PIR vs Mineral Wool Insulation: Which Should You Use? (UK)
Short answer: use PIR board where space is tight (between rafters, under a floor, on the inside of a solid wall), because it reaches the same U-value in about half the thickness. Use mineral wool where there is room (the loft floor, stud walls, between joists), because it is non-combustible, absorbs sound, fits uneven gaps and usually costs less per m². To take a roof from U 2.0 to the 0.16 target you need 130 mm of PIR, 205 mm of mineral wool slab or 255 mm of loft roll. The figures come from the insulation thickness calculator.
Thickness to reach each target, from an uninsulated element (U 2.0)
Rounded up to the next 5 mm, before the thermal bridging of joists and studs (which adds some):
| Target U-value (Approved Document L) | PIR board | Mineral wool slab or batt (λ 0.035) | Mineral wool loft roll (λ 0.044) |
|---|---|---|---|
| Roof or loft (0.16) | 130 mm | 205 mm | 255 mm |
| Floor, or a new wall (0.18) | 115 mm | 180 mm | 225 mm |
| Existing floor upgraded (0.25) | 80 mm | 125 mm | 155 mm |
| Existing wall, internal or external (0.30) | 65 mm | 100 mm | 125 mm |
From a partly insulated element (U 0.5)
| Target U-value | PIR board | Mineral wool slab or batt (λ 0.035) | Mineral wool loft roll (λ 0.044) |
|---|---|---|---|
| Roof or loft (0.16) | 95 mm | 150 mm | 190 mm |
| Floor, or a new wall (0.18) | 80 mm | 125 mm | 160 mm |
| Existing floor upgraded (0.25) | 45 mm | 70 mm | 90 mm |
| Existing wall, internal or external (0.30) | 30 mm | 50 mm | 60 mm |
λ (lambda) is the conductivity the maker declares, and a lower figure insulates better. PIR boards are usually around 0.022, mineral wool slabs and batts about 0.032 to 0.040 and loft rolls about 0.040 to 0.044. Use the figure printed on your product in the calculator.
Where each one makes sense
| Where | Usual choice | Why |
|---|---|---|
| Loft floor (cold loft) | Mineral wool roll | There is room for the depth. An empty 35 m² loft takes 13 rolls of 100 mm to reach 270 mm (see the loft insulation calculator). |
| Between and under rafters (room in the roof) | PIR board | Rafters are shallow and you need a ventilation gap, so the thinner board is often the only way to reach the target. |
| Solid ground floor, under screed | PIR board | It takes the load of the screed; ordinary mineral wool does not. |
| Suspended timber floor, between joists | Either | Mineral wool supported on netting fills uneven gaps; PIR cut to fit on battens works too. |
| Inside a solid brick wall | PIR insulated plasterboard | Every millimetre comes out of the room: 65 mm of PIR reaches 0.30, against 100 mm of mineral wool slab. |
| Stud partition between rooms | Mineral wool (acoustic) | It is there for sound, not heat. |
How they compare
- Thickness: PIR is roughly half the thickness of loft roll for the same result, which matters wherever space is limited.
- Fire: most mineral wool is non-combustible (Euroclass A1). PIR is combustible; check its classification, and ask building control before using it in the walls of flats or tall buildings.
- Sound: mineral wool absorbs airborne sound; PIR does little.
- Fit: mineral wool squeezes into uneven gaps. PIR must be cut accurately and the gaps filled as the maker says, or heat leaks round the edges.
- Moisture: foil-faced PIR with taped joints controls vapour. Mineral wool needs a separate vapour control layer on the warm side in walls and sloping ceilings, and must not get wet.
- Handling: mineral wool irritates skin and eyes, so wear gloves, long sleeves, goggles and a mask. PIR makes fine dust when cut; a fine-toothed saw keeps it down.
Before you buy
Take the U-value you have from a survey or EPC, check the target for your part of the UK (these are for England), and add some depth for bridging. The insulation thickness guide has more targets, the loft insulation guide counts the rolls, and how to calculate room heat loss shows what the insulation saves. The roof, loft and energy planner puts it in order. Building work on walls, roofs and floors often needs Building Regulations approval: check with building control first.
Frequently asked questions
Is PIR better than mineral wool?
It insulates better per millimetre: PIR at λ 0.022 needs 130 mm to take a U 2.0 roof to 0.16, while loft roll at λ 0.044 needs 255 mm. Mineral wool is non-combustible, absorbs sound, copes with uneven joists and usually costs less per m², so it is better where space is not tight.
How much PIR equals 270 mm of loft insulation?
About 135 mm of PIR at λ 0.022 gives the same thermal resistance as 270 mm of mineral wool roll at λ 0.044, because PIR conducts half as much heat. Check the λ printed on both products.
Is PIR insulation fire resistant?
PIR is a foam plastic and is combustible, though it is formulated to resist ignition; check the fire classification of the product. Most mineral wool is non-combustible (Euroclass A1). For walls of flats and taller buildings the Building Regulations restrict combustible materials, so ask building control.
Which is better for soundproofing?
Mineral wool. Its open fibres absorb sound in a stud wall or between floor joists, which is why acoustic slabs are made from it. PIR does little for airborne sound.
Do I need a vapour barrier with mineral wool?
Usually, on the warm side, in walls and sloping ceilings, because mineral wool lets vapour through. Foil-faced PIR with taped joints acts as a vapour control layer itself. Follow the maker's details, especially for internal wall insulation.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.
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