A loft conversion turns a cold roof void into a heated room, so the insulation moves from the loft floor up into the roof slopes, dormers and knee walls. It's also notifiable building work, so building control or an approved inspector will check what you install. This guide covers what goes where, and which of our products fit each part.
Where the insulation goes
- Sloping roof (rafters): the main area of heat loss. Usually PIR boards between the rafters with a second layer underneath.
- Flat ceiling above the collar ties: can be insulated at ceiling level with loft roll, like a normal loft.
- Dormer cheeks and roof: treated like a small timber-frame wall and a small flat roof.
- Knee walls (the short walls under the slope): insulate either the wall itself, or the roof slope and floor behind it, but keep the insulation continuous.
- The new floor: doesn't need thermal insulation when the room below is heated, but benefits from acoustic insulation, and building control will usually expect it to provide fire resistance.
Insulating between and under the rafters
The most common approach is a two-layer build-up:
- PIR pitched roof boards cut to fit tightly between the rafters, with any gaps sealed using foil tape or low-expansion foam.
- A continuous layer of insulated plasterboard fixed across the underside of the rafters. This covers the timbers, which would otherwise act as cold bridges, and gives you the finished ceiling in the same step.
Whether you need a ventilation gap above the boards depends on the roof underlay. With traditional bitumen felt, a 50mm air gap between the insulation and the felt is the usual rule. With a modern breathable membrane you may be able to use more of the rafter depth; check the membrane manufacturer's guidance and your drawings. If the rafters are shallow, battens fixed below them make room for more insulation without losing too much head height.
Target U-values
In England, building control typically asks for a converted roof to reach between 0.15 and 0.18 W/m²K, depending on how the work is classed under Approved Document L. The thickness you need depends on the board's thermal conductivity, the rafter depth and the under-rafter layer. Send us your drawings or target U-value and we'll work out a build-up.
Dormers
Dormer cheeks are small timber-frame walls. Insulate between the studs with timber frame boards, add a vapour control layer on the warm side and line with insulated plasterboard. Flat dormer roofs are best built as a warm roof, with PIR boards above the deck so the timber stays warm and dry.
The new floor: sound and fire
New floor joists are usually laid alongside or above the old ceiling joists. Filling the joist void with acoustic partition roll or mineral wool roll cuts noise between floors, and fire-rated plasterboard on the ceiling below helps meet the fire requirements building control sets for the conversion. For the floor deck, see our OSB vs plywood guide.
Typical materials list
- PIR pitched roof boards for between the rafters
- Insulated plasterboard for under the rafters and on knee walls
- Foil tape and a vapour control layer (see membranes)
- Timber frame boards for dormer cheeks
- Acoustic roll or loft roll for the floor void
- Fire-rated plasterboard for the ceiling below and around the new stair
- Fixings, tapes and beads
Common mistakes
- Leaving gaps between boards and rafters. Even small gaps undo a lot of the insulation's performance.
- Not covering the rafters, which leaves cold stripes that can show up later as lines of condensation.
- Blocking ventilation at the eaves when the roof design needs it.
- Ordering too late. A full conversion uses a lot of board, so check stock before the roof is opened up.
Loft conversions near you
We deliver loft conversion packages across most of England. See our local pages for Birmingham and the West Midlands, where interwar semis are often converted, Essex, with its chalet bungalows, and London.
Send your drawings or room dimensions to sales@nextdayinsulationltd.co.uk and we'll put together a full materials list and price. Delivery is free on orders over £600 ex VAT.