Insulation R-Value & U-Value Calculator
Calculate the thermal resistance (R-value) and U-value of any wall, floor, or roof construction. Check compliance with UK Building Regulations Part L.
U-Value and Insulation Guide
R-Value and U-Value Explained
Thermal resistance (R-value): R = thickness (m) / thermal conductivity (λ W/mK). Units: m²K/W. Higher R = better insulation. R-values of multiple layers ADD together. U-value (thermal transmittance): U = 1 / R_total. Units: W/m²K. Lower U = better insulation. The U-value tells you how many watts escape through 1 m² of construction for every 1°C temperature difference. A wall with U = 0.3: if inside is 20°C and outside is 0°C (ΔT = 20°C): heat loss = 0.3 × 20 = 6 W/m².
UK Building Regulations Part L (2022)
Minimum U-values for new builds (England, Part L 2021): External walls: 0.18 W/m²K. Roofs (pitched, insulation at ceiling): 0.16 W/m²K. Ground floors: 0.13 W/m²K. Windows and doors: 1.4 W/m²K. Existing building extensions: walls 0.28, roofs 0.18, floors 0.18. Typical uninsulated cavity wall: U = 1.5 W/m²K. With 100mm mineral wool cavity fill: U ≈ 0.45 W/m²K. With 100mm PIR full-fill: U ≈ 0.22 W/m²K. For Part L 2021 compliance, 150mm mineral wool in a cavity wall achieves approximately U = 0.18 W/m²K.
Thermal Conductivity (λ) Values
Common building material conductivities: brick: 0.77 W/mK. Concrete block (dense): 1.5 W/mK. Concrete block (aerated): 0.18 W/mK. Timber: 0.13 W/mK. Plasterboard: 0.25 W/mK. Render: 1.0 W/mK. Air cavity (unventilated 25mm): R = 0.18 m²K/W. External surface resistance: Rse = 0.04 m²K/W. Internal surface resistance: Rsi = 0.13 m²K/W. These surface resistances must be added to get total R of a complete construction — they represent the thermal film on surfaces.
Avoiding Thermal Bridges
A thermal bridge is a path of significantly lower thermal resistance through a construction — a 'cold spot'. Common thermal bridges: structural timbers within an insulated frame (timber has λ = 0.13, much higher than insulation). Concrete columns in insulated walls. Lintels over windows. Floors meeting walls. Roof/wall junctions. Thermal bridges can dramatically reduce the effective U-value below the theoretical value. Continuous insulation (no breaks) prevents thermal bridging. Structural therm
Recommended for this calculator