Celotex PIR boards

Who makes Celotex PIR insulation boards

Celotex is a British PIR manufacturer with a long history. The name began life in London in 1925 as the UK arm of an American insulation business, and the company has been based in Hadleigh, near Ipswich in Suffolk, since 1980. Hadleigh remains its main factory, with a second manufacturing site at Eggborough in Yorkshire, and Celotex PIR insulation boards are stocked by builders merchants across England, Scotland and Wales.

Ownership has changed twice in recent years. Saint-Gobain, the French building materials group, bought Celotex in 2012. In December 2023 Saint-Gobain agreed to sell a 75 percent stake in the business to SOPREMA, and the deal completed in early 2024 with Saint-Gobain keeping a 25 percent holding. SOPREMA is a family-owned French group founded in Strasbourg in 1908 and operating in more than 90 countries. From 1 January 2026 SOPREMA UK has been bringing the Celotex name under the SOPREMA brand, so boards leaving Hadleigh now carry either the Celotex or the SOPREMA name on the foil while the product, the factory and the specification stay the same.

Celotex boards are manufactured under ISO 9001:2015 quality management and ISO 14001:2015 environmental management systems, and the company's PIR insulation has long held British Board of Agrement certification for its roof, wall and floor applications, so the test data behind the 0.022 W/mK figure has been independently checked.

The general purpose Celotex PIR board

The board PIR Boards supplies is the standard general purpose rigid PIR insulation board, 2400 x 1200mm, 2.88 square metres per sheet, with a low emissivity aluminium foil facing bonded to both sides of a rigid polyisocyanurate foam core. It has the same specification as the other brands we stock: a declared thermal conductivity of 0.022 W/mK at every thickness from 20mm to 200mm, straight edges for butt jointing, and a foil face that doubles as a vapour control layer once the joints are taped.

PIR earns its place by doing more insulating per millimetre than almost any other mainstream material. Compared with mineral wool at around 0.035 to 0.044 W/mK, a PIR board of a given thickness gives roughly 60 to 100 percent more thermal resistance, so it fits inside rafter depths, floor voids and stud walls where fibre would need to be far thicker.

Where Celotex PIR insulation boards are used

Celotex PIR insulation boards are used throughout the building envelope, and the thickness is chosen to suit the element and the target U-value. Pitched roofs are the classic application: 100mm to 140mm between the rafters with a ventilation gap left behind, then 25mm to 50mm across the underside of the rafters to deal with the cold bridge through the timber. Loft conversions follow the same approach, and the thin boards make it possible to insulate a dormer or sloping ceiling without losing headroom.

Flat roofs take a single thick layer, typically 120mm to 150mm, laid over a vapour control layer on the deck in a warm roof build-up. Ground floors use 100mm to 150mm over the slab or between the joists, with an upstand around the perimeter before the screed or chipboard deck goes down. Timber frame walls take 50mm to 100mm between the studs, often with a further thin layer across the face, and masonry walls are lined internally with PIR board behind a batten and plasterboard finish. Extensions and garden rooms usually need all three.

As a guide: 20mm to 40mm for reveals, dormer cheeks, thermal breaks and the second layer in roofs; 50mm to 75mm for stud walls and internal lining; 100mm to 150mm for floors, flat roofs and between rafters; 150mm to 200mm where one layer has to achieve the U-value by itself.

How PIR Boards supplies Celotex PIR insulation boards

PIR Boards sells by thickness, not by brand. Celotex is one of the manufacturers we supply, and every brand we stock makes its general purpose board to the same specification: 2400 x 1200mm, foil faced on both sides, 0.022 W/mK. When your order comes in, we dispatch the brand that is best stocked at the depot serving your postcode that day. Sometimes that is Celotex and sometimes it is another brand with identical declared performance.

We work this way because it is better for the buyer. The U-value of a 100mm board at 0.022 W/mK does not change with the logo on it, so your calculations, your building control submission and your fixing details all stand. What does change is the price and the lead time. By drawing on several manufacturers, PIR Boards keeps trade prices down, holds the less common thicknesses in stock and can promise delivery in 3 working days on average.

If a drawing names Celotex PIR insulation boards, the specifier almost always means the general purpose board at this specification, and an equal-performance board is normally accepted. Where a contract requires a particular brand by name, speak to us before you order. We would rather say so in advance than send you a board you cannot use.

How many boards to order

Each 2400 x 1200 board covers 2.88 square metres. Take the area of the element, divide by 2.88, round up and add around 10 percent for cutting waste. A 5m x 6m floor is 30 square metres, which is 10.4 boards, so 11, and adding waste gives 12 boards. For between-rafter and between-stud work allow 15 percent, because the boards are ripped into strips and the leftover from each sheet is seldom usable as a full strip.

Where a roof or wall uses two layers, work out each layer on its own and add them together. PIR offcuts are rarely wasted: they close the ends of cavities, pack out around steels and lintels, and make thermal breaks behind radiators and at window reveals.

Cutting, fitting and taping Celotex PIR board

PIR board cuts with ordinary hand tools. Up to 50mm a sharp knife scored deeply on both faces and snapped over a straight edge will do; above that a fine-toothed handsaw, insulation saw or circular saw gives a straighter edge. Measure between rafters or studs at several points and cut each strip a few millimetres over so it pushes in tight. Fill any remaining gaps with expanding foam and trim it flush once cured.

Lay sheets across a floor or under rafters in a staggered pattern so no four corners meet, butt the edges tightly and fix under-rafter boards through to the timber before the plasterboard goes on. Then tape every joint on the warm side with a foil tape of at least 75mm width, pressed firmly onto clean dry foil. This turns the facing of each board into one continuous vapour control layer and is the single most important detail for avoiding condensation inside the roof or wall.

Why the 0.022 W/mK figure matters

Thermal conductivity, often called the lambda value, is the rate at which heat passes through a material. The lower the figure, the better. In a U-value calculation each layer's thickness in metres is divided by its conductivity to give its thermal resistance, the resistances of all the layers are added, and the total is inverted to give the U-value in W/m2K. A 100mm board at 0.022 W/mK adds about 4.5 m2K/W of resistance; at 0.025 W/mK it would add 4.0, which can be the difference between meeting and missing a Building Regulations target.

Every brand PIR Boards supplies, Celotex among them, declares 0.022 W/mK on its general purpose board. That is what allows us to sell by thickness and what allows you to treat the brands as interchangeable in a calculation.

Storage, handling and delivery

Keep PIR insulation boards flat, under cover and off the ground, on bearers close enough together to stop the pack sagging. Leave the wrapping on until you need the boards and cover opened packs against rain. The foam does not rot and a shower will not ruin it, but boards that have stood in water need time to dry before they are fitted and taped.

PIR Boards delivers Celotex PIR insulation boards and the other brands we supply across the UK mainland. Order before 2:30pm on a working day for delivery in 3 working days on average. Dates can vary by area and availability, and we will let you know if your postcode or thickness needs longer. Delivery is free on UK mainland orders over £450 + VAT. Arrange access for a lorry and someone to check the quantity and thickness as the boards come off.