Thermal resistance and U-values
U-value of a wall, roof or floor (BS EN ISO 6946 layer sum)
- where
- total thermal resistance (m²K/W)
- internal surface resistance (m²K/W)
- layer thicknesses (mm)
- layer thermal conductivities (W/mK)
- unventilated air layer resistance (m²K/W)
- external surface resistance (m²K/W)
- U-value (W/m²K)
Explain this equation
Works out the U-value of a wall, roof or floor made of up to five layers. Use it to compare build-ups and check against Building Regulations targets.
Each layer adds its own resistance, thickness over λ (with mm converted to m). The thin films of still air on each surface add R_si inside and R_se outside, and a sealed air gap adds R_g. Adding all of these gives the total R. The U-value, the heat that flows through each square metre for every degree of temperature difference, is its inverse.
Layers in series: resistances add, just as with resistors in a single loop.
Sense check: a 215 mm solid brick wall plastered both sides is about 2 W/m²K; adding 60 mm of PIR board with plasterboard brings it to about 0.30.
Watch out: this ignores timber studs, fixings and air gaps around boards. Use the bridged and corrections calculators where those apply.
Adds up the resistance of each layer plus the two surface resistances to give the U-value of an element with uniform layers. Use it for walls, roofs and floors by choosing the right surface resistances.
Enter your values to see the result.
Good to know
Surface resistances from BS EN ISO 6946 (as set out in BR 443): Rsi = 0.13 m²K/W for walls (horizontal heat flow), 0.10 for roofs and ceilings (heat flow upwards) and 0.17 for floors (heat flow downwards); Rse = 0.04 in all three cases. The example is a 215 mm solid brick wall with 13 mm plaster and 20 mm render, lined internally with 60 mm PIR and 12.5 mm plasterboard: about 0.30 W/m²K, the Part L target for internal wall insulation. Layer order does not change the U-value, only the condensation risk. For ground floors use the BS EN ISO 13370 calculator instead.
Common mistake
Using the wall Rsi of 0.13 for a loft or floor, or counting a ventilated cavity or loft void as an insulating layer. Layers outside a well-ventilated air space are ignored and Rse is replaced by Rsi for that side.
Limits
Uniform layers only. Battens, studs or joists running through the insulation need the bridged-layer (combined method) calculator, and fixings and air gaps need the ISO 6946 Annex F corrections. A Building Regulations submission should be a full calculation to BR 443.
Sources
- BS EN ISO 6946:2017 Building components and building elements – Thermal resistance and thermal transmittance – Calculation methods (surface resistances; combined method; Annex F corrections for air voids and mechanical fasteners)
- BRE BR 443 (3rd edition, 2019) Conventions for U-value calculations (Rsi 0.13 / 0.10 / 0.17 and Rse 0.04 m²K/W; air-gap levels 0 / 0.01 / 0.04; timber fractions; ground λ clay 1.5, sand/gravel 2.0, rock 3.5 W/mK; ignore corrections under 3%)
- BS EN ISO 10456 Hygrothermal properties – tabulated design values (as reproduced in the SEAI DEAP guide: gypsum plaster 1200 kg/m³ 0.43; plasterboard 0.25; cement–sand render 1.00; brick outer leaf 0.77 W/mK; softwood 0.13 W/mK as used by BR 443)
- British Gypsum product data sheet: Gyproc WallBoard 12.5 mm (λ = 0.19 W/mK)
- Kingspan and Celotex PIR board data sheets (declared λ typically 0.022 W/mK for foil-faced PIR; phenolic about 0.018–0.020); use the declared value on the product you are fitting
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.