Loft Insulation Calculator UK
Insulating a loft is the cheapest big win in a UK home, and this calculator tells you exactly how much to buy. Give it your loft area and whatever depth is already up there, and it works out the shortfall to the current recommended 270 mm, how many rolls of mineral wool that takes, the rough 2026 material cost, and the U-value band you'll end up in. It assumes the standard UK two-layer method — one layer between the joists, a second across them — and adds a sensible waste allowance so you're not driving back to the merchant for one more roll. Enter your figures below to see rolls, cost and depth.
2026 UK typical — edit to match your merchant.
Enter your loft area to calculate.
How this calculator works
The tool starts from the gap between what you have and what you want. Most UK lofts are aiming for 270 mm of mineral wool, the depth recommended in current guidance, so if you've got 100 mm up there already you need another 170 mm on top. It converts that shortfall into layers based on your roll thickness, multiplies by your loft area, adds 10% for waste and offcuts around joists and pipes, then rounds up to whole rolls. Cost is simply rolls times your price per roll, which you can edit to match your merchant. Takeaway: you're buying to fill a gap, not to hit a total — existing insulation counts, so measure it before you order.
How much loft insulation do you need in the UK?
Current UK guidance points to 270 mm of mineral wool loft insulation, up from the 100 mm that was normal in homes built or upgraded decades ago, and the 25 mm or so you'll find in genuinely old, untouched lofts. That jump matters: going from 100 mm to 270 mm roughly halves the heat lost through the roof. The 270 mm figure is for mineral wool at a thermal conductivity around 0.044 W/m·K — higher-performance materials like PIR boards or sheep's wool hit the same U-value in less depth, but roll mineral wool is what most people lay because it's cheap and easy. Takeaway: 270 mm is the number to aim for with standard mineral wool; if you top up an old 100 mm loft you're adding two-thirds again, not starting from scratch.
The two-layer method (and why it matters for quantity)
UK lofts are almost always done in two layers. The first goes between the joists, which are usually 100 mm deep, so it fills that space. The second goes across the joists at right angles, covering the timber itself — because the joists are a thermal bridge that leaks heat if you leave them exposed. This calculator counts layers for exactly that reason: a 270 mm target on 170 mm rolls is two passes, not one, so you need roughly double the area in rolls. It's also why "how many rolls" is never just area divided by roll coverage. Takeaway: budget for two layers unless you're only topping up a thin single layer — the cross-layer over the joists is where a lot of the real saving comes from.
A point most loft insulation calculators skip
Plenty of online tools quietly assume you're starting from a bare loft and quote you the full 270 mm, which over-sells rolls to anyone topping up. The honest calculation subtracts what's already there. But there's a second thing the tidy-number tools miss: you should not insulate under a cold-water tank, and you must not cover downlighters, extractor motors or cabling without the right clearances, or squash the insulation into the eaves and block the ventilation gap that stops condensation. A loft crammed wall-to-wall and 270 mm thick over the light fittings is a damp problem waiting to happen. Takeaway: the quantity is the easy part — leave the tank base clear, keep clearances around fittings, and don't choke the eaves ventilation.
Worked example — topping up a Sheffield semi
Take a three-bedroom semi in Sheffield with a 50 m² loft that already has the old 100 mm of insulation between the joists, aiming for the recommended 270 mm with 170 mm mineral-wool rolls covering 5.44 m² each at £22 a roll. The shortfall is 170 mm, so that's one cross-layer over the joists. Area 50 m² times one layer, plus 10% waste, divided by 5.44 = 11 rolls, about £242 in materials. The finished depth is 270 mm, landing in the "Recommended" U-value band at roughly 0.16 W/m²K. If the same loft were bare, it'd be two layers — around 25 rolls and £550. Takeaway: topping up an old 100 mm loft is a sub-£300 material job for a typical semi; starting from bare roughly doubles it.
UK loft insulation depth reference
| Situation | Typical depth | What it means |
|---|---|---|
| Current recommendation | 270 mm | The target this tool defaults to |
| 1990s–2000s building standard | 100–200 mm | Common in homes upgraded once already |
| Pre-1990 / untouched | 0–25 mm | Effectively uninsulated — big saving available |
| New-build (Approved Document L) | Designed to a U-value, often ~250–300 mm equivalent | Specified by U-value, not depth |
| Loft conversion (warm roof) | Insulation moves to the rafter line | Different job — see the loft conversion cost tool |
Takeaway: match your top-up to what you actually have, and remember a loft conversion insulates the slope, not the floor.
Frequently asked questions
How much loft insulation do I need in the UK?
Current UK guidance recommends 270 mm of mineral wool loft insulation. If your loft already has some — often 100 mm in homes upgraded in the past — you only need to add the difference, usually a single 170 mm layer laid across the joists.
How many rolls of loft insulation do I need?
It depends on your loft area, the target depth and how much is already there. As a rough guide, a bare 50 m² loft going to 270 mm needs around 22–25 rolls of 170 mm mineral wool; a 50 m² loft topping up from 100 mm needs around 11. This calculator works out the exact number with a waste allowance.
What is the recommended depth for loft insulation?
270 mm of mineral wool is the current recommended depth. Older homes were built to 100 mm or less, so many lofts benefit from a top-up layer to reach today's standard.
Can I just lay new insulation over old?
Yes, as long as the existing insulation is dry and in good condition. Lay the new layer across the joists at right angles to the old layer between them. If the old material is damp, compressed or contaminated, take it up first.
Should I insulate under the water tank?
No. Leave the area directly under a cold-water storage tank clear so a little warmth rises to stop it freezing in winter. Insulate the tank sides and lid separately instead.
How much does it cost to insulate a loft in the UK?
For materials alone, a typical semi top-up is often £200–£350 and a bare loft £450–£700, depending on area and product. Supply-and-fit by an installer commonly adds roughly £6–£12 per square metre on straightforward lofts.
Will 270 mm of insulation stop my loft ventilating?
Only if it's laid badly. Keep the insulation clear of the eaves so air can still pass into the roof space, and don't cover downlighters or extractor fans without the correct clearances. Blocking ventilation is a common cause of loft condensation.
Related tools
- U-Value Calculator UK — full layer-by-layer U-value for a roof, wall or floor to BS EN ISO 6946
- Dew Point Calculator UK — check condensation and mould risk once you know your build-up
- Loft Conversion Cost Calculator UK — if you're converting rather than just insulating the floor
- Plasterboard Calculator UK — sheets for boarding out a room once insulated
Guides worth reading
- How Much Loft Insulation Do I Need? UK Guide — the 270 mm target, the two-layer method and a worked cost example
Disclaimer: This calculator gives indicative quantities and 2026 UK material costs for planning only. It is not a quote and does not replace professional advice. Actual requirements depend on your existing insulation, joist depth, loft ventilation, the specific product and its stated coverage, and any fittings that need clearance. Building work should follow Approved Document L (conservation of fuel and power) and manufacturer fitting instructions. Keep insulation clear of the eaves, cold-water tank base and light fittings, and get advice if your loft has damp, spray-foam or electrical concerns.