What has to be insulated
- Roof slopes to 0.18 W/m²K or better
- Dormer walls and flat roofs
- Gable and party walls where they become room walls
- The new floor, for sound as much as heat
- Any residual cold voids behind dwarf walls
Keeping the headroom
This is where multifoil earns its place in conversions: a PIR-between, multifoil-under build-up reaches the target at 65–80mm below the rafters, against 100mm+ for PIR alone. On a low roof that is the difference between a bedroom and a cupboard. Multifoil vs PIR.
Tell us about the roof and we will show the insulation route that keeps the height.
Cost
Insulation is normally priced inside the conversion. As a standalone element, allow £3,000–£8,000 for a typical semi's roof slopes, dormers and floor.
Ventilation and the membrane
If the roof is being re-covered, a breathable membrane and a warm-roof build-up simplify everything. If the tiles stay, the eaves-to-ridge ventilation must be designed in. Ask the conversion company to show the ventilation strategy, not just the insulation thickness.
Questions homeowners ask
Does a loft conversion need building regulations approval?
- GOV.UK — Room in roof insulation best practice — checked 2 September 2026