Thermal resistance and U-values
Insulation thickness needed for a target U-value
- where
- minimum insulation thickness (mm)
- insulation thermal conductivity (W/mK)
- target U-value (W/m²K)
- U-value of the existing element (W/m²K)
- resistance of other new layers (m²K/W)
- thickness to order, next stock size (mm)
- stock thickness step (mm)
- U-value with stock thickness (W/m²K)
Explain this equation
Works out how much insulation to add to an existing wall, roof or floor to reach a target U-value, and rounds up to a stock thickness.
1/U_t is the total resistance needed. The existing element already provides 1/U_e (which includes its surface resistances), and any new board or air gap provides R_a. What is left must come from the insulation; resistance times λ gives thickness, and 1000 converts to mm. Rounding up to the next stock step gives the board to buy, and the last line works out the U-value you actually get with it.
Resistances add, so the insulation has to supply the gap between what you have and what you need.
Sense check: a solid wall at 2.0 needs about 62 mm of λ 0.022 board to reach 0.30; ordering 70 mm gives about 0.27.
Watch out: thick internal insulation on solid walls raises condensation and moisture risks. Check the build-up for condensation before specifying.
Finds how thick the new insulation must be to bring an existing wall, roof or floor down to a target U-value, then rounds up to a stock thickness.
Enter your values to see the result.
Good to know
1/Ue is the total resistance of the existing element including its surface resistances, so the new insulation only has to supply the difference. Typical uninsulated U-values: 215 mm solid brick about 2.0, 225 mm solid stone 2.0–2.3, unfilled 1930s–70s cavity wall about 1.5 W/m²K. Targets for upgrading existing dwellings (Approved Document L): internal or external wall insulation 0.30, cavity fill 0.55, roof 0.16, floor 0.25 W/m²K. Add the plasterboard or a sealed air gap behind a lining as Ra if you want to count them.
Common mistake
Forgetting that studs, battens and fixings through the insulation raise the real U-value. Size from the bridged or corrected U-value, or allow 10–20% extra thickness when insulation sits between timbers.
Limits
Assumes the new insulation is a continuous, unbridged layer. Check the final build-up with the U-value, bridged-layer and correction calculators, and check interstitial condensation risk before specifying internal insulation.
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%)
- Approved Document L Volume 1: Dwellings, 2026 edition (England): Table 3.2 new or replacement elements in existing dwellings (roof 0.15, wall 0.18, floor 0.18 W/m²K); Table 3.3 existing elements: threshold / improved U-values roof 0.35/0.16, cavity fill 0.70/0.55, internal or external wall insulation 0.70/0.30, floor 0.70/0.25; para 3.14 15-year payback and 0.7 backstop. In force 24 March 2027; the same values appear in Tables 4.2 and 4.3 of the 2021 edition, which applies until then
- 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.