How the calculation works
- Layers. Each layer’s thermal resistance is its thickness divided by its conductivity (λ). The inside and outside surface resistances are added, and the U-value is one over the total.
- Timber framing. Studs, rafters, joists and battens bridge the insulation, so those layers are worked out with the combined method (the average of an upper and a lower limit), using BR 443’s timber fractions.
- Masonry. In lightweight block walls the mortar joints bridge the blocks, so those leaves are worked out with the combined method too, using BR 443’s mortar fraction.
- Cavities and air gaps. Unventilated cavities and service voids are worked out from their width and the emissivity of their faces, so a foil facing counts. A well-ventilated cavity, and everything outside it, is left out.
- Corrections. Gaps in the insulation, metal fixings through it, and rain on inverted roofs, as BS EN ISO 6946 Annex F sets out.
Ground floors, a slab on the ground or a suspended floor over a ventilated void, are worked out to BS EN ISO 13370 from the extension’s floor area and exposed perimeter. Floors over a passage or open air are covered too.
For a worked example, read how to calculate the U-value of a timber frame wall.
Save your build-ups and use them in compensatory calculations
With a free Offset account you can save up to 6 build-ups, or as many as you like on a paid plan. Share them with your practice, and include them in a compensatory heat loss calculation, where the U-value fills in for you.