How the loft calculation works
The calculator sizes your loft insulation to a U-value, the measure Approved Document L uses. You can pick one of two levels:
- Building Regs (Part L): U-value 0.16 W/m²K. This is the improved value in Table 4.3 for upgrading an existing roof. The same row covers a loft insulated at ceiling level, a room in the roof insulated at the rafters, and a flat roof.
- New-build standard: U-value 0.15 W/m²K. This is the Table 4.2 value for a new roof in an existing home.
From U-value to millimetres
The U-value of the whole loft is:
U = 1 ÷ (Rsi + Σ thickness/λ + Rse)
For a loft, the inside surface (Rsi) is 0.10, the outside surface (Rse) is 0.04, and the ceiling and roof space add another 0.43, all in m²K/W. A U-value of 0.16 means a total R of 1 ÷ 0.16 = 6.25, so the insulation has to supply 6.25 − 0.57 = R 5.68 m²K/W.
At λ 0.044, that would be 250 mm if nothing got in the way. But the first layer sits between the ceiling joists, and timber conducts heat about three times better than mineral wool. The calculator assumes 100 mm joists covering 12.8% of the loft floor, so the first 100 mm of roll does about a fifth less than it would on its own. That's why the answer comes out at 270 mm, as 100 mm between the joists plus 170 mm across them. Knauf and the RdSAP 10 specification both give U-value 0.16 for that build-up, and we've calibrated the calculator to match.
Your existing insulation
If you enter a depth, the calculator values old mineral wool or polystyrene at λ 0.040, the figure the RdSAP 10 specification uses, and bridges it with the joists in the same way. New layers then go on top. If you already have enough, it says so.
Choosing the rolls
Loft Roll 44 comes in 100, 150, 170 and 200 mm. The calculator tries every combination of up to three layers that reaches the target and picks the fewest layers, then the thinnest stack. Rolls per layer = loft area × 1.1 ÷ the area one roll covers, rounded up:
| Roll thickness | Covers per roll |
|---|---|
| 100 mm | 13.89 m² |
| 150 mm | 9.18 m² |
| 170 mm | 8.01 m² |
| 200 mm | 6.84 m² |
Worked example: a 45 m² loft
Start with an empty loft floor of 45 m², for example 9 m × 5 m. With 10% waste, you need to cover 49.5 m².
- U-value now: 1.75 W/m²K
- Add: 100 mm + 170 mm = 270 mm
- To buy: 4 rolls of 100 mm and 7 rolls of 170 mm
- Material cost: £369–£1,021
- U-value after: 0.16 W/m²K
Most lofts already have something up there. Here's the same loft with different depths of old mineral wool:
| Already there | U-value now | Add | Rolls | Material cost | U-value after |
|---|---|---|---|---|---|
| Nothing | 1.75 | 100 + 170 mm | 4 + 7 | £369–£1,021 | 0.16 |
| 25 mm | 0.95 | 150 + 100 mm | 6 + 4 | £660–£928 | 0.16 |
| 50 mm | 0.65 | 150 + 100 mm | 6 + 4 | £660–£928 | 0.14 |
| 75 mm | 0.49 | 200 mm | 8 | £263–£394 | 0.15 |
| 100 mm | 0.40 | 170 mm | 7 | £234–£693 | 0.16 |
| 120 mm | 0.33 | 150 mm | 6 | £524–£600 | 0.16 |
| 150 mm | 0.27 | 150 mm | 6 | £524–£600 | 0.14 |
| 200 mm | 0.20 | 100 mm | 4 | £136–£328 | 0.14 |
| 270 mm | 0.15 | Nothing: already meets 0.16 | – | – | 0.15 |
Rolls only come in set thicknesses, so you sometimes end up a little better than the target. The price ranges are wide because retailers charge very different amounts for the same roll, and multibuy offers set the low end. Check the shelf price and enter it under More options.
On the New-build standard setting (U-value 0.15), the empty loft needs 300 mm: 8 rolls of 200 mm and 4 rolls of 100 mm, for £399–£722, reaching U-value 0.14.
If you'd rather pay someone, the Energy Saving Trust puts the installed cost at about £750 to insulate a typical loft from nothing to 270 mm, and about £600 to top up from 120 mm. For hard-to-reach lofts, see the blown loft insulation calculator.
How to lay loft insulation
First layer between the joists, second layer across
Roll the first layer between the joists, snug against them with no gaps. Lay the second layer at right angles across the joists, so it covers the timber as well as the first layer. That cross layer does the most work, because nothing bridges it.
Don't squash it
Mineral wool insulates because of the air trapped in it. Squashed under boards or storage, it gets thinner and loses part of its R-value. If you want to store things in the loft, raise the boards on loft legs (stilts fixed to the joists) so the full depth fits underneath.
Keep the eaves ventilation clear
A cold loft needs air to move through it to carry moisture away. Stop the insulation short of the eaves so it doesn't block the vents or the gap at the edge of the roof. Don't push it hard into the eaves.
Insulate and draught-proof the hatch
Fix a piece of insulation board or roll to the top of the loft hatch and fit draught strip around the frame. A bare hatch is a big hole in an otherwise well-insulated ceiling.
Cold water tank and pipes
Don't insulate the ceiling directly under the cold water tank. Leaving it bare lets a little warmth up to help stop the tank freezing. Insulate the top and sides of the tank instead, and lag any pipes in the loft, which will be colder once the insulation is down.
Downlights and cables
Recessed downlights get hot. Don't cover them unless they're designed for it, or fit a suitable cover. Lift electric cables out of the way and lay them on top of the insulation where you can, since cables buried in insulation can run hot. If you're unsure, ask an electrician.
Vermiculite
If you find pebbly loose fill, it may be vermiculite, which can contain asbestos. The calculator warns you if you choose it. Don't disturb it: the HSE lists loose-fill insulation among the places asbestos can be found, so have it checked by a professional before any work in the loft.
Stay safe
Step only on the joists or on boards laid across them, since the ceiling won't take your weight. Wear an FFP2 mask, goggles, gloves and long sleeves when handling mineral wool, and use a good work light.
Next steps
- See how thick each product needs to be in loft insulation thickness
- Check the full table of Part L U-values
- Work out any build-up by hand with the U-value calculator
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- BRE BR 443: Conventions for U-value calculations (2019)
- RdSAP10 Specification (BRE, February 2024)
- Energy Saving Trust: Roof and loft insulation
- Knauf Insulation: Loft Roll 44 (Combi-cut)
- Knauf Insulation: Loft Roll installation guide (2025)
- Wickes: loft insulation (prices)
- Toolstation: insulation (prices)
- HSE: Asbestos — location of materials (loose-fill)