The formulas
The converter above uses three relationships.
Thickness to R-value
- R = thickness (m) ÷ λ
- 200 mm of a material with λ 0.035 W/m·K: 0.200 ÷ 0.035 = R 5.71 m²K/W
R-values are in m²K/W. Thicknesses go into the formula in metres, so divide millimetres by 1,000 first.
R-value to U-value
- U = 1 ÷ R, and R = 1 ÷ U
- R 4 m²K/W = U 0.25 W/m²K
Whole element: roof, wall or floor
- U = 1 ÷ (Rsi + Σ thickness/λ + Rse)
This is the method in BS EN ISO 6946, which UK U-value calculations follow, with conventions from BRE's BR 443. Rsi and Rse are the inside and outside surface resistances: the thin layers of still air that cling to each face.
| Surface resistance | Value (m²K/W) | Where the calculators use it |
|---|---|---|
| Rsi, heat flowing up | 0.10 | Lofts and roofs |
| Rsi, heat flowing sideways | 0.13 | Walls |
| Rsi, heat flowing down | 0.17 | Floors |
| Rse, outside | 0.04 | Lofts, walls and floors |
The converter also shows the US R-value. US product labels use different units: 1 US R-value = 0.1761 m²K/W, so R 5.71 m²K/W is about R-32 in US terms.
Worked example: a loft, step by step
Here's how the loft insulation calculator reaches U-value 0.16 with 100 mm of Loft Roll 44 between the joists and 170 mm across them.
- Surfaces and ceiling. Rsi 0.10 + Rse 0.04 + 0.43 for the ceiling and roof space = 0.57 m²K/W. On its own, that's an uninsulated loft at 1 ÷ 0.57 = U 1.75.
- The layer between the joists. Timber joists take up 12.8% of the area in our model. The calculator blends the two conductivities by area (a parallel-path calculation): 0.872 × 0.044 + 0.128 × 0.13 = λ 0.055. So 100 mm gives 0.100 ÷ 0.055 = R 1.82, not the 2.27 it would give on its own.
- The layer across the joists. Nothing bridges it: 0.170 ÷ 0.044 = R 3.86.
- Total. 0.57 + 1.82 + 3.86 = 6.25 m²K/W.
- U-value. 1 ÷ 6.25 = 0.16 W/m²K, the Part L value for an upgraded loft.
Worked example: a filled cavity wall
A cavity wall with a 65 mm cavity filled with blown mineral wool at λ 0.040:
- Rsi 0.13 + Rse 0.04 + 0.51 for the rest of the wall = 0.68 m²K/W, or U 1.47 unfilled.
- Cavity fill: 0.065 ÷ 0.040 = R 1.63.
- Total: 0.68 + 1.63 = 2.31 m²K/W.
- U-value: 1 ÷ 2.31 = 0.43 W/m²K, inside the Part L value of 0.55 for cavity fill.
The cavity wall insulation calculator does this for every cavity width from 50 to 100 mm.
Layer R-value or element U-value?
Type 270 mm and λ 0.044 into the converter and you get R 6.14, which is 1 ÷ 6.14 = U 0.163. That's close to the loft calculator's 0.16, but by coincidence: the joists take away about 0.45, and the surfaces and ceiling add 0.57. In a floor or wall the two don't cancel out. A Part L U-value always means the whole element, so use the calculators when you're checking against the Building Regulations, and the converter for single layers and product comparisons.
Common mistakes
- Adding U-values. R-values of layers add up; U-values don't. Convert each layer to R, add them, then take 1 ÷ the total.
- Forgetting the surfaces. Leaving out Rsi and Rse makes an element look worse than it is.
- Ignoring timber. Insulation between joists, rafters or studs performs worse than its label, because heat flows through the wood.
- Millimetres in the formula. 100 mm is 0.1 m. Divide by 1,000 before dividing by λ.
- US and UK R-values. An American R-30 is about 5.3 m²K/W. Check which unit a figure uses.
For the targets you're aiming at, see Part L U-values. For thicknesses at a glance, see loft insulation thickness.
Sources
- BRE BR 443: Conventions for U-value calculations (2019)
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- RdSAP10 Specification (BRE, February 2024)
- Knauf Insulation: Loft Roll 44 (Combi-cut)
- SOPREMA UK: SOPRATHERM GA4000 (previously Celotex GA4000)
- Knauf: Supafil 40 cavity wall insulation