Loft Insulation Calculator
Calculate how many rolls of loft insulation you may need for any loft. Covers cold loft floor and warm roof rafter methods with the 2026 Building Regs 270mm standard and eaves ventilation estimate.
Full system achieves the 2026 Building Regs target of 0.16 W/m²K. The cross-layer prevents cold bridging through timber joists.
Knauf Earthwool 100mm: 1200×370mm roll, 8.35 m² coverage. Standard between-joist base layer product.
Total length of eaves where insulation meets the roof edge — not the full room perimeter. For a standard pitched roof with two eaves, enter the combined length of the two long sides only (e.g. 8m + 8m = 16m). Gable ends have no eaves. Leave blank to skip eaves boards.
Each recessed downlight needs a fire-rated insulation cap (fire hood) to allow heat to escape. Leave blank or 0 to skip.
Insulating the loft floor makes the space above significantly colder in winter. An uninsulated cold water tank and pipes will be at much higher risk of freezing.
Safety gear checklist
Installation Guide
How to Insulate a Loft
The 270mm Two-Layer System
The 2026 UK Building Regulations Part L and Energy Saving Trust recommend 270mm total depth for mineral wool loft insulation, achieving a U-value of approximately 0.16 W/m²K. This is always installed in two layers to prevent cold bridging through the timber joists.
Layer 1 — 100mm between joists: Cut to fit snugly between the ceiling joists. Mineral wool should friction-fit with no gaps at the edges. Work from the eaves toward the access hatch — never trap yourself in a corner.
Layer 2 — 170mm across joists: Laid at 90° over the top of the joists and first layer. This continuous layer covers the joists themselves, eliminating the thermal bridge that would otherwise allow heat to escape through the timber. This layer does not need to be cut between joists.
Ventilation — Never Block the Eaves
In a cold loft, air must flow from the outside in at the eaves, across the loft floor, and out at the ridge. This prevents condensation forming on the cold roof timbers which would cause rot. The Building Regulations require a minimum 25mm clear air gap between the insulation and the underside of the roof deck at the eaves.
Fit eaves ventilation boards (also called roll-stop or eaves trays) at the edge of the insulation to hold the mineral wool back and maintain the airflow channel. Never pack insulation tight to the eaves fascia.
Warm Roof — Loft Conversions
For a habitable loft conversion, the insulation moves from the floor to between and above the rafters — creating a "warm roof" where the loft space stays at room temperature. This requires high-performance PIR boards (λ ≈ 0.022 W/mK) to achieve the required U-value of 0.18 W/m²K within the limited rafter depth.
Ventilation gap: A 50mm clear gap must be maintained between the top of the insulation and the underside of the roof deck above, running from eaves to ridge. Without this gap, moisture from the living space can condense on the cold roof deck, causing rot and mould.
Safety & Building Regulations
Always wear an FFP2 or FFP3 dust mask, safety goggles and gloves when handling mineral wool. Never step between joists — use a walking board (a plank of wood spanning at least two joists) to distribute your weight. A single misplaced foot will go straight through the plasterboard ceiling.
Upgrading an existing loft from less than 100mm to 270mm is generally permitted development with no planning permission required. However, insulating a new loft conversion requires Building Regulations approval under Approved Document L.
FAQ
Common Questions
270mm total depth is the UK standard for mineral wool loft insulation, achieving a U-value of approximately 0.16 W/m²K. This is laid in two layers: 100mm between ceiling joists and 170mm at 90° across the joists. The cross-layer prevents cold bridging through the timber. Many older homes only have 100mm and would benefit significantly from a top-up layer.
Divide your loft floor area by the roll coverage (shown on the packaging) and add 10% for wastage. Knauf Earthwool 100mm covers 8.35 m² per roll; the 170mm covers 5.57 m² per roll. A typical 48 m² semi-detached loft needs approximately 6 rolls of 100mm and 9 rolls of 170mm for the full system.
Yes, if you already have 100mm between the joists you can lay a 170mm cross-layer directly on top. This is the top-up layer option in our calculator. Do not remove existing insulation — just add the cross-layer over it. If the existing insulation is compressed, damaged or less than 50mm, remove and replace it first.
Yes — the UK government's Great British Insulation Scheme (2024–2026) provides free or heavily subsidised loft insulation for homes with an EPC rating of D, E, F or G. The ECO4 scheme also offers free insulation to households on certain benefits. Contact your energy supplier or check the gov.uk website for eligibility. DIY installation does not qualify for government grants.
Yes. Cables buried in insulation cannot dissipate heat and may overheat, creating a fire risk. Before laying insulation, clip all cables to the top faces of the joists so they sit above the insulation layer, or reroute them through the joists. Recessed light fittings projecting into the loft space must be covered with fire-rated insulation covers (available at builders merchants).