Doing it yourself? Our loft insulation guide covers the two-layer method, storage without squashing it, and a planner priced at UK merchants that shows how fast a DIY top-up pays for itself.
How the loft insulation calculator works
The calculator finds the depth you still need (target minus existing), then picks the most practical combination of standard roll thicknesses to reach it. If there's nothing there, it suggests a 100mm layer between the joists plus a top layer across them.
Typical roll coverage
| Roll thickness | Coverage per roll | Use |
|---|---|---|
| 100mm | 11.08 m² (9.72 × 1.14m) | Between joists |
| 170mm | 6.47 m² (5.68 × 1.14m) | Top-up over 100mm (= 270mm) |
| 200mm | 5.51 m² (4.83 × 1.14m) | Deep top-up |
Figures are for the Knauf loft rolls stocked by Wickes and B&Q. Other brands vary, so use the coverage on your chosen product if it differs. A 1.14m roll cuts neatly into two or three pieces to fit common joist spacings.
Safety and good practice
- Wear coveralls, gloves, goggles and a mask: mineral wool is an irritant.
- Only walk on the joists, or on boards laid across them.
- Keep eaves vents clear so the roof space stays ventilated; use eaves trays if needed.
- Don't cover recessed downlights unless they're rated for it (use fire-rated covers), and lay cables on top of the insulation where possible.
Worked examples: an 8m × 6m loft
The loft floor is 48m². Each layer is ordered on 48 × 1.05 = 50.4m², to allow for trimming round joists and the hatch.
- About 100mm there now (level with the joists): one 170mm layer across the joists brings it to 270mm. 50.4 ÷ 6.47 = 7.8, so 8 rolls of 170mm.
- Nothing there: a 100mm layer between the joists (5 rolls at 11.08m²) plus a 170mm layer across them (8 rolls) = 13 rolls.
- A thin 50mm layer: you need another 220mm. The most practical mix is again 100mm + 170mm, 13 rolls, finishing at 320mm, which is above the minimum and does no harm.
At around £27 a roll on offer, the top-up in the first case costs about £216 in materials. Our loft insulation guide works out how quickly that pays back in heating savings for your home.
Storage and boarding
Boards laid straight on the joists squash a 270mm layer down to joist depth, losing much of its benefit. If you want to store things, fit raised loft legs to lift the boards above the full depth of insulation, and keep the stored weight sensible. Board only the area you need near the hatch. The rest of the loft can stay at full depth.
Don't forget
- The hatch: insulate the back of the hatch door and fit a draught seal. An uninsulated hatch is a big cold spot.
- Pipes and tanks: once the loft is colder, lag water pipes and the cold water tank (sides and top, not underneath) so they don't freeze.
- Ventilation: keep the eaves clear so air can move through the roof space. Blocked eaves can cause condensation on the rafters.
- Grants: government-backed energy schemes sometimes fund loft insulation for eligible homes. Check GOV.UK or your energy supplier before you buy.
Common mistakes
- Squashing the insulation under boards or stored boxes.
- Gaps at the edges. Even small gaps at the eaves and round the hatch let heat escape. Butt rolls tightly together.
- Both layers the same way. The top layer should run at right angles to the joists to cover them and reduce cold bridging.
Once the loft is done, check your radiators with the radiator BTU calculator, or the whole system with the boiler size calculator.