U-Value and R-Value: Insulation Units Explained

Insulation, windows and even duvets are rated by how well they slow heat down. The trouble is that the US, Europe and bedding manufacturers each use different units for the same idea.

R-value: resistance

The R-value measures how strongly a layer resists heat flow. Higher is better.

• US R-value: ft²·°F·h/BTU, printed on US insulation as "R-13", "R-38" and so on.
• SI R-value (RSI): m²·K/W, used in Canada, Australia, New Zealand and Europe.

1 US R-value = 0.1761 RSI, and 1 RSI = 5.678 US R-value

US rating	RSI (m²·K/W)
R-11	1.94
R-13	2.29
R-19	3.35
R-30	5.28
R-38	6.69
R-49	8.63

U-value: transmittance

The U-value is the heat that gets through per square metre per degree of temperature difference. Lower is better. Windows and whole wall assemblies are usually rated this way.

For a whole assembly, U is the reciprocal of total R:

U = 1 ÷ R (in matching unit systems)

A wall with a total RSI of 4 has a U-value of 0.25 W/(m²·K).

To switch unit systems:

1 W/(m²·K) = 0.1761 BTU/(h·ft²·°F)

A worked heat-loss example

Heat loss (W) = U × area (m²) × temperature difference (K).

A 10 m² window with U = 1.4 W/(m²·K), when it is 20 °C inside and 0 °C outside:

1.4 × 10 × 20 = 280 W leaking through that window.

Use a temperature difference conversion here, not an ordinary temperature conversion. A 20 °C difference is a 36 °F difference.

Tog and clo

• Tog rates duvets and sleeping bags: 1 tog = 0.1 m²·K/W. A 10.5 tog duvet has an RSI of 1.05.
• Clo rates clothing: 1 clo = 0.155 m²·K/W, roughly a business suit.

Check the code, not just the number

Building regulations set minimum values that depend on climate zone, building type and year, and they change regularly. Use conversions to compare products, but confirm compliance against your local code or with a qualified professional.

Related conversions

• US R-value to RSI
• U-value: W/(m²·K) to BTU/(h·ft²·°F)
• Thermal conductivity
• Tog to US R-value