U-Value Calculator — Calculate Thermal Transmittance for Walls, Roofs & Floors

This calculator determines the U-value (thermal transmittance) of a building element using the standard method from BS EN ISO 6946. The U-value tells you how much heat passes through 1 m² of the element for every 1 degree Kelvin temperature difference.

The formula

U-value = 1 / (Rsi + R1 + R2 + ... + Rn + Rse)

Where each layer's thermal resistance R = thickness (m) / thermal conductivity (W/mK). Rsi is the internal surface resistance and Rse (0.04) the external surface resistance. Per BR 443 the internal surface resistance depends on heat-flow direction — wall (horizontal) 0.13, roof (upward) 0.10, floor (downward) 0.17 m²K/W — and this calculator applies the correct value for the element type you select.

UK Building Regulations Part L 2021 targets

New-build limiting values (AD L 2021 Vol 1 Table 4.1):

• External walls: 0.26 W/m²K
• All roof types (pitched or flat): 0.16 W/m²K
• Ground floor: 0.18 W/m²K
• Party wall: 0.20 W/m²K

Upgrading a retained existing element (AD L 2021 Vol 1 Table 4.3 column b):

• Wall — internal or external insulation: 0.30 W/m²K
• Wall — cavity insulation: 0.55 W/m²K
• Roof: 0.16 W/m²K
• Ground floor: 0.25 W/m²K

Thermal conductivity values used

• Brick: 0.77 W/mK
• Concrete block: 1.13 W/mK
• Lightweight block: 0.19 W/mK
• Mineral wool: 0.035 W/mK
• PIR board (Celotex/Kingspan): 0.022 W/mK
• EPS: 0.034 W/mK
• Plasterboard (standard wallboard): 0.21 W/mK (0.25 for higher-density acoustic/fire board)
• Timber: 0.13 W/mK
• Cavity air gap: R = 0.18 m²K/W (unventilated, BR 443 §4.7.1)

These are representative design values from BR 443 (Conventions for U-value calculations) and BS EN ISO 10456. Always use the manufacturer's declared lambda where available, as actual performance varies with density, moisture content and installation quality.