PIR board is the high-performance option: a rigid foam board that reaches the same U-value as mineral wool in about half the thickness. That makes it the go-to where space is tight, but it costs more. This independent guide explains how it performs, where it is worth it, and what it costs.
By Adam Gough, Founder & Lead Adviser, Insulation Advice Centre
Last reviewed: 18 August 2026. Typical lambda values are published manufacturer and CIBSE figures; check your product's declared value. Costs are Checkatrade (2026). Guide only, not a quote.
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PIR (polyisocyanurate) is a rigid foam insulation board, usually faced with foil on both sides. It is the material you reach for when performance per centimetre matters more than the lowest price. Because its thermal conductivity is low, it delivers a strong U-value in a slim board, which is why it dominates in roofs, floors and tight spaces. This guide explains where PIR earns its higher cost, and where a cheaper material would do the same job.
The key number is thermal conductivity, or lambda. PIR has a typical lambda of around 0.022 W/mK, compared with about 0.040 for mineral wool. A lower lambda means the same thermal resistance in less thickness, so PIR reaches a given U-value in roughly half the depth of mineral wool. In a loft floor with plenty of headroom that thinness is not worth paying for; but on a roof slope, a floor, or an internal wall where every centimetre of space counts, it is exactly what you need. You can compare any build-up with our free U-value calculator, entering your board's declared lambda for accuracy.
Where PIR is usually not worth it: a standard cold-loft floor with room to spare, where cheaper mineral wool reaches the same target for less. As always, the honest rule is best for this application, because of these conditions.
Often yes, but with care. Adding a PIR layer over or between existing insulation can lift performance, for example boarding a loft on raised legs over the existing quilt, or overlaying a floor. The things to get right are avoiding trapped moisture, keeping any required ventilation paths clear, and making sure the construction still manages condensation risk, which for a warm roof means the right build-up and vapour control. Because getting the moisture detail wrong can cause problems, this is a case to take proper advice on rather than simply stacking boards. We can match you with installers who understand the build-up.
It depends on the target U-value for the element and the board's declared lambda, not on a fixed number. As a rough illustration, because PIR is about twice as effective per millimetre as mineral wool, it reaches a typical loft or roof target in roughly half the depth mineral wool would need. The honest way to size it is to work back from the target U-value for your application using the actual product's lambda, which is exactly what our U-value calculator does. Always verify the target against the current Building Regulations for your project rather than assuming a single thickness is compliant everywhere.
PIR costs more per square metre than mineral wool, which is the trade-off for its slim, high performance.
| Item | Typical UK figure | Source |
|---|---|---|
| PIR board, material only | Around £35 per m² | Checkatrade, 2026 |
| Mineral wool quilt, for comparison | Around £10 per m² | Checkatrade, 2026 |
The material premium is often justified where space is tight or where a slim build-up avoids other costs. For the full picture across materials and applications, see our insulation cost guide and insulation comparison. Always compare a few quotes.
Against mineral wool, PIR wins on thinness and loses on price. Against multifoil, PIR is simpler to specify and judge, because it has a clear declared lambda, whereas multifoil must be assessed as a tested build-up rather than by a single value. For the full side-by-side, see our insulation comparison and multifoil guide.
If PIR looks right for your roof, floor or wall, or you are weighing it against a cheaper material, we can help you decide and match you with vetted, accredited installers so you can compare no cost quotes side by side. Tell us about your project in under a minute, or call us free on 0800 4947163.
The questions UK homeowners ask us most about PIR board.
PIR (polyisocyanurate) is a rigid foam insulation board, usually foil-faced on both sides. It has a low thermal conductivity (lambda around 0.022), so it reaches a given U-value in about half the thickness of mineral wool. That makes it ideal for roofs, floors and tight spaces, though it costs more per square metre.
It depends on the job. PIR is about twice as effective per millimetre, so it wins where space is tight, such as a roof slope, floor or internal wall. But it costs more, so for a standard loft floor with depth to spare, mineral wool usually gives the same U-value for less. Choose by the application.
There is no single figure; it depends on the target U-value and the board's declared lambda. Because PIR is roughly twice as effective per millimetre as mineral wool, it reaches a typical loft or roof target in about half the depth. Work back from the target using the actual product's lambda, and verify against the current Building Regulations. Our U-value calculator does the maths.
Often yes, but with care. Adding PIR over or between existing insulation can improve performance, but you must avoid trapping moisture, keep required ventilation clear, and manage condensation risk with the right build-up and vapour control. Getting the moisture detail wrong can cause problems, so take proper advice rather than just stacking boards.
PIR board is around 35 pounds per square metre for the material, compared with about 10 pounds for mineral wool quilt (Checkatrade, 2026). The premium is often justified where space is tight or a slim build-up avoids other costs. Compare a few quotes for your specific job.
We supply and arrange certified SuperFOIL installations, and we will still tell you if PIR is the right board for your job. Tell us about your roof, floor or wall and we will help you weigh PIR against the alternatives, then match you with vetted, accredited installers so you can compare no cost quotes.
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