Insulation thickness calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Element | Wall |
| The work | New element in an existing home (e.g. an extension) |
| Where the home is | England |
| The other layers, inside to outside | Brick: material Brick (outer leaf), thickness 103 mm, thermal conductivity λ 0.77 W/mK, studs or joists run through this layer No; Aircrete block: material Aircrete block, thickness 100 mm, thermal conductivity λ 0.15 W/mK, studs or joists run through this layer No; Plaster: material Gypsum plaster, thickness 13 mm, thermal conductivity λ 0.57 W/mK, studs or joists run through this layer No |
| λ of the insulation you'll use | 0.022 W/mK |
| The insulation goes between studs or joists | No |
| Target U-value | 0.18 W/m²K |
| Thickness you can buy at or above the answer | 120 mm |
| Inside surface resistance Rsi | 0.13 m²K/W |
| Outside surface resistance Rse | 0.04 m²K/W |
Insulation: at least 100.4 mm
| Result | Value |
|---|---|
| Minimum insulation thickness | 100.4 mm |
| Target U-value | 0.18 W/m²K |
| U-value with the thickness you'll buy | 0.155 W/m²K |
| U-value without the insulation | 1.007 W/m²K |
Show the working
- Target: 0.18 W/m²K (your figure)
- Without the insulation: R = 0.993 m²K/W, U = 1.007 W/m²K
- Thickness: 0.022 × (1 ÷ 0.18 − 0.993) = 100.4 mm
- With 120 mm: U = 0.155 W/m²K
How it works
- Resistance of the other layers R_other = Rsi + Σ d/λ + Rse
- Thickness d = λ_insulation × (1 ÷ target U − R_other)
- When the insulation sits between studs or joists, the thickness is found by trying values until the combined-method U-value (BS EN ISO 6946) equals the target
- Then buy the next thickness up and check its U-value
Corrections for air gaps, wall ties and fixings (ΔU) aren't included: allow a little extra or ask the maker for a full calculation.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Inside surface resistance Rsi: Wall | 0.13 m²K/W | Our assumption Not yet checked against the source | Our choice: change it to suit your job Inside surface resistance for a wall (horizontal heat flow) as used in brief v2 I1 test UV1 (BS EN ISO 6946). Researcher to cite the clause. |
| Outside surface resistance Rse: Wall | 0.04 m²K/W | Our assumption Not yet checked against the source | Our choice: change it to suit your job Outside surface resistance for a wall as used in brief v2 I1 test UV1 (BS EN ISO 6946). Researcher to cite the clause. |
| Inside surface resistance Rsi: Roof | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 I1: Rsi by element (BS EN ISO 6946), upward heat flow. Not stated in the brief; Researcher to fill. |
| Outside surface resistance Rse: Roof | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 I1: Rse by element (BS EN ISO 6946). Not stated for roofs in the brief; Researcher to fill. |
| Inside surface resistance Rsi: Floor | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 I1: Rsi by element (BS EN ISO 6946), downward heat flow. Not stated in the brief; Researcher to fill. |
| Outside surface resistance Rse: Floor | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 I1: Rse by element (BS EN ISO 6946). Not stated for floors in the brief; Researcher to fill (ground floors are treated differently, BS EN ISO 13370). |
Regulations and standards
- Approved Document L 2021 Vol 1, Table 4.2 (new elements) and Table 4.3 (renovated elements), England
- BS EN ISO 6946 (U-value method)
Explained: Approved Document L (conservation of fuel and power), BS EN ISO 6946 (U-values).
Our Part L figures are for England. Wales, Scotland and Northern Ireland set their own limits, which we're still adding.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Thermal conductivity (λ) of common materials | BS EN ISO 10456 design values / maker data sheets (Researcher to cite per row) | UK-wide | 0/12 values entered Not yet checked against the source |
| Part L limiting U-values by nation, element and work | Approved Document L 2021 Vol 1 (England); Wales, Scotland and Northern Ireland equivalents (Researcher) | UK-wide | 3/36 values entered Not yet checked against the source |