Loft Insulation Calculator — How Many Rolls Do I Need?

The Energy Saving Trust recommends around 270mm of mineral wool insulation at ceiling level, which gives a U-value of about 0.16 W/m²K with standard λ 0.044 W/mK mineral wool. Building Regulations (Approved Document L) set a U-value target rather than a fixed depth, so 270mm is the typical depth that meets it. Many energy advisors suggest 300mm or more for best performance. If you have less than 270mm, it is worth topping up. Use our Insulation Thickness Calculator to check the exact depth needed for your target U-value.

The first layer (typically 100mm) is laid between the ceiling joists to fill the gaps. The second layer (typically 170mm) is laid across the top of the joists at 90 degrees. This cross-layer method avoids thermal bridging through the timber joists and gives much better overall insulation performance.

According to the Energy Saving Trust (figures based on fuel prices as of July 2026), insulating a gas-heated home from a completely uninsulated loft to 270mm saves around £360 a year for a detached house, £350 for a detached bungalow, £190 for a semi-detached house and £140 for a mid-terrace. If you already have a partial layer, topping up from 120mm to 270mm saves much less, around £35 for a detached house or bungalow, £17 for a semi and £13 for a mid-terrace, because most of the easy heat loss is already captured. The exact savings depend on your property type, heating system and energy prices. Try our EPC Rating Estimator to see how loft insulation affects your energy rating.

Yes, laying mineral wool rolls in a loft is one of the simplest DIY insulation jobs. You need a dust mask, gloves, goggles, and long sleeves as mineral wool fibres can irritate skin and lungs. Ensure you do not block eave ventilation, cover electrical cables, or compress the insulation around obstacles.

Mineral wool loft rolls (Knauf, Superglass, Isover) are usually 1140–1200mm wide and combi-cut, meaning each roll is partially perforated so it can be split into two 570mm or three 380mm strips to fit between joists, or laid full-width across them. A thicker roll holds less length, so coverage per roll falls as thickness rises — for example Knauf Loft Roll 44 covers about 13.89 m² per roll at 100mm but about 8.01 m² per roll at 170mm. Always check the pack coverage (m²) for the specific product you are buying.

Multiply your loft length by its width to get the floor area in m², then divide by the coverage (m²) printed on the roll pack and round up to whole rolls. Going from a bare ceiling to the 270mm recommended depth normally takes two layers, a 100mm layer between the joists plus a 170mm layer across them, so you need one set of rolls per layer. As a rough guide, a typical 48 m² semi-detached loft needs around 4 rolls of 100mm plus around 6 rolls of 170mm before wastage, but the exact number depends on the product. For an L-shaped loft, use the Add section button to enter each rectangle separately. Enter your dimensions, current depth and roll coverage above and this loft insulation calculator does the maths for you, adding a cutting allowance.

Split the loft into rectangles and enter each one as its own section using the Add section button; the calculator adds the areas together for you. Roll counts are worked out per section and rounded up before being added, which covers the extra cutting where runs stop and restart at the junction between sections and never under-orders. Use three sections for a T or U shaped loft, and keep the wastage allowance on for cutting around obstacles.

Yes. A cold loft must stay ventilated so moisture cannot build up and cause condensation, mould and timber rot. Approved Document C of the Building Regulations (resistance to moisture) is met for a cold pitched roof by following BS 5250, which calls for cross-ventilation: a continuous eaves opening equivalent to a 25mm gap for shallow pitches (and 10mm for pitches over 15 degrees), plus additional high-level ventilation equivalent to a 5mm continuous gap at the ridge where the pitch is above 35 degrees or the roof span is over 10 metres. In practice this means never push insulation tight into the eaves — keep a clear air path at the wall plate (eaves ventilation trays make this easy) so the roof void can breathe.

Not unless they are designed for it. Covering standard recessed downlights traps heat and is a fire risk, so insulation must be kept clear of fittings that are not rated for insulation contact — use IC-rated (insulation-coverable) downlights or fit proprietary fire-rated loft caps over each one and follow the maker's clearance instructions. Cables also need care: BS 7671 (the IET Wiring Regulations, 18th Edition) treats cables buried in thermal insulation as a derating condition, because insulation stops them shedding heat, so heavily loaded circuits may need checking by an electrician. Lay insulation above cables where you can rather than burying them, and never block the loft hatch or recessed extract fans.

Sometimes, but the schemes change often, so always check the current rules before relying on them. The Great British Insulation Scheme closed to new applications on 31 March 2026; the Energy Company Obligation (ECO4) still runs through participating energy suppliers, and the Warm Homes: Local Grant is delivered by councils in England. Eligibility usually depends on your income, the benefits you receive, your home's EPC rating and sometimes your council tax band. Because amounts and qualifying criteria are updated regularly, check the latest position on gov.uk and Ofgem rather than any third-party figure.