What Is an R-Value (Thermal Resistance)?

The R-value (thermal resistance) measures how well a material resists the flow of heat. It is expressed in m²K/W (square metres Kelvin per watt). A higher R-value means better insulation. R-values are the building blocks used to calculate U-values.

How R-Value Is Calculated

The R-value of a material layer is calculated by dividing its thickness (in metres) by its thermal conductivity (lambda, λ):

R = thickness (m) / λ (W/mK)

For example, 100 mm of PIR insulation with a lambda of 0.022 W/mK has an R-value of 0.1 / 0.022 = 4.55 m²K/W.

R-Value vs U-Value

R-values and U-values are inversely related. The U-value of a complete construction (wall, floor, or roof) is calculated by adding up the R-values of all layers (including surface resistances) and taking the reciprocal: U = 1 / R_total. A high total R-value gives a low (good) U-value.

Why R-Values Matter

• They allow you to compare the insulating performance of different materials at different thicknesses
• They are additive — you can add R-values of multiple layers together to find total thermal resistance
• They are used in U-value calculations required by Part L of the Building Regulations
• Insulation product datasheets typically quote both lambda (λ) values and R-values at standard thicknesses

Typical R-Values

• 100 mm PIR board (λ = 0.022): R = 4.55 m²K/W
• 100 mm mineral wool (λ = 0.035): R = 2.86 m²K/W
• 100 mm EPS (λ = 0.032): R = 3.13 m²K/W
• 102.5 mm brick (λ = 0.77): R = 0.13 m²K/W
• 12.5 mm plasterboard (λ = 0.19): R = 0.07 m²K/W