Rigid board (PIR) loft insulation
Foil-faced foam boards that insulate roughly twice as well per millimetre as mineral wool. They are used where there isn’t room for deep rolls: between rafters, in loft rooms and on hatches.
| What it is | Rigid foam boards, usually PIR (polyisocyanurate), faced with foil |
|---|---|
| Comes as | Boards, typically 1200 × 2400mm, in many thicknesses |
| Typical lambda | Around 0.022 W/mK |
| Fitted | Cut to fit between and under rafters or joists |
| Fire | Combustible; must be installed to the manufacturer’s and building regulations’ requirements |
| Best for | Warm roofs, loft rooms, shallow spaces and hatches |
Where boards make sense
Rolls are the cheapest way to insulate a loft floor when there is depth to spare. Boards come in when there isn’t: a loft that is being turned into a room and needs insulation in the slope of the roof, a shallow loft where the joists are already near the ceiling of a dormer, or anywhere you want insulation that stays rigid. They are also used to insulate the back of loft hatches.
Cold loft or warm loft?
Insulating the loft floor keeps the roof space cold and the house warm, and is what most people need. Insulating the rafters instead makes the loft part of the heated envelope, a “warm roof”. That is done when the loft is used as a room or for storage you want kept warm. It is a bigger job, needs careful attention to ventilation and vapour control, and in a conversion has to meet building regulations. Boards are the usual material for it.
Things to know
- Boards must be cut accurately and the joints sealed, or warm air sneaks round them
- Ventilation gaps behind the boards, between them and the roof, must be kept where the roof needs them
- They cost more per square metre than rolls, but less depth is needed
Thermal conductivity (the “lambda” value, in W/mK) tells you how well a material resists heat: the lower the number, the less thickness you need. Figures here are typical ranges; the product’s own declaration of performance gives the exact value.
Common questions
Can I use PIR boards on the loft floor?
Is insulating the rafters better than the floor?
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