Understanding PIR Insulation: A Comprehensive Guide (2026 Update)

What PIR insulation actually is

PIR stands for polyisocyanurate, a rigid closed-cell foam developed as a step on from polyurethane (PUR). The chemistry is processed with excess isocyanate, which produces a more thermally stable and more fire-resistant structure than PUR.

A PIR board is a low-density foam core, around 30 kg/m³, with approximately 95% of its cells closed, faced on both sides. Most general purpose boards use a composite aluminium foil facing. Other facings exist for specific jobs: glass tissue to receive flat roof membranes, and kraft paper where the board is bonded to plasterboard. The facing matters more than most buyers realise, and it is covered further down.

Why everyone calls it Celotex

Celotex has genericised. It is used the way Hoover is used for vacuum cleaners: as the name of the category rather than the name of the manufacturer. Ask for Celotex on a trade counter and what you are usually asking for is foil-faced PIR board, whoever made it.

There is an odd asymmetry in this. Kingspan is a name of comparable size in the market, but the trade tends to use "Kingspan" to mean Kingspan and "Celotex" to mean PIR. One is used as a brand, the other as a category.

Product codes are used as nouns

The other layer of shorthand is codes. TP10, GA4000, XR4000, TF70 and K118 all get used as if they were product names in their own right, because on a busy counter they are quicker to say than a full description. Worth knowing: within the Kingspan Therma range, TP10, TF70 and TW55 are widely understood in the trade to be the same board manufactured under different codes for different applications rather than three different specifications. If one is out of stock, the alternative is usually a like-for-like swap, though the specification should always be checked against the job.

The rest of the shorthand

You will also hear PIR called the yellow board, foil board, insulation board, or simply "iso", which is more common in American sources than British ones. All of them mean the same thing. None of them tell you the grade, the facing or the compressive strength, which is why an order placed on shorthand alone is an order worth double-checking.

The Celotex to Soprema naming gap

Saint-Gobain agreed the sale of a majority stake in its UK Celotex PIR business to Soprema in December 2023, and the Celotex brand is being phased out in favour of Soprema branding from January 2026. Product codes are retained and specifications are unchanged, so GA4000 is still GA4000. In practice the trade will keep saying Celotex for years after the labels change, so both names are worth carrying on any order or specification. Read our full breakdown of the Celotex rebrand for what has and has not changed.

"My spec says Celotex. Can I use something else?"

This is the question that lands most often, and the answer is almost always yes, with one condition attached.

Where a specification names Celotex, it is usually describing foil-faced general purpose PIR of a given thickness rather than insisting on that manufacturer. All the major brands manufacture to BS EN 13165, all declare 0.022 W/mK on their general purpose boards and all carry third-party certification. A like-for-like swap between brands at the same thickness and grade will not change the thermal performance of the build-up.

The condition is that you match the grade, not just the thickness. A general purpose board and a flat roof board are both "PIR" and both might be 100mm, but they differ on facing, compressive strength and often lambda. Swapping a tissue-faced flat roof grade for a foil-faced general purpose board because both came up under the same brand search is how membranes fail to bond and how screed cracks over insulation that was never rated for it.

What PIR does well, and what it does not

Two boundaries are worth stating plainly, because a lot of published copy blurs them. Compressive strength is a board property, not a load-bearing figure for the finished build-up, and it should always be checked as part of the full floor, wall or roof construction. U-values are also build-up properties. A 100mm board does not achieve a U-value on its own; a construction does.

The low-emissivity benefit of a foil facing is conditional rather than automatic. How much of it you get depends on the depth of the adjacent air space and whether the construction is a floor, wall or roof, because that determines the direction of heat flow.

What you are paying for with PIR

PIR costs more per square metre than mineral wool, EPS or XPS. Ask why PIR insulation is so expensive on a trade counter and you will usually get a shrug, so here is the actual answer. Four things sit behind the price:

• Manufacturing complexity. The reaction is more energy-intensive and more tightly controlled than blowing EPS, and it needs specialist plant.
• Certification burden. BBA scope, LPCB approval to LPS 1181 Part 1 and FM approval all sit behind the mainstream boards, and testing and maintaining those approvals is a real cost carried in the price.
• Raw materials. Isocyanate and polyol pricing is tied to petrochemical supply, and pentane blowing agents add handling and storage requirements at the plant.
• Logistics. PIR is bulky rather than heavy, so pallets take up disproportionate lorry space and boards need careful handling to avoid crushed edges.

The PIR brands stocked at Insulation UK

One point on ownership that is not obvious from the labels: EcoTherm is a Kingspan Group brand, made at the same Pembridge site as the Therma range. Two of the six names above therefore come from one owner and one factory, which is worth knowing if you are comparing quotes and assuming you are comparing independent suppliers.

