Insulation & Energy
Attic / Loft Insulation Calculator
Calculates rolls of mineral-wool loft insulation
Updated 28 May 2026 · Live
What this tool does
Estimates the number of rolls of mineral-wool loft insulation needed to cover a loft area, with a 5% trade allowance for offcuts and overlaps.
Formula Used
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How the attic / loft insulation calculator works
Divide the loft area by the coverage of one roll, multiply by 1.05 to allow for offcuts and overlaps at trusses, and round up to whole rolls. Multiply the roll count by the supplier price for an indicative cost. Every figure is an estimate — site conditions and the specific product will move the final number.
Typical UK loft roll dimensions and coverage
A standard UK mineral-wool loft roll measures around 1140 mm wide by 5.45–6.00 m long, covering roughly 6.0–6.8 m² per roll. Shorter rolls (≈3.10 m) covering ≈3.50 m² are common for thicker 200 mm and 270 mm products. Check the product label — coverage is printed on the wrapper. The calculator default of 6.22 m² reflects a typical 100–170 mm loft roll; adjust it to match the roll you intend to buy.
Loft insulation depths and indicative R-values
Approved Document L 2021 sets a target U-value of 0.16 W/m²K for new and refurbished loft floors. Mineral wool at λ ≈ 0.044 W/m·K gives indicative thermal resistances of R ≈ 2.3 m²K/W at 100 mm, R ≈ 4.5 m²K/W at 200 mm, and R ≈ 6.1 m²K/W at 270 mm. Reaching the 0.16 W/m²K target in a cold-loft build-up typically needs around 270–300 mm of mineral wool, laid in two layers — the second course cross-laid over the joists to break the thermal bridge.
What this tool does not do
It does not size the second cross-laid layer separately, factor eaves ventilation, assess condensation risk, replace a professional quote, or confirm Building Regulations compliance. Those remain the responsibility of a suitably qualified designer, surveyor, or your building control officer.
On-site considerations
Maintain at least a 50 mm clear ventilation gap at the eaves so the cold roof void breathes — closing it off invites interstitial condensation. Keep insulation away from recessed downlights unless they are rated for direct cover. Vapour control layers, where specified, must be continuous and sealed at every service penetration.
Building Regulations and compliance
New and refurbished building fabric must meet the U-value targets in Approved Document L. SAP 10.2 is the current calculation methodology for dwellings. A pre-application enquiry to the local authority can give early clarity, which tends to be less costly than retrospective correction.
Before you order
Mineral wool stored outside gets damp and loses performance — bring rolls inside the weathertight envelope before installation. Cross-check the calculator's output against a supplier quote to catch differences in product specification (thickness, λ-value, roll coverage) and confirm delivery timescales against your programme.
Adjusting the defaults
Every input is editable. Enter your own loft area, the coverage printed on your chosen product, and the supplier price — the output recalculates instantly.
Using this calculator alongside other BuildMetricLab tools
Pair the roll count with our wall cavity insulation, floor insulation, and material-cost calculators to build a complete fabric upgrade estimate. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the maths.
Sources & methodology
Rolls = ⌈(Loft area ÷ Coverage per roll) × 1.05⌉. The 1.05 factor adds a 5% trade allowance for offcuts at trusses and end-of-run overlaps. Indicative cost = rolls × price per roll (whole-roll basis — no fractional billing).
Frequently asked questions
Are attic / loft insulation calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
How much loft insulation do I need to meet Approved Document L?
Approved Document L 2021 sets a U-value target of 0.16 W/m²K for loft floors. In a cold-loft build-up using mineral wool (λ ≈ 0.044 W/m·K), reaching that target typically needs around 270–300 mm laid in two layers — the second cross-laid over the joists to break the thermal bridge. The calculator counts the rolls; thickness is a product choice you make at the supplier.
Does the cost include partial rolls?
No. The calculator rounds up to whole rolls and bills the cost on that whole-roll count — suppliers do not sell mineral wool by the offcut, so the figure mirrors what you would actually pay.
Does this replace professional advice?
No. This tool is a planning estimator. For works that affect structure, Building Regulations, Party Wall, gas, electrics, drainage to a sewer, or similar, consult a suitably qualified professional.
Can I change the unit prices?
Yes — every price field is editable. Plug in your supplier's quote to get a total that matches your project.
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