How the insulation calculator works
Every insulation product has a thermal conductivity, written λ (lambda) or k, which is the heat in watts that flows through one metre of the material for each degree of temperature difference. The lower the lambda, the better the material. Thermal resistance, the R-value, grows in direct proportion to thickness, so the thickness for any target is a single multiplication.
Thickness (m) = target R (m²·K/W) × λ (W/m·K) · U = 1 ÷ RUS R-values use different units: divide them by 5.678 to get the metric figure before multiplying. The calculator then rounds up to whole layers of the product you buy, reports the R-value actually installed, and counts packs or boards for every layer, with 5% extra for cutting around joists, pipes and corners.
Worked example
A 50 m² loft floor is to reach R 7 m²·K/W, a common target for renovation grants in France, with glass wool rated λ 0.040. The thickness needed is 7 × 0.040 = 0.28 m, so 280 mm. Rolls of 140 mm exactly make it in two layers, installing R = 0.28 ÷ 0.040 = 7.0 and a U-value of 1 ÷ 7 = 0.143 W/m²·K for the insulation layer. Each layer needs 50 × 1.05 = 52.5 m², so two layers need 105 m², and at 5.4 m² per roll that is 19.4, rounded up to 20 rolls.
Material performance at a glance
| Material | λ (W/m·K) | US R per inch | Thickness for R 5 m²·K/W | Thickness for R-30 |
|---|---|---|---|---|
| Standard fibreglass batt | 0.044 | 3.3 | 220 mm | 9.2 in |
| Glass wool roll or blown cellulose | 0.040 | 3.6 | 200 mm | 8.3 in |
| Rock or mineral wool batt | 0.035 | 4.1 | 175 mm | 7.3 in |
| EPS polystyrene board | 0.036 | 4.0 | 180 mm | 7.5 in |
| XPS extruded board | 0.029 | 5.0 | 145 mm | 6.0 in |
| PIR or polyiso board | 0.022 | 6.5 | 110 mm | 4.6 in |
Typical targets: the US Department of Energy recommends R-30 to R-60 in attics depending on climate zone and R-13 to R-21 in wood-frame walls; UK guidance for lofts is 270 mm of mineral wool, around U 0.16; French grants ask for R 7 on loft floors, R 6 under sloping roofs and R 3.7 on walls. Your local building code sets the legal minimum, so check it before buying.
Getting the rated performance in practice
- Read the declared lambda. Two rolls of the same thickness can differ by 15% in R-value. Use the λ or R printed on the label rather than the generic value in the menu.
- Cross the second layer. In a loft, lay the first layer between the joists and the second at right angles over them, so the timber no longer forms a cold line every 40 to 60 cm.
- Never compress it. A 200 mm roll squashed into a 150 mm gap loses much of its value, so flooring over a loft needs raised supports rather than boards laid straight on the insulation.
- Keep ventilation and clearances. Leave the eaves vents open, keep insulation away from flues and chimneys, and only cover downlights rated for it or fitted with a proper cover.
- Protect yourself. Mineral fibres irritate skin and lungs: wear gloves, long sleeves, goggles and an FFP2 or N95 mask when cutting and laying.
Frequently asked questions
How thick should loft insulation be?
Around 270 to 300 mm of glass or mineral wool, which gives roughly R 6 to 7 m²·K/W. With λ 0.040 glass wool, 280 mm reaches R 7.
How many inches of insulation is R-38?
R-38 needs about 10.5 inches of glass wool at R-3.6 per inch, about 11.6 inches of standard fibreglass batt, or roughly 7.6 inches of rigid XPS board.
How do I convert a U-value into an R-value?
R = 1 ÷ U. A target of U 0.20 W/m²·K therefore means R 5 m²·K/W, which is 200 mm of λ 0.040 insulation or 110 mm of PIR board.
What is the difference between metric and US R-values?
They measure the same thing in different units: 1 m²·K/W equals 5.678 ft²·°F·h/BTU. A US R-19 batt is therefore about R 3.3 in metric.
How many rolls of insulation do I need for a 50 m² loft?
For R 7 with two layers of 140 mm glass wool, you insulate 105 m² including 5% margin; at 5.4 m² per roll that is 20 rolls. Check the roll label, as coverage falls when thickness increases.
Last reviewed: 2026-09-27 · How we calculate