The formulas
R = Thickness (m) ÷ Thermal conductivity λ (W/m·K)
R(US) = R(metric) × 5.678
U = 1 ÷ (R total + 0.17)
The 0.17 m²·K/W adds the inside and outside surface resistances for a wall. Lower U-values mean less heat loss.
Material comparison (25 mm / 1 in)
| Material | λ (W/m·K) | R (metric) | R (US) |
|---|---|---|---|
| PIR / polyiso | 0.022 | 1.14 | R-6.5 |
| XPS | 0.034 | 0.74 | R-4.2 |
| EPS | 0.037 | 0.68 | R-3.8 |
| Sheep's wool | 0.038 | 0.66 | R-3.7 |
| Closed-cell rubber | 0.036 | 0.69 | R-3.9 |
λ values are typical; check the product's declared value. In vans, flexible materials are easier to fit around curves, while rigid boards give the most insulation per millimetre on flat panels. A vapour control layer on the warm side helps prevent condensation behind linings.
Frequently asked questions
What's the best insulation for a campervan?
Rigid PIR board gives the most insulation per thickness on flat areas, with flexible wool or closed-cell rubber for curved panels and cavities.
How do I convert R-value to U-value?
U = 1 ÷ R. Add about 0.17 m²·K/W to R for the wall surfaces first for a realistic U-value.