Attic Insulation Calculator — Rolls, Depth & R-Value
Calculate how many rolls or batts of insulation you need for an attic or loft floor, and check the R-value the depth achieves. Enter the attic area and target depth, and get roll count per layer with waste included.
How much loft insulation do I need?
Divide the loft floor area by the coverage printed on the pack, then add 5–10% for cutting around joists and pipes. Rolls commonly cover 5–8 m² (54–86 sq ft), and less as they get thicker — a 200 mm roll is shorter than a 100 mm one of the same width, so doubling the depth more than doubles the packs.
How deep should loft insulation be?
Far deeper than a wall, because nothing has to fit inside a stud cavity. 270–300 mm — near enough 11–12 in — is the usual target for mineral wool at 0.034–0.040 W/mK, roughly R-40 to R-49. Lay it in two layers with the second crossing the joists: that covers the timber, which is otherwise a cold bridge every 400 mm.
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How the attic insulation is calculated
- 1Attic floor area = length × width (measure at joist level)
- 2Rolls per layer: area ÷ coverage per roll (from the product label)
- 3Two crossed layers beat one thick one — the second layer covers the joists and kills thermal bridging
- 4R-value: total thickness (m) ÷ thermal conductivity λ (mineral wool ≈ 0.037 W/mK, about R-3.9 per inch)
- 5Add 5–10% waste for cuts around joists, pipes and the hatch
- 6Keep eaves ventilation paths open — don't pack insulation into the eaves
Worked example
Attic: 60 m² (645 sq ft). Two crossed layers of 150 mm (6 in) mineral wool (λ = 0.037), roll coverage 5.5 m², or 59 sq ft.
11 rolls per layer = 22 rolls total. Combined R-value: 0.30 ÷ 0.037 = 8.1 m²K/W — approximately R-46 in US units.
Frequently asked questions
How much insulation should an attic have?
Common targets: 270–400 mm of mineral wool in UK/EU guidance, and R-38 to R-60 in US climate zones (roughly 250–400 mm depending on material). More depth is one of the cheapest energy upgrades a house can get.
Can I add new insulation on top of old?
Yes — as long as the old material is dry and mould-free, lay the new layer across it at right angles. Use unfaced material on top; a vapour barrier between layers traps moisture.
Should insulation be compressed to fit?
No — insulation works by trapping air, and compressing 200 mm into a 100 mm space — 8 in into 4 in — gives roughly the R-value of 100 mm. Buy the thickness that fits, or raise the floor level if you need loft storage.
Blown-in or rolls for attic insulation?
Rolls/batts are the DIY option and what this calculator counts. Blown-in cellulose or fibreglass fills irregular spaces faster but needs a machine — its coverage comes from the bag chart at your target depth.
Why keep the eaves clear of insulation?
Roof ventilation enters at the eaves; blocking it causes condensation on the cold roof timbers, and wet timber rots. Use eaves baffles to hold the ventilation path open where insulation meets the roof slope.