How to use this calculator
- 1Measure the area to insulate, for example the loft or attic floor.
- 2Enter your target R-value from your building rules or energy advice (use the unit menu for US R or m²K/W).
- 3Choose the insulation material, or enter the λ value printed on your product.
- 4Topping up a loft? Tick the box and enter the depth of the old insulation that stays.
- 5Choose the product you plan to buy — its thickness and coverage fill in together — or enter your own pack's figures.
- 6Read the thickness needed, the number of layers and the packs to buy.
The formula
- 01Thickness needed = (target R − R of old insulation) × λ
- 02R of old insulation = old depth ÷ old λ
- 03Layers = thickness needed ÷ pack thickness, rounded up
- 04R-value you get = layers × pack thickness ÷ λ
- 05Packs = area × layers × (1 + extra %) ÷ area per pack, rounded up
The R-value (thermal resistance) of a layer of insulation is its thickness divided by its thermal conductivity, λ (lambda): R = thickness ÷ λ. So the thickness you need is simply the target R-value multiplied by λ. Materials with a lower λ, such as PIR board, reach the same R-value with much less thickness than mineral wool.
Europe uses metric R-values (m²·K/W) and λ in W/(m·K); the US uses R-values in ft²·°F·h/BTU. One US R equals 0.176 m²K/W, so R-49 is about 8.6 m²K/W. US products give an R-value per inch; the calculator converts between the two.
Material λ values vary between products. For each material we use the less favourable end of the published range, so the thickness is not under-estimated. Use the value printed on your own product when you have it.
Worked example
To reach 6 m²K/W with glass mineral wool at λ 0.044 you need 264 mm of insulation.
With 150 mm rolls that is 2 layers (300 mm, giving 6.82 m²K/W). For 40 m² plus 10% extra, buy 10 packs covering 9.177 m² each.
Tips before you buy
- Lay the first layer between the ceiling joists and the second across them. This covers the joists, which would otherwise let heat escape.
- If you want to store things in the loft, use raised loft legs or a platform so boards do not squash the insulation.
- Seal gaps around loft hatches, pipes and cables before insulating — warm air leaking through them wastes heat and carries moisture into the cold loft.
- For blown insulation, fit depth rulers across the attic before blowing so you can check the settled depth everywhere.
- Packs are compressed; open them where they will be laid and let the roll recover its full thickness before judging the depth.
Frequently asked questions
How thick should loft insulation be?
In the UK the usual recommendation for a loft is 270 mm of mineral wool, which is about R 6 m²K/W. In the US, ENERGY STAR recommends R-49 to R-60 for an uninsulated attic in most climate zones — roughly 16–20 in of fiberglass batts.
How do I convert R-value to thickness?
Multiply the R-value by the material's thermal conductivity. For R 6 m²K/W with glass wool at λ 0.044: 6 × 0.044 = 0.264 m, or 264 mm. With US R-values, divide by the R per inch: R-49 ÷ 3.0 per inch ≈ 16.3 in.
What is the difference between R-value and U-value?
R-value measures one layer's resistance to heat flow; higher is better. U-value measures how much heat passes through a whole wall, roof or floor; lower is better. Building rules in Europe often set U-values, which include all the layers of the construction.
What is a good λ (lambda) value?
Lower is better. Loft rolls are typically 0.040–0.044 W/m·K, stone wool slabs about 0.035–0.038, EPS about 0.033–0.038 and PIR boards about 0.022. The λ value is printed on European packs.
Can I put new insulation on top of old?
Usually yes, if the old insulation is dry and in good condition. Tick "Topping up old insulation" and enter its depth: its R-value is taken off your target, so only the new layer is counted. Remove anything damp or mouldy, and find the cause first.
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Sources
- Knauf Insulation Loft Roll 40 and 44 data sheet, June 2024 (λ 0.044; 100/150/170/200 mm: 13.885/9.177/8.014/6.840 m² per pack)
- Owens Corning R-30 unfaced fiberglass batt, 10 in, 53.33 ft² per bag
- Loose-fill insulation: settled R-value per inch — Building America Solution Center
- Energy Saving Trust: roof and loft insulation (270 mm recommended depth)
- ENERGY STAR recommended home insulation R-values
- US DOE Guide to Home Insulation (R-values of batts and boards)
Formulas last reviewed: September 30, 2026 ·How we check our formulas