Room-in-roof insulation

The five surfaces

1. Sloping ceilings — between and under the rafters, with the ventilation gap kept.
2. Dwarf (knee) walls — the short vertical walls; insulate the wall itself, or the slope behind it to the eaves.
3. Flat ceiling — the small loft above the room; insulate on its floor like a normal loft.
4. Gable walls — where the room has an exposed external wall.
5. Floor at the eaves — the residual void behind the dwarf walls.

Government best-practice guidance for room-in-roof insulation sets out these elements and the moisture risks in each. Any installer quoting the sloping ceilings alone is quoting a fifth of the job.

Materials

Sloping ceilings: PIR between rafters plus multifoil or PIR under; multifoil where headroom is tight. Dwarf walls: PIR or mineral wool between studs with a vapour control layer on the room side. Flat ceiling: mineral wool. The eaves void behind the dwarf walls must stay ventilated.

Cost and funding

£2,500–£7,000 for a typical semi's loft room, more if the room is stripped back to the rafters and re-finished. Room-in-roof insulation is a recognised measure under ECO4 and can be offered under Warm Homes: Local Grant for eligible households. Current routes.

Questions homeowners ask