The three positions
| Position | Material | Pros | Cons |
|---|---|---|---|
| Between rafters | PIR, mineral wool | Uses rafter depth; no loss of headroom | Rafters bridge the insulation; gap needed to the felt |
| Under rafters | Multifoil, PIR | Continuous layer; closes bridges; vapour control | Loses headroom |
| Over rafters | PIR, warm roof | Best performance; no internal work | Only when re-roofing |
The standard from-inside build-up
50–75mm PIR between rafters leaving a 25–50mm gap to the felt, then a certified multifoil or 25–50mm PIR under the rafters, taped continuous, then a service batten and plasterboard. Achieves 0.18 W/m²K in most rafter depths. Foil roof insulation walks through the multifoil version.
What will fit in your rafters?
Check my homeTell us the roof and the room and we will show the build-up.
Cost
£45–£110 per m² of slope from inside, depending on materials and finish. See foil insulation cost and insulation costs.
Questions homeowners ask
How much rafter depth do I need?
Typical rafters are 100–150mm. With 50–75mm PIR between and a gap to the felt, plus a layer under, most reach 0.18 W/m²K. Shallower rafters push toward multifoil under.
Sources and when they were last checked
- GOV.UK — Room in roof insulation best practice — checked 2 September 2026
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026