The short answer
If your loft floor has less than about 150mm of insulation, top it up to 270mm of mineral wool: one layer between the joists, a second across them. It costs £450–£1,600 installed for a typical semi, or a few hundred pounds in materials if you do it yourself, and it is the measure most likely to be fully funded if you are eligible. If the loft is already at 270mm, more loft insulation is not your best next spend — walls or floors are.
Which type
| Type | Where | Cost | Notes |
|---|---|---|---|
| Mineral wool roll (glass or rock) | Loft floor | Lowest | The standard. Itchy to fit; wear a mask |
| Sheep's wool / natural fibre | Loft floor | Higher | Pleasant to handle, breathable, dearer |
| Blown mineral or cellulose fibre | Loft floor, awkward lofts | Low–mid | Installer only; fills gaps well |
| PIR board | Under boarding, rafters | Mid | Thin; rigid; more cutting |
| Multifoil | Rafters; over joists under boarding | Mid–high | Not for open floors. Why |
Floor or rafters?
Insulating the floor keeps the loft cold and the house warm — cheapest and standard. Insulating the rafters makes the loft itself warm, which you want if it is a room, a future conversion, or has tanks and pipes that freeze. The two are not the same job and should not be quoted as if they were. Roof insulation covers the rafter route.
Answer six questions and we will show the likely route and an indicative cost before asking who you are.
The details that get missed
- Ventilation. Keep the eaves clear so air moves across the loft. Blocked eaves plus new insulation equals condensation on the felt.
- Tanks and pipes. Insulate pipes and the tank sides; do not insulate under the tank.
- Downlights. Fire-rated covers or a gap; never bury an uncovered downlight.
- The hatch. Insulate the hatch and draughtproof it. A bare hatch is a chimney.
- Boarding. Boards laid on joists crush the wool. Use raised loft legs. Loft boarding.
Cost and funding
Installed: £450–£1,600 for a typical semi, more with removal of old material or tank work. See loft insulation cost. Loft top-ups are a core measure under Warm Homes: Local Grant and ECO4 for eligible households. See free loft insulation — who may qualify.
Loft guides
- How deep should loft insulation be?
- Insulation and boarding
- Removing old loft insulation
- Loft insulation installers
Questions homeowners ask
How do I know how much loft insulation I have?
Can I put new insulation over old?
Does loft insulation cause condensation?
Is loft insulation worth it if I already have some?
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026
- Checkatrade — Loft insulation cost guide — checked 2 September 2026
- MyBuilder — Loft insulation cost guide — checked 2 September 2026