0.13
New Build
W/m²K maximum
0.18
Extension
W/m²K maximum
0.25
Renovation
W/m²K maximum
Our AI calculator handles all common UK floor constructions with correct ground contact and ventilation calculations
Concrete slab on ground with insulation above, below, or within the slab. Calculated using BS EN ISO 13370 with characteristic floor dimension based on area/perimeter.
Typical U-value: 0.10 - 0.18 W/m²K
Timber joists with insulation between, suspended over a ventilated void. Requires void thermal resistance and wind speed correction factors.
Typical U-value: 0.15 - 0.22 W/m²K
Precast concrete beams with infill blocks, typically with insulation above. Common in modern UK construction. Bridging fraction included.
Typical U-value: 0.12 - 0.18 W/m²K
Concrete floor suspended over a ventilated or unventilated void. Often used over basements or sloping sites.
Typical U-value: 0.13 - 0.20 W/m²K
Ground floor U-values are calculated differently from walls and roofs because heat flows into the ground, not directly to outside air. The key parameter is the characteristic floor dimension:
B' = A / (0.5 × P)
Where A = floor area (m²) and P = exposed perimeter (m)
The ground thermal conductivity is taken as 2.0 W/mK for clay or silt soils (most common). The calculation accounts for:
Placed beneath the floor slab at the perimeter, extending inward. Minimum 1m width recommended, with 50mm PIR (λ = 0.022) typically used.
Reduces perimeter heat loss by 15-25%
Applied to the foundation wall from floor level to base of foundation. Reduces heat loss through the wall-floor junction thermal bridge.
Reduces perimeter heat loss by 20-30%
For new builds in the UK under Part L 2021, ground floors should achieve a U-value of 0.13 W/m²K. For extensions, the target is 0.18 W/m²K, and for renovations, 0.25 W/m²K is acceptable. Floor U-values are typically harder to achieve than walls due to ground contact.
Ground floor U-value depends on the perimeter-to-area ratio (P/A). Larger floors lose proportionally less heat because more floor area is further from the cold perimeter. A small floor extension may have a worse U-value than a larger floor with the same construction.
Solid floors (concrete slab on ground) are calculated using BS EN ISO 13370 with ground thermal conductivity of 2.0 W/mK. Suspended floors include a ventilated void beneath, requiring additional resistance calculations for the air space and wind speed corrections.
Edge insulation significantly reduces heat loss at the floor perimeter where temperatures are lowest. Horizontal edge insulation should extend at least 1m inward, while vertical edge insulation should extend the full depth of the foundation. BR 443 provides methods for including edge insulation benefits.