Insulation Calculator — Walls, Lofts, Rolls & Batts
Calculate how many rolls or batts of insulation you need for walls, ceilings, floors and roofs. Compare mineral wool, glass wool and rigid foam options with R-value and U-value guidance for your climate zone.
How many rolls of insulation do I need?
Divide the area to insulate by the coverage printed on the pack, then add 5–10% for cuts around joists, pipes and awkward corners. Loft rolls commonly cover 5–8 m² (54–86 sq ft) each, and thicker rolls cover less — a 200 mm (8 in) roll is shorter than a 100 mm (4 in) one of the same width, so doubling the depth more than doubles the packs.
Is insulation sold by area or by thickness?
Both, and they answer different questions. The area on the pack decides how many packs to buy; the thickness decides how well the wall or roof performs. Mineral wool runs about 0.034–0.040 W/mK, which is roughly R-3.6 to R-4.2 per inch. Area sets the cost, thickness sets the result.
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How the insulation calculator works
- 1Calculate total area to insulate (m²)
- 2Divide by coverage per roll or batt (from product data sheet)
- 3Apply 5–10% waste for cuts around joists, pipes and obstacles
- 4R-value check: thickness (m) ÷ thermal conductivity λ (W/mK) — mineral wool λ ≈ 0.034–0.040, which is R-3.6 to R-4.2 per inch
- 5U-value = 1 ÷ total R-value of the whole wall/roof assembly
- 6Round up to nearest whole roll or pack
Worked example
Attic floor: 40 m² (430 sq ft). Two layers of 100 mm (4 in) mineral wool (λ = 0.037), roll coverage 5.76 m² — 62 sq ft — each.
You need 8 rolls per layer = 16 rolls total. Combined R-value: 5.4 m²K/W (100 mm ÷ 0.037 × 2) — about R-31 in US units — achieving a U-value of approximately 0.18 W/m²K.
Frequently asked questions
How thick should loft insulation be?
Most energy codes recommend 270–400 mm of mineral wool in loft spaces — 11–16 in, roughly R-40 to R-60 in US terms. This is typically installed as two layers laid at right angles to eliminate cold bridges through joists.
What is the R-value of 100 mm (4 in) mineral wool?
At λ = 0.037 W/mK, 100 mm of mineral wool gives R = 0.1 ÷ 0.037 = 2.7 m²K/W — about R-15 in US units, or R-3.9 per inch. Rigid EPS foam at λ = 0.038 gives a similar R-value in the same thickness.
What is the difference between R-value and U-value?
R-value measures thermal resistance — higher is better. U-value measures heat loss through the whole assembly — lower is better. U = 1 ÷ R-total.
Should I use mineral wool or rigid foam for wall insulation?
Mineral wool batts are easier to install between studs and provide fire resistance and acoustic damping. Rigid foam boards offer higher R-value per mm and are better for continuous external insulation.
How much insulation do I need for a 50 m² (540 sq ft) floor?
At 200 mm (8 in) depth with rolls covering 5.76 m² (62 sq ft) each, you need approximately 9–10 rolls per layer (9 before waste, 10 with 10% waste allowance). For two layers (crossing direction): 18–20 rolls.