U-value ≤ 0.30 for external walls, ≤ 0.20 for roofs and ground floors, energy frame 30 kWh/m²/year for dwellings — the concrete BR18 figures the carpenter needs to know, and how to calculate them.
Introduction
The climate envelope — exterior walls, roof/ceiling, windows and terrain deck — is the carpenter's domain. BR18 Chapter 11 sets the minimum U-values allowed in new construction and an energy frame that limits total heating demand.
U-value requirements (new construction)
| Building part | Minimum requirements (new construction) | Indicative tightening (renovation) |
|---|---|---|
| Exterior wall | U ≤ 0,30 W/m²K | U ≤ 0.18 W/m²K |
| Ceiling/roof against unheated room | U ≤ 0,20 W/m²K | U ≤ 0.12 W/m²K |
| Ground deck against earth | U ≤ 0,20 W/m²K | U ≤ 0.10 W/m²K |
| Windows and exterior doors | U ≤ 1.80 W/m²K | U ≤ 1,40 W/m²K |
Source: BR18 §§ 250–298, verified via bygningsreglementet.dk.
The U-value formula
Example: lightweight timber frame wall (from outside in): 22 mm facade board (λ = 0.12), 9 mm wind barrier board (λ = 0.14), 195 mm mineral wool insulation (λ = 0.036), 12.5 mm plasterboard (λ = 0.22). R_total = 0.04 + 22/120 + 9/140 + 195/36 + 12.5/220 + 0.13 = 0.04 + 0.18 + 0.06 + 5.42 + 0.06 + 0.13 ≈ 5.89 m²K/W → U ≈ 0.17 W/m²K. Well below the 0.30, and close to the stricter renovation requirement.
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