Wall insulation thickness calculator for a target U-value
For a homogeneous wall model, added insulation thickness is λ × max(0, 1/Utarget − Rexisting − Rsurfaces). With target U = 0.20 W/(m²·K), existing layer R = 0.50, surface resistances totalling 0.17 and λ = 0.040 W/(m·K), the illustrative result is 173.2 mm.
Calculate with your assumptions
Prepare the wall data
Sum the thermal resistances of the existing homogeneous material layers, using thickness in metres divided by the relevant design lambda. Enter surface resistances separately. If your existing R already includes them, do not add them again. If you start from a reliable existing whole-wall U, 1/U is total resistance; avoid mixing that value with a layer-only R.
Surface resistances and target
Surface resistances depend on heat-flow direction and applicable boundary conditions. The example total 0.17 m²·K/W represents commonly used values 0.13 + 0.04 for a vertical wall under the stated convention, not a universal input for every construction. Enter a target established for your location and project; the calculator does not determine a legal requirement.
Check thickness and the chosen product
Required additional R in the example is 5.00 − 0.50 − 0.17 = 4.33 m²·K/W. Multiplying by 0.040 gives 0.1732 m, or 173.2 mm. Available product thicknesses are a separate purchasing decision. If you round up, recalculate the modelled U using the actual product thickness and design lambda.
Whole-wall limitations
Studs, mortar paths, metal fixings, cavities, gaps, workmanship, moisture and thermal bridges can require a more detailed method or corrections. This series-layer model does not evaluate condensation, fire requirements, attachment design or the U-value of a framed or heterogeneous wall. A result of zero means the supplied simplified model already meets the supplied target.
Examples with stated assumptions
| Inputs and method | Result |
|---|---|
| Additional R: 5.00 − 0.50 − 0.17 | 4.33 m²·K/W |
| Thickness: 4.33 × 0.040 | 173.2 mm |
| Modelled U after 180 mm | ≈ 0.1934 W/(m²·K) |
Result scope
The method is a homogeneous layer calculation. BR 443 explains conventions and corrections; the result does not certify full-wall performance or regulatory compliance.