How much loft insulation do I need?
Enter the loft floor area and we'll work out how much mineral-wool insulation you need to reach the recommended 270 mm, plus the cost — with the calculation shown.
1 m² covered per m².
| Loft insulation (270mm mineral wool) | 10 × roll (≈5.4 m²) | £220 |
- 1Loft area: 50 m²
- 2Usage rate: 1 m² covered per m²
- 3Allowing for waste: 50 × 1 × 1.05 waste = 53 m²
- 4Packs: 53 m² ÷ 5.4 per pack = 10 × roll (≈5.4 m²)
- Building Regs recommend ~270mm. Two-layer (170+100) gives best coverage.
- Indicative trade-counter price; confirm with a current supplier quote.
How to work out loft insulation
- 1Measure the loft area
Length × width of the loft floor. It's usually a little less than the ground-floor footprint.
- 2Aim for 270 mm depth
Current guidance is around 270 mm of mineral wool. If you already have some, top up the difference rather than starting from scratch.
- 3Two layers, crossed
Lay the first layer between the joists and the second layer across them to cut cold bridging through the timber.
- 4Keep eaves clear
Don't block the eaves ventilation, and don't compress the insulation — loft boarding needs raised battens to keep the depth.
Loft insulation is the definitive quick win — cheap, fast, and one of the few improvements you can do yourself in an afternoon. Working from the loft area to the 270 mm target tells you exactly how many rolls to buy, so you top up to standard in one trip rather than guessing.
Loft insulation quantities (indicative)
| Property | Loft area | Notes |
|---|---|---|
| Flat / small | ~25 m² | Quick DIY job |
| 3-bed house | ~45 m² | A few hours' work |
| Large / detached | ~70 m² | Larger roll count |
| Target depth | 270 mm | Two crossed layers |
Coverage per roll depends on the product width and thickness. Add a little for offcuts. Indicative — confirm with quotes.
Frequently asked questions
How much loft insulation do I need?+
Enough to reach about 270 mm across the whole loft floor. Measure the loft area, and if you already have some insulation, only top up the shortfall — the calculator works out the rolls and cost.
How deep should loft insulation be?+
Current guidance is around 270 mm of mineral wool for a cold loft. Older homes often have 100 mm or less, so topping up to 270 mm is a cheap, high-impact upgrade.
Does loft insulation improve the EPC?+
Yes — increasing loft insulation to the recommended depth is one of the most cost-effective EPC improvements, which matters for lettings under minimum energy standards.
Related tools & guides
Want to know how these figures are calculated? See our cost methodology.
Cost figures shown are indicative estimates, not quotations. You are responsible for verifying all costs (obtain contractor quotes) and any figures submitted to a lender. ScopeWise is a documentation tool, not financial, tax, structural or planning advice. HMO compliance prompts are guidance only — confirm requirements with your local council, as standards and licensing vary by authority.