U-Values Explained with Practical Examples
A straightforward guide to U-values - what they measure, what the regulations require, and real-world examples for common building elements.
What is a U-Value?
A U-value (thermal transmittance) measures how effective a building element is as an insulator. It represents the rate of heat transfer through a structure, measured in W/m²K (watts per square metre per degree Kelvin).
Key Principle: Lower U-values = Better Insulation
A wall with U-value 0.18 loses heat half as fast as one with U-value 0.36.
Part L 2021 Requirements
Building Regulations Part L sets maximum (limiting) U-values. However, to achieve the overall energy targets, you typically need to do better than the limits.
| Element | Limiting Value | Typical Target |
|---|---|---|
| External Walls | 0.26 W/m²K | 0.18-0.22 W/m²K |
| Ground Floors | 0.18 W/m²K | 0.13-0.15 W/m²K |
| Roofs | 0.16 W/m²K | 0.11-0.13 W/m²K |
| Windows | 1.6 W/m²K | 1.2-1.4 W/m²K |
| External Doors | 1.6 W/m²K | 1.0-1.4 W/m²K |
Real-World Examples
Wall U-Value Examples
Traditional Cavity Wall (uninsulated)
Brick-cavity-block, no insulation
Filled Cavity Wall (100mm PIR)
Brick-100mm full-fill-block
Timber Frame Wall (140mm + 50mm)
Typical timber frame with service void
Floor U-Value Examples
Solid Concrete Floor (100mm XPS)
Slab with 100mm insulation below
Beam & Block (150mm mineral wool)
Suspended floor with insulation between joists
Roof U-Value Examples
Cold Roof (300mm loft insulation)
Mineral wool at ceiling level
Warm Flat Roof (120mm PIR)
Insulation above deck
How U-Values Affect SAP Ratings
Better U-values directly improve your SAP rating by reducing calculated heat loss. The impact depends on element area - improving large wall areas has more effect than small windows.