Another thing that catches people out, this time on flat roofs: "a Kingspan board" could mean Thermaroof TR24, TR26 or TR27, and those are not interchangeable because they suit different waterproofing systems. The same applies across brands. The grade matters, not just the name on the facing.

Where PIR is used

• Pitched roofs. Between and over rafters, in warm and cold roof build-ups.
• Flat roofs. Warm deck constructions under single ply, built-up felt, mastic asphalt or cold liquid applied systems, using tissue or bitumen-faced grades only.
• Floors. Solid concrete, block and beam and suspended timber, under screed, with or without underfloor heating.
• Timber and steel frame walls. Between studs or as insulating sheathing.
• Masonry cavity walls. Supplied in a smaller 1200 x 450mm format to suit typical wall tie spacings, often with a tongue and groove edge. Traditionally partial fill with a 50mm residual cavity, though full fill boards reducing the air gap to around zero to 10mm have become common over the last decade.
• Internal wall lining. As insulated plasterboard laminate, bonded and fixed in one operation.
• Garden rooms, garage conversions, loft rooms and van conversions. The fibre-free handling and slim build-up are the reasons PIR dominates this end of the market.

Application-specific grades and the trade-off nobody explains

Grades differ on five axes, and compressive strength is only one of them.

• Facing. Foil, glass tissue, mineral-coated fleece, bitumen, plasterboard or OSB. This determines membrane and adhesive compatibility, and it is the most common cause of a wrong order.
• Compressive strength. Broadly 120 to 150 kPa across the mainstream. Floors and trafficked roofs need the higher grades.
• Thermal conductivity. 0.022 W/mK on general purpose foil-faced boards, rising to around 0.027 W/mK on some thinner tissue-faced flat roof grades.
• Format and edge profile. 2400 x 1200mm square edge as standard, 1200 x 1200mm and 1200 x 600mm on flat roof grades, 1200 x 450mm on cavity boards.
• Approvals. BBA scope, LPCB approval to LPS 1181 Part 1 and FM approval, which specifiers and insurers often ask for by name.

Here is the part that is usually reported backwards. Flat roof grades are not weaker than general purpose boards, they are stronger, commonly sitting at the top of that 120 to 150 kPa range. What they give up is thermal efficiency per millimetre. A tissue-faced flat roof board declaring 0.027 W/mK needs noticeably more depth than a 0.022 W/mK general purpose board to reach the same thermal resistance. The specialisation buys compressive strength, membrane compatibility and fire approvals, and the cost is paid in lambda.

One further nuance. Some manufacturers state that thermal conductivity is a property of the material and therefore independent of thickness, so a declared 0.022 W/mK applies at 25mm and at 150mm alike. Others declare lambda in thickness bands, so a single flat roof product might declare three different values across its range. Both statements can be true, because the declared value covers the board as a whole, including facings and edge effects, and how those influence the result varies by product. Read the Declaration of Performance for the specific board rather than assuming the range shares one figure.

Compare the PIR ranges we stock

There is a PIR board for virtually every part of the building envelope, from stud walls and floors to roofs and lofts. The ranges below are grouped by manufacturer so you can compare thickness, thermal resistance, compressive strength and approved applications side by side.

Celotex

EcoTherm

Kingspan

The four Therma boards above are worth reading together. TP10, TF70 and TW55 declare identical thickness ranges, R-values, lambda and compressive strength, which is the evidence behind the trade shorthand: they are the same board under application-specific codes. The three Thermaroof grades are genuinely different products, and TR24 and TR27 pay for their flat roof specialisation in lambda.

Recticel

Unilin (formerly Xtratherm)

PIR against the alternatives

PIR costs more per square metre than mineral wool or EPS but needs less thickness to reach the same thermal resistance. Where depth is free, cheaper materials often make more sense. Where depth is constrained, PIR usually wins. The head-to-heads below take that further.

