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Insulation R-Value Calculator

R value, U value and heat loss for an insulation thickness. Doubling insulation halves the remaining loss, not the original loss, which is why the first 50 mm saves far more than the second.

Also called: u value calculator, insulation thickness calculator.

mm
m²
K
R value
2.7

R 2.7 m²K/W, a U value of 0.37 W/m²K. Over 50 m² with a 15 K difference that is 277.5 W of loss. Doubling the thickness halves the remaining loss, not the original loss, so each added layer saves less than the last.

U value
0.37
R value, imperial units
15.35
Heat loss
277.5
Thermal conductivity
0.04
R at double thickness
5.41
Loss saved by doubling
138.75
On diminishing returns
Doubling the thickness halves the remaining loss, not the original loss, so each added layer saves less than the last.
How this is calculated
Method and background

How this is calculated

R value is thickness over conductivity and rises linearly. Heat loss depends on U, its reciprocal, so the relationship between insulation and saving is hyperbolic rather than linear. Going from no insulation to 50 mm might cut loss by seventy percent; going from 50 to 100 mm cuts the remainder by half again, which is a much smaller absolute saving. That is the whole economics of insulation depth, and it is why there is an optimum rather than more always being better.

R rises linearly with thickness while heat loss falls with its reciprocal, which is where the diminishing returns come from
d
Thickness
lambda
Conductivity

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

100 mm of mineral wool

Insulation thickness
100 mm
Material
Mineral wool
Area
50 m²
Temperature difference
15 K

R value2.7

0.1/0.037; 0.37 x 50 x 15

Open this example

doubling halves the remaining loss

Insulation thickness
200 mm
Material
Mineral wool
Area
50 m²
Temperature difference
15 K

R value5.41

boundary: exactly half the 100 mm figure

Open this example

Method and limits

What it assumes

  • Steady-state conduction through the insulation only.

What it deliberately does not model

  • Thermal bridging through studs and fixings can dominate the real performance.
  • Air leakage often loses more heat than conduction, and no R value addresses it.
  • Imperial R values are about 5.68 times the metric figure, so the two are easily confused.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Is twice the insulation twice as good?
No. Doubling thickness halves the remaining loss, so the second 50 mm saves much less than the first. That is why there is an economic optimum.
Why do US R values look so much bigger?
Different units. An imperial R value is about 5.68 times the metric one for the same material, so R-13 in the US is about R-2.3 metric.