An uninsulated roof loses around a quarter of a home's heat, according to the Energy Saving Trust. Topping up a loft is one of the easiest energy-saving jobs you can do yourself: no special tools, no mess downstairs, and most lofts are done in a day. This guide covers what to check first, the two-layer method to 270mm, how to keep storage space without squashing it, and the mistakes that cause damp. The loft planner counts the rolls, prices them at UK merchants, and shows how quickly a DIY top-up pays for itself.
1. Is it worth it? The honest numbers
The Energy Saving Trust publishes typical figures for a GB home (last updated May 2026):
| Job | Installer cost (GB) | Typical saving |
|---|---|---|
| Top up from about 120mm to 270mm | About £600 | About £17 a year (£20 in Northern Ireland) |
| Insulate from nothing to 270mm | About £750 | Much more: an empty loft is where most of the heat goes |
The part most guides skip: at £17 a year, paying someone £600 for a top-up takes about 35 years to pay back. Doing it yourself, you only pay for the rolls: for a typical 40–50m² loft, that's a couple of hundred pounds at today's prices, which pays back in roughly 11–13 years. Mineral wool lasts around 40 years, so a DIY top-up comes out well ahead, and the house is warmer from the first night. If your loft has less than about 100mm, or none at all, the saving is much bigger and it's worth doing whichever way you do it. If you're on the borderline, check the grants below before you spend anything.
Grants: the government's Warm Homes Plan is bringing fully funded insulation for people on low incomes and low-interest loans for other homeowners. In Scotland ask Home Energy Scotland, in Wales Nest, and in Northern Ireland the NI Energy Advice Service. Check before you buy rolls.
Just need the numbers? The loft insulation calculator works out the rolls for any loft size and existing depth.
2. Check your loft first
- Measure what's there. Push a ruler down through the insulation to the ceiling board in a few places. Joists are usually about 100mm deep, so insulation that's level with the tops of the joists is about 100mm.
- Look for damp. Dark stains on the timber or roof felt, mould, or wet insulation mean there's a leak or a condensation problem. Fix that first, or get advice: insulating over damp makes it worse.
- Check the ventilation. Air needs to flow in at the eaves (where the roof meets the walls). If the old insulation is stuffed into the eaves, pull it back.
- Find the cables and lights. Lift cables up so they'll sit on top of the new insulation, where they stay cool. Don't cover recessed downlights unless they're rated for it: fit fire-rated downlight covers first.
- Lofts that need a professional: flat roofs, lofts used as a room, lofts with damp or structural worries, and anything where the insulation needs to go at the sloping roof instead of the floor.
3. Plan and price your loft
Enter your loft size and what's there now. The planner picks the cheapest combination of standard roll thicknesses to reach your target, prices the rolls at the cheapest UK merchant we've found, and adds raised legs and boards for a storage area if you want one.
Assumes 5% extra for cuts. Roll coverage comes from the actual products priced (it varies a lot between brands: a 100mm roll covers anything from about 8m² to 11m²). Storage boarding assumes 1220 × 320mm chipboard loft panels with 10% extra, on 175mm raised legs at about 4 per m² (on joists at 400mm centres, about every 600mm along each joist). The payback uses the Energy Saving Trust's £17 a year figure for a top-up from about 120mm, so it's only shown when you already have 100–150mm.
4. How the layers go in
- Layer 1, between the joists: 100mm rolls, cut to fit the width of the gaps (many rolls are pre-cut into strips for standard joist spacings). If there's already insulation level with the tops of the joists and it's dry, keep it: that's your first layer.
- Layer 2, across the joists: lay the top-up layer at right angles to the first, butted tightly together with no gaps. This covers the joists, which otherwise act as cold bridges.
- Thickness for the top-up: 170mm over 100mm makes 270mm. Over 150mm, a 170mm layer takes you to 320mm; over 200mm, a 100mm layer is enough.
5. Step by step
- Clear and prepare. Move stored boxes out, put down a temporary board to kneel on, and set up a light. Wear coveralls, gloves, goggles and an FFP2 mask.
- Lift the cables so they'll end up on top of the insulation, and fit covers over any recessed downlights.
- Start at the eaves and work back to the hatch, so you never have to step over finished insulation. Keep 50mm or so of clear air gap at the eaves, or fit eaves trays.
- Lay the first layer between the joists if you need it, pressing it gently into place, not compressed.
- Lay the top layer across the joists, unrolling it in strips, cutting with the insulation knife against a straight board. Butt the edges tightly.
- Leave the area directly under the cold water tank uninsulated, so warmth from the house stops it freezing. Insulate the tank itself (but not underneath it) and lag the pipes.
- Insulate and draught-proof the hatch. Glue a piece of rigid insulation board to the top of the hatch and fit a draught strip around the frame. An uninsulated hatch is a big hole in your insulation.
- Fit the storage area on raised legs if you need one (next section).
6. Keeping storage space without squashing it
The most common loft-insulation mistake is laying boards straight onto the joists over a 270mm blanket. Insulation works by trapping air: squash 270mm down to 100mm and it loses most of its value. It can also cause condensation under the boards.
- Raised loft legs screw onto the tops of the joists and hold the boards above the insulation. 175mm legs on 100mm joists give about 275mm of space, which clears a 270mm layer.
- Board only what you need. A walkway from the hatch and a storage area near it is usually enough. Leave the rest of the loft at full depth.
- Trussed roofs (the W-shaped timbers in most homes built since the 1960s) aren't designed for heavy loads. Keep storage light and close to the walls, and use legs made for trussed roofs.
7. Mistakes to avoid
- Blocking the eaves. No airflow means condensation and damp in the roof timbers.
- Squashing it with boxes or boards on the joists.
- Insulating under the cold water tank, so it freezes in a cold snap.
- Burying cables, which then run hot, or covering downlights that aren't rated for it.
- Leaving gaps between rolls: every gap is a cold spot.
- Forgetting the hatch.
- Insulating a damp loft without fixing the cause first.
- Treading between the joists. Your foot goes through the ceiling. Always kneel or stand on the joists or a board laid across them.
8. Loft insulation checklist
Print this page (the planner is left out of the printout) and tick these off as you go:
- Existing depth measured, loft checked for damp, grants checked
- Rolls, coveralls, masks, gloves, goggles and a light bought
- Boxes cleared, kneeling board and light in place
- Cables lifted, downlight covers fitted
- Eaves kept clear (or eaves trays fitted)
- First layer between the joists (if needed)
- Second layer across the joists, edges butted tight
- Area under the cold water tank left clear; tank and pipes lagged
- Hatch insulated and draught-stripped
- Storage area on raised legs, rest of the loft at full depth
Warmer house? Check each room's radiator with the radiator BTU calculator, and see the boiler size calculator if you're replacing the boiler.