PIR Insulation Board 75mm: The Combination Thickness That Fills the Gaps

Specify 100mm PIR for your walls when 75mm would have passed building control, and you've overspent by 30% on insulation for no measurable benefit. Specify 75mm for your floor slab when it needed 100mm, and you'll fail the building control inspection and have to rip up the screed. 75mm PIR is the thickness that sits between the obvious choices, and getting it right means understanding exactly where the regulations draw the line.

What it is and what it's for

PIR (polyisocyanurate) is the rigid foam insulation board with aluminium foil on both faces. If you've read the 50mm PIR page, you already know the basics: closed-cell foam core, lambda value of 0.022 W/mK, foil faces that act as a vapour barrier when you tape every joint. The material is identical. What changes at 75mm is where it fits in your build.

At 75mm thick, a PIR board delivers an R-value (thermal resistance) of 3.40 m2K/W. That's 51% more thermal resistance than a 50mm board. The standard board is 2400 x 1200mm (2.88 m2), weighing around 8 kg. Still comfortable for one person to carry, but noticeably heavier than the 4.5 kg 50mm boards.

The 75mm thickness exists because building regulations don't have a single number for insulation. Approved Document L (2021, with 2023 amendments) sets different U-value targets for different building elements. And 75mm PIR passes some of those targets comfortably, misses others completely, and works brilliantly as part of a combination system for the rest.

That table tells you where 75mm belongs. It's the right thickness for walls in most domestic extensions, where it exceeds the requirement with room to spare. It fails for floors and roofs when used alone, but 75mm boards are commonly combined with other thicknesses to build up to the required U-value in those applications.

Why 75mm instead of 50mm or 100mm

This is the question most insulation guides dodge. Here's when 75mm is the right choice.

Cavity walls where 50mm leaves too little margin. A 50mm partial-fill board achieves around 0.22 W/m2K against the 0.26 wall limit. That passes, but your building control officer might ask whether the calculation accounts for thermal bridging through wall ties and mortar joints. At 75mm, you hit 0.17 W/m2K. Nobody questions that figure. The extra 25mm buys you compliance certainty, and the cost difference per board is around £5-7.

Timber frame walls between 75mm studs. Standard 75mm (3-inch) timber studs are common in internal partition and timber frame construction. The PIR sits flush between the studs, filling the cavity exactly. Add a 25-50mm continuous layer over the stud faces to tackle thermal bridging (heat escaping through the timber itself, which conducts roughly five to six times more heat than PIR), and your wall build-up hits 0.15-0.18 W/m2K.

As the supplementary layer in roof builds. Between-rafter insulation alone rarely achieves the 0.16 W/m2K roof target because the timber rafters bridge the insulation. The common solution is 100mm between rafters plus 50-75mm under rafters as a continuous layer. Using 75mm under the rafters instead of 50mm pushes the overall U-value down to around 0.13 W/m2K, giving margin for real-world imperfections.

Floor build-ups where height is constrained. In renovations where raising the finished floor level is limited, 75mm PIR under screed achieves around 0.23 W/m2K. That fails the 0.18 notional target but may pass under area-weighted SAP calculations (a method where better-than-required performance in one element compensates for shortfalls in another). Your architect or SAP assessor decides whether this trade-off works. For new-build extensions without height constraints, 100mm is the standard floor choice.

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Types, sizes, and specifications

Every mainstream 75mm PIR board shares the same core performance. Lambda 0.022 W/mK, R-value 3.40 m2K/W, compressive strength 120-140 kPa, standard size 2400 x 1200mm. The brands are functionally interchangeable for domestic use. Forum consensus is blunt: "It's all basically the same stuff."

Celotex GA4075 is the default choice. Widest merchant distribution, widest thickness range, and consistently the best value from online specialists. Kingspan TW55 has marginally higher compressive strength (140-180 kPa depending on the production batch) but costs 35-40% more for the same thermal performance. The Kingspan TP10, sold at Wickes for around £45 per board, is a roof-specific product with enhanced specifications that don't matter for general wall or floor use.

Recticel typically comes in 5-10% below Celotex. Ecotherm sits in the same range. Both are perfectly good boards that meet the same BS EN 13165 standard.

Fire classification

PIR boards carry a Euroclass E or F fire rating (or the older BS 476-7 Class 1, which broadly maps to the same performance). These are combustible materials. The December 2022 amendments to Approved Document B (fire safety) ban combustible insulation in external walls of buildings over 11 metres. Your domestic extension or loft conversion sits well below that threshold. PIR is fine for every typical house project.

The concern about PIR and fire comes from high-rise cladding headlines. Different world. Your single-storey extension is not a tower block. PIR must be covered by plasterboard or another non-combustible lining, and it must never be left exposed on site near heat sources or hot work.

How to work with it

The 50mm PIR page covers the fundamentals of cutting, fitting, taping joints, and installation methods in detail. Everything there applies equally to 75mm boards. Rather than repeat it, here's what changes at this thickness.

Cutting

A standard handsaw cuts 75mm PIR cleanly. Unlike 50mm boards, you can't score-and-snap 75mm with a utility knife (the blade doesn't penetrate deep enough for a clean break). Mark your cut line on the foil face, saw through, and clean up the edge with a surform if needed.

Cut boards 5mm oversize for friction fits between studs and rafters. A PIR board that's 3mm too small with air gaps on both sides defeats