U-Value & R-Value Converter

Worked example

With these inputs:

• I want to convert: R-value → U-value
• Value to convert: 19
• Layer 2 — thickness: 1 in
• Layer 2 — material: Polyiso rigid foam board (R-6/in)

Formula

U = 1 ÷ R (and R = 1 ÷ U) · RSI = R × 0.1761 · Assembly R = sum of every layer's R, plus air films

Frequently asked questions

Is a lower U-value better?

Yes. U measures heat that gets through, so lower is better — the opposite of R-value, where higher is better.

Why is metric R-value so much smaller?

Different units. R-19 imperial is RSI 3.35 metric. Multiply imperial R by 0.1761 to convert.

How do I convert R-value to U-value?

U equals 1 divided by R, in the same unit system. R-20 is U-0.05. The catch is which R you start from: the R of the insulation alone is not the R of the assembly, so converting a batt rating gives a U-value better than the wall will ever achieve.

What is RSI and how does it relate to R-value?

RSI is the metric equivalent, in square-metre-kelvins per watt. Multiply an imperial R-value by 0.1761 to get RSI, so R-20 is about RSI 3.52. Canadian and European specifications use RSI, which is why a product can appear to have wildly different ratings in different markets when it is the same material.

Why do windows use U-value instead of R-value?

Convention, mostly, plus the fact that window performance is dominated by the frame, spacer and glazing as a system rather than by a thickness of material. Since U-values are small numbers where lower is better, they compare window assemblies more conveniently than the fractional R-values would.