Energy requirements and U-values in BR18
0.20 for roof and 0.30 for wall — how you read building elements' minimum requirements
En stor del af moderne tømrerarbejde handler om at holde på varmen. Bygningsreglementet stiller konkrete krav til, hvor meget varme hver enkelt bygningsdel må lukke ud, og disse krav bestemmer i praksis, hvor tykt der skal isoleres i tag, væg og gulv. Forstår man U-værdien, forstår man, hvorfor konstruktionerne ser ud, som de gør.
§What is a U-value?
The U-value indicates how much heat flows through one square metre of a building element when the temperature difference between inside and outside is one degree. It is measured in watts per square metre per kelvin, W/(m²K). The lower the U-value, the better the building element insulates. A thick, well-insulated roof construction has a low U-value; a thin, poorly insulated wall has a high one.
§BR18's minimum requirements for new construction
BR18 sets minimum requirements (U-value requirements) for individual building parts in new construction. These minimum requirements must always be met regardless of how the building's overall energy limit is calculated. The most important for the carpenter are the requirements for roof, external wall and ground floor.
| Building part | Minimum U-value requirement (new construction) |
|---|---|
| Ceiling and roof | 0,20 W/(m²K) |
| Exterior walls and basement walls against ground | 0,30 W/(m²K) |
| Ground slab and intermediate floor against the elements | 0,20 W/(m²K) |
| Floor separation/partition toward unheated space | 0,40 W/(m²K) |
| Windows and glass walls (to the outside) | 1,80 W/(m²K) |
§Tightened requirements for renovation
With renovation and retrofit of existing building parts there are often stricter requirements than for new construction because you take the opportunity to raise the standard. For heated rooms the roof requirement can be right down to about 0.12 W/(m²K) for exterior walls about 0.18 and for basement slabs about 0.10. In practice this means even thicker insulation when you are going to open the structure anyway.
§From U-value to insulation thickness
- 01The lower the U-value requirement, the thicker the insulation layer must be
- 02The insulation's lambda value (thermal conductivity) determines how thick it needs to be to meet the requirement
- 03Thermal bridges at rafters, beams and joints reduce the actual U-value and must be limited
- 04Good insulation helps nothing if air tightness fails—the two go together
For the carpenter the U-value is the link between regulatory requirements and practical craft. It determines how much insulation is needed and thus how strong rafters and how deep constructions must be. Being able to look up the right minimum requirements and translate them into a structure that both insulates and is airtight is a core skill in modern timber building.