Rigid insulation is sold in boards, and its performance is printed as a thermal conductivity, λ. A house with 40 m of walls, 3 m high and 15 m² of windows and doors has 105 m² to insulate: with 5% extra, 154 boards of 1200 × 600 mm. At 100 mm and λ = 0.035, each board has R = 2.86 m²·K/W, or R-16.2 in US units. Type the walls or the area, the board size, its thickness and λ, and the calculator gives the boards, the volume and the R- and U-values.
R-value by thickness and λ
| λ (W/m·K) | 50 mm | 100 mm | 150 mm | US R per inch |
|---|---|---|---|---|
| 0.022 | 2.27 (R-12.9) | 4.55 (R-25.8) | 6.82 (R-38.7) | R-6.56 |
| 0.031 | 1.61 (R-9.2) | 3.23 (R-18.3) | 4.84 (R-27.5) | R-4.65 |
| 0.035 | 1.43 (R-8.1) | 2.86 (R-16.2) | 4.29 (R-24.3) | R-4.12 |
| 0.040 | 1.25 (R-7.1) | 2.50 (R-14.2) | 3.75 (R-21.3) | R-3.61 |
Values in m²·K/W, with the US R-value in brackets. R grows in a straight line with thickness and falls as λ rises, so 100 mm of a λ 0.022 board does the work of about 160 mm at λ 0.035.
Where λ comes from
The label or the datasheet gives λ, often as λD (the declared value). Lower is better. As a guide, white EPS is often around 0.035–0.038, graphite (grey) EPS around 0.031, mineral wool slabs 0.035–0.040 and foil-faced PIR around 0.022, but use your product’s figure. In the US, boards are usually sold by their R-value per inch or in total, and the calculator shows both.
Board sizes
| Board | Area |
|---|---|
| 1200 × 600 mm | 0.72 m² |
| 1000 × 500 mm | 0.50 m² |
| 2400 × 1200 mm | 2.88 m² |
| 4 × 8 ft | 32 ft² (2.97 m²) |
| 2 × 8 ft | 16 ft² (1.49 m²) |
External wall insulation also needs adhesive, anchors, mesh, a base profile along the bottom (the perimeter, in the results) and a render; the system’s datasheet gives those quantities per m². The calculator works offline, so you can plan from the scaffold.
Frequently asked questions
How do I calculate how many insulation boards I need?
Wall area = perimeter × height − windows and doors. Add about 5% for cuts and divide by the area of one board. 40 m × 3 m − 15 m² = 105 m²; with 5%, 110.3 m², or 154 boards of 1200 × 600 mm.
How do I work out the R-value of a board?
R = thickness in meters ÷ λ, the thermal conductivity on the label. 100 mm of board at λ = 0.035 W/m·K gives R = 0.1 ÷ 0.035 = 2.86 m²·K/W, or R-16.2 in US units.
How do I convert a metric R-value to US R-value?
Multiply by 5.678. RSI 2.86 m²·K/W is R-16.2 ft²·°F·h/BTU. To go back, divide by 5.678.
What does a lower λ mean?
Better insulation for the same thickness. At λ = 0.022 (typical of PIR), 100 mm gives R-25.8 (US); at λ = 0.040, the same thickness gives only R-14.2.
Is the R-value of the board the R-value of the wall?
No. The whole wall also counts the masonry, the plaster and the thin air layers at each surface, so its total R is higher and its U-value lower. Thermal bridges such as anchors and studs work the other way.
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