taking a shortcut through a more conductive path — studs, joists, mortar joints, lintels. It is not captured here and it pushes a real wall's U-value above what the layers alone suggest, sometimes by a third.
Should I include surface resistances?
A full calculation does. Still air films on each face add roughly 0.13 m²K/W inside and 0.04 outside, which matters most for well-insulated build-ups where they are a larger share of the total.
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