What is thermal resistance (R-value)?
Thermal resistance, written R, indicates how well a layer of material or a building element resists the flow of heat. The unit is m²K/W. The higher the thermal resistance, the better the insulation.
Calculation
For a uniform layer:
R = d / λ, where d is the thickness in metres and λ the thermal conductivity in W/(m·K).
Examples:
– 120 mm PIR with λ = 0.023: R ≈ 5.2 m²K/W.
– 100 mm brick with λ ≈ 0.8: R ≈ 0.13 m²K/W.
That difference shows why uninsulated masonry barely insulates.
Adding layers
The resistances of successive layers are added together, plus the surface resistances inside and outside (Rsi and Rse). For walls with horizontal heat flow, ISO 6946 uses Rsi = 0.13 and Rse = 0.04 m²K/W. An unventilated cavity also has a small resistance of its own.
Link with the U-value
The U-value (thermal transmittance) is the reciprocal of the total resistance: U = 1 / RT, in W/(m²K). The lower the U-value, the better. Building regulations in the UK set limits as U-values; in the Netherlands they are set as Rc-values.
Tips
- Compare insulation boards by R-value at the thickness you need, not by thickness alone.
- Calculate the whole build-up: timber studs, fixings and wall ties reduce the effective resistance.
- Thermal bridges at junctions often matter more than a few extra centimetres of insulation.
Related terms
- Thermal conductivity
- Thermal insulation
- Thermal bridge
- Heat flow
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