🔧 Interactive U-Value Calculator (Layer Method)
Select building element, add layers from inside to outside. Surface resistances (Rsi, Rse) are automatically included per BS EN ISO 6946.
📐 U-Value Formula & Thermal Resistance
Rtotal – total thermal resistance (m²K/W). d – layer thickness (m). λ – thermal conductivity (W/m·K). A lower U‑value means better insulation and less heat loss.
🧊 What Is a U‑Value?
The thermal transmittance (U‑value) measures how much heat passes through 1 m² of a building element when the temperature difference between inside and outside is 1°C. Units: W/m²K. Lower U‑values reduce heating demand and improve EPC ratings.
🇬🇧 UK Building Regulations Part L – Limiting U‑Values (2022 Edition)
| Element | New Build (W/m²K) | Extension / Renovation (W/m²K) |
|---|---|---|
| Wall | 0.18 | 0.26 |
| Roof (pitched) | 0.11 | 0.15 |
| Flat roof | 0.13 | 0.15 |
| Floor | 0.13 | 0.18 |
| Windows / glazed doors | 1.2 | 1.4 |
| Rooflights | 1.7 | 1.7 |
These values are from Approved Document L, Volume 1 (Dwellings) and Volume 2 (Buildings other than dwellings). SAP calculations use these as notional targets; actual designs often aim lower for better EPC ratings.
🧱 Insulation Materials & Thermal Conductivity (λ)
| Material | λ (W/m·K) | Typical thickness for 0.18 wall |
|---|---|---|
| PIR rigid board | 0.022 | ~110 mm |
| Phenolic foam | 0.018–0.020 | ~90 mm |
| Mineral wool (stone/glass) | 0.035–0.040 | ~175 mm |
| EPS (expanded polystyrene) | 0.038 | ~190 mm |
| XPS (extruded polystyrene) | 0.033 | ~165 mm |
| Spray foam (closed‑cell) | 0.022–0.028 | ~110 mm |
🧱 Typical UK Wall Buildup Example
Cavity wall: brick outer (102mm, λ=0.77) + cavity (50mm) + block inner (100mm, λ=0.15) + PIR insulation (90mm, λ=0.022) + plasterboard (12.5mm, λ=0.16). With Rsi=0.13, Rse=0.04, U‑value ≈ 0.16 W/m²K – compliant.