What Thickness of Loft Insulation Do I Need? The 270mm Rule
If you're insulating a loft at ceiling level (a "cold loft"), the figure you'll hear most is 270mm. This guide explains where that number comes from, how it relates to Building Regulations, and which combinations of loft roll get you there. It also covers topping up existing insulation. If you already know the depth and just need quantities, see how many loft insulation rolls do I need.
Quick answer
- Aim for at least 270mm of mineral wool loft insulation. The Energy Saving Trust describes 270mm as the minimum recommended depth.
- Lay it in two layers: one between the joists and one across them at right angles.
- Typical build-up with our rolls: 100mm between the joists plus 200mm across = 300mm.
- Topping up: if you already have around 100mm that is dry and in good condition, a 200mm layer across the joists takes you to 300mm.
Where does 270mm come from?
Building Regulations set U-values (the rate of heat loss) rather than depths. In England, Approved Document L (Volume 1, 2026 edition) gives a target U-value of 0.16 W/m²K for renovation work on a roof insulated at ceiling level. Its example of a typical way to achieve that is loft insulation of at least 250mm of mineral fibre, laid between and across the ceiling joists. Wales has its own Approved Document L, so check the Welsh guidance for projects there.
The Energy Saving Trust recommends a minimum depth of 270mm for mineral wool, and that is the figure most people now work to. Going deeper than 270mm improves performance further, as long as you keep the eaves ventilation clear.
Why depth matters
Mineral wool insulates by trapping air, and the thicker the layer, the higher its thermal resistance (R-value). URSA's datasheet shows this for its 10 DIVERSO loft roll:
| URSA 10 DIVERSO thickness | Thermal resistance (m²K/W) |
|---|---|
| 150mm | 3.40 |
| 200mm | 4.50 |
URSA 10 DIVERSO has a declared thermal conductivity (λ) of 0.044 W/mK and is Euroclass A1 (non-combustible). ODE Lunaflex Loft Roll Ultra 42 has a λ of 0.042 W/mK, according to ODE's Environmental Product Declaration. The lower the λ, the better the insulation per millimetre.
Ways to reach 270mm or more with our rolls
Coverage is per roll. Prices are ex VAT, at the time of writing:
| Roll | Coverage | Price per roll | Approx. cost per m² |
|---|---|---|---|
| ODE Lunaflex Ultra 42, 100mm | 10.83m² | £22.47 | £2.07 |
| ODE Lunaflex Ultra 42, 150mm | 7.98m² | £25.87 | £3.24 |
| ODE Lunaflex Ultra 42, 200mm | 6.84m² | £26.73 | £3.91 |
| URSA 10 DIVERSO, 150mm | 8.55m² | £30.57 | £3.58 |
| URSA 10 DIVERSO, 200mm | 6.27m² | £29.73 | £4.74 |
| Knauf Loft Roll 44 Combi-Cut, 100mm | 13.89m² | £30.00 | £2.16 |
| Knauf Loft Roll 44 Combi-Cut, 150mm | 9.18m² | £30.00 | £3.27 |
| Knauf Loft Roll 44 Combi-Cut, 170mm | 8.01m² | £30.00 | £3.75 |
| Knauf Loft Roll 44 Combi-Cut, 200mm | 6.84m² | £30.00 | £4.39 |
Common combinations:
- 100mm + 200mm = 300mm. The usual choice for a loft with 100mm-deep joists. The 100mm roll fills between the joists and the 200mm roll goes across them.
- 100mm + 170mm = 270mm. Knauf Loft Roll 44 comes in 170mm, so 100mm between the joists and 170mm across them gives exactly 270mm.
- 150mm + 150mm = 300mm. Suits deeper joists. Use 150mm between and 150mm across.
- 100mm + 150mm = 250mm. This matches the Approved Document L example but falls short of 270mm, so we'd suggest going to 300mm if you have the headroom.
With URSA 10 DIVERSO, two 150mm layers give a combined thermal resistance of 6.80 m²K/W (3.40 + 3.40), using URSA's published figures.
Topping up existing loft insulation
- Measure what's there. Push a ruler down through the insulation to the ceiling in a few places. Many older lofts have around 100mm or less.
- Check its condition. If it's damp, badly compressed or contaminated, remove it and start again. Fix any damp problem first.
- Add the difference. Lay the top-up layer across the joists, at right angles to the existing layer, until you reach at least 270mm in total.
- Don't squash it. Compressed mineral wool loses much of its performance. If you want to board the loft for storage, raise the boards above the full depth of insulation.
- Keep the eaves clear. Leave the ventilation gap at the eaves open so air can move through the roof space.
For step-by-step laying advice, see our guide to installing loft insulation.
What about Knauf and Isover loft rolls?
We now stock Knauf Insulation Loft Roll 44 Combi-Cut in 100, 150, 170 and 200mm (λ 0.044 W/mK, Euroclass A1). It is part-cut along its length, so you can split it into 570mm or 380mm widths to suit your joist spacing. Isover also makes glass mineral wool loft rolls, and you'll see them specified on some jobs; we don't stock Isover at the moment. The same rules apply whichever brand you use: check the roll's declared λ and R-value on the manufacturer's datasheet, and lay enough to reach at least 270mm. The ODE, URSA and Knauf rolls above are the loft rolls we sell now.
When PIR board makes more sense
If you need to keep the loft floor usable and can't build up to 270mm of wool, PIR board gives much more insulation per millimetre. 100mm of Recticel Eurothane GP has the same R-value (4.50 m²K/W) as 200mm of URSA 10 DIVERSO. See our guide to what thickness of PIR insulation you need, or check a build-up with the PIR U-value calculator.
Frequently asked questions
Is 200mm of loft insulation enough?
It is well below the 270mm the Energy Saving Trust recommends. If you have 200mm or less, it's usually worth topping up.
Is 300mm too much?
No. 270mm is a minimum recommendation, and deeper insulation cuts heat loss further. Just keep the eaves ventilation clear and don't compress the rolls.
Can I lay new insulation on top of old?
Yes, as long as the existing insulation is dry and in reasonable condition. Lay the new layer at right angles to the joists.
Which way do I lay the second layer?
Across the joists, at right angles to the first layer. This covers the tops of the joists, which would otherwise be a cold bridge.
Order loft insulation
Browse our loft insulation rolls, or use the free insulation calculator to work out how many rolls you need. Prices are shown ex VAT. We are delivery only (no collection) and deliver to mainland England and Wales, with a £400 ex VAT minimum order and free delivery on every order (see delivery information). Call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm), email sales@insulationwarehousedirect.co.uk, or order online.
Sources: Energy Saving Trust, Roof and loft insulation; Approved Document L, Volume 1, 2026 edition (GOV.UK), Table C1; URSA 10 DIVERSO datasheet; ODE Lunaflex Loft Roll Ultra 42 EPD; Recticel Eurothane GP. Figures are indicative and correct at the time of writing.