PIR vs EPS

PIR vs HP+

PIR vs XPS

PIR vs phenolic

What to expect when it arrives

• It is bulky, not heavy. Pallet counts roughly halve as thickness doubles, so a pallet of 25mm boards holds far more sheets than a pallet of 150mm. Deliveries are kerbside on a pallet, so somewhere to put it needs sorting before the lorry turns up. A 2400 x 1200mm board catching the wind overhead will pull you off balance, which is a two-person job on anything but a still day.
• Thicknesses. We stock PIR from 15mm through to 200mm. Thin boards are for lining and thermal upgrade work where depth is at an absolute premium; the heavier thicknesses tend to go into floors, flat roofs and anything working to a current Part L target.
• Cutting. A fine-tooth saw or a sharp blade against a straightedge gives the cleanest edge. Dust is minimal compared with fibrous insulation and there is no skin irritation, though board edges can be sharp so gloves are sensible.
• Who to listen to on site practice. The Insulation Manufacturers Association, known for 40 years as BRUFMA, is the representative body for the UK PIR and PUR industry and publishes free Best Practice Guides covering design and installation. Their guidance is a better citation than any merchant blog, including this one. On cutting, the IMA position is more conservative than common site habit: power tool cutting generates non-hazardous dust but should be kept to a minimum, carried out in well ventilated conditions, with a dust mask to BS EN 149 worn where possible.
• Storage. Store dry, flat and clear of the ground, taking out only what can be installed in a single working period. Keep boards out of direct sunlight, because UV exposure degrades foil facings over a matter of weeks, and away from ignition sources. Handle carefully: crushed edges create gaps that let air move through the insulation layer.
• Offcuts. PIR cannot be recycled through most local waste streams, so cutting patterns are worth planning before the first board goes on the trestles. Small offcuts are useful for filling gaps and detail work rather than going in the skip.

Fire and regulation: where things stand

PIR boards commonly declare Euroclass F for reaction to fire on their Declarations of Performance. On the BS EN 13501-1 scale, A1 is the highest classification and F the lowest, and smoke emission classifications are not provided for products rated E or F overall. Reaction to fire is a material classification and is not the same thing as fire resistance, which belongs to the tested system rather than the board, so the specification for the full construction is what governs.

On height, Soprema limits PIR in external wall constructions to 11m, which is a manufacturer position rather than a statutory ban. In England, regulation 7(2) of the Building Regulations 2010 has prohibited combustible materials in the external walls of relevant buildings with a storey 18m or more above ground level since 2018, extended in 2022 to hotels, hostels and boarding houses at the same threshold, alongside guidance recommending A1 or A2-s1,d0 materials in residential external walls above 11m. Fire safety is devolved, so Wales, Scotland and Northern Ireland each differ, and the Building Safety Regulator consulted on revisions to Approved Document B, including the scope of the ban, through the first half of 2026. Treat any published height limit, including this one, as current rather than settled.

Separately, the Future Homes Standard introduces Approved Document L 2026, targeting carbon emissions from new homes 30 to 35% better than Part L 2021. It comes into force on 24 March 2027 with a transitional period running to 24 March 2028. The commercial effect is more fabric: thicker insulation and wider cavities are already being designed in ahead of the deadline.

Correct at the time of writing, August 2026. Check the current Approved Documents and manufacturer certification before specifying.

Why Insulation UK recommends Recticel

We will start with the part most suppliers skip. On thermal performance, the leading PIR brands are equivalent. Recticel, Kingspan and Sopratherm all manufacture to BS EN 13165 and all declare 0.022 W/mK on their general purpose boards, with comparable compressive strength at equivalent grades. Anyone claiming one of them is thermally better than the others is making a claim you can check on a Declaration of Performance in about ninety seconds.

So the recommendation rests on the things that do differ:

• UK manufacture. Recticel Insulation UK operates from Stoke-on-Trent as part of the Belgium-based Recticel Group, one of the world's largest producers of PIR.
• Circularity. In May 2026 Recticel opened an industrial-scale recycled polyol plant at Wevelgem in Belgium, the first investment of that scale by a PIR manufacturer in a reactor for recycled polyol. It is designed to recycle up to 4,000 tonnes of PIR waste a year, with the recycled polyol expected to deliver a 30 to 50% reduction in CO2 emissions against virgin polyol and to replace around a third of the virgin polyol used in the boards without altering performance. Boards produced using the recycled material will be available from all Recticel plants, including in the UK.
• Technical support. Recticel's team advises on product selection and provides condensation risk analyses, U-value calculations and installation guidance, which is worth having on anything more complicated than a straight loft.
• Range coherence. General purpose, cavity, flat roof and insulated plasterboard boards sit in one range, with tapered schemes through Gradient, so a single specification can be carried across a whole project.
• Third-party recognition. The Recticel Group received an EcoVadis Silver Medal for the first time in early 2026.

If a job specifies a different brand, buy the specified brand. If the choice is open, Recticel is where we would point you, on manufacturing, circularity and support rather than on a thermal advantage that does not exist. Read the full Recticel Eurothane GP range review for the general purpose board most projects start with.

Common mistakes worth avoiding

• Unstaggered board joints, which line up weaknesses through the layer.
• Untaped joints where the design calls for them to be sealed.
• Crushed edges from poor handling, leaving gaps that allow air movement.
• Relying on the foil facing instead of installing a separate vapour control layer.
• The wrong facing for the membrane or adhesive being used, particularly on flat roofs.
• The wrong compressive grade under screed or a trafficked roof.
• Substituting phenolic for PIR, or the reverse, because both came up under a search for the same brand name.

PIR insulation FAQs