PIR vs mineral wool insulation: which to use where

Quick answer: PIR board at 0.022 W/mK needs about half the thickness of mineral wool at 0.044 for the same result, and costs roughly six times as much per square metre. Use PIR where depth is fixed, such as between rafters. Use mineral wool everywhere else, especially where fire matters, because it’s Euroclass A1 and PIR is Euroclass F.

The thickness trade, in millimetres

Strip out the marketing and this comes down to one sum. Thermal resistance is thickness divided by lambda. To hit a given U-value you need a given resistance, so the lower the lambda the thinner the layer. We haven’t tested any of these products; the figures below are published data sheets and merchant pricing, checked 27 July 2026.

So a PIR board saves you 110mm of depth over a glass wool roll and costs about six times as much per square metre. That’s the whole decision. If the 110mm is free, mineral wool wins on cost by a distance. If the 110mm costs you headroom, floor area or a rafter you can’t deepen, board wins regardless of price.

Cost, done properly

Compare at equal resistance, not equal thickness. At R = 5.0, 110mm of GA4000 costs roughly £15 per square metre while 220mm of Knauf loft roll costs roughly £5. Board is still around three times dearer once you correct for thickness, which is a smaller gap than the headline suggests but still decisive in an open loft.

The gap closes further on labour. Rolls go down fast in a clear loft. Cutting board to fit between irregular rafters is slow, wasteful and unforgiving, and every gap you leave is a bypass that undoes the lambda advantage. Our guide to U-values and insulation performance covers what gaps and thermal bridging do to the finished number.

Fire, where the two aren’t close

Mineral wool, both glass and stone, is Euroclass A1 to BS EN 13501-1. That’s the top of the scale, and it means no contribution to fire. PIR boards such as Sopratherm GA4000 and Kingspan Thermapitch TP10 are Euroclass F, the bottom. Phenolic insulated plasterboard such as Kingspan Kooltherm K118 achieves B-s1,d0, which is much better but still combustible.

In a house this drives three practical rules. PIR needs a lining, normally 12.5mm plasterboard, between it and the room. PIR shouldn’t be left exposed in a loft, garage or plant space. And in the external wall of a relevant building 18 metres or higher in England, Euroclass F is excluded outright under regulation 7(2). Soprema goes further than the regulation and tells specifiers to keep its PIR out of external walls above 11 metres.

None of that makes PIR unsafe in a bungalow floor. It does mean that if you’re choosing between two materials that both work, the A1 one carries less risk. More detail in insulation fire safety for UK homeowners.

Moisture

PIR is closed cell and foil faced, so it’s close to vapour tight. Mineral wool is vapour open. Neither is better in the abstract. They’re different design approaches, and mixing them up causes damp.

Put PIR in a wall with a masonry outer leaf and a foil face on the warm side and you have a vapour control layer built in. That works. Put PIR in a solid stone wall with no cavity and you’ve created a cold face behind an impermeable board. Interstitial condensation is a real risk. Mineral wool in the same wall lets moisture move, which is why breathable build-ups on old buildings use wool or wood fibre instead of foam.

Rigid board also can’t follow an irregular surface. Old joists aren’t parallel and old rafters aren’t straight. Wool squashes into the gap. Board doesn’t, and the air gap you leave behind it can convect. That’s a bigger real-world penalty than the lambda difference in a lot of retrofits.

Sound

Not a contest. Mineral wool absorbs airborne sound, PIR reflects it. Rockwool RWA45 at 45 kg/m³ is the standard fill for an acoustic partition. Nobody specifies PIR to quieten a room, and a stud wall packed with foam board will perform worse acoustically than the same wall with a cheap wool batt.

So which one

• Cold loft at ceiling level, headroom available: mineral wool rolls, 270mm, two layers crossed.
• Between rafters in a loft conversion where depth is fixed: PIR, or phenolic if the rafters are shallow.
• Solid floor under screed: PIR, because it takes compression and wool doesn’t.
• Suspended timber floor: mineral wool between the joists on netting, because it fits irregular spacings.
• Stud partitions and party walls: mineral wool, denser the better.
• Solid stone or brick walls being lined internally: wool or wood fibre on a breathable build-up, not foil-faced foam.

If you take one number away, take this one. At equal thermal resistance PIR costs about three times what mineral wool costs, and it takes longer to fit. Pay that when the millimetres are worth more to you than the money. Everywhere else, buy wool and buy more of it.

For the fuller material comparison including EPS, see mineral wool, PIR and EPS compared, and phenolic foam boards for the step up from PIR. Whole-job costs are modelled in the calculator and the insulation product guide.

FAQs

Is PIR twice as good as mineral wool?

Per millimetre, yes, near enough. PIR at 0.022 W/mK against glass wool at 0.044 is exactly double. Per pound it’s the other way round: at equal thermal resistance, board costs roughly three times more, and that’s before the extra labour of cutting it in.

Can I mix PIR and mineral wool in the same construction?

Yes, and it’s common between rafters, with board below the rafters and wool between them. Get the vapour control right: the foil face of the board becomes the vapour control layer, so it belongs on the warm side with the wool behind it.

Which is better for a loft conversion?

PIR or phenolic, because rafter depth is fixed and you need the resistance in the space available. Mineral wool at 0.044 W/mK can’t reach a modern U-value in 150mm of rafter, but a 100mm PIR board plus insulated plasterboard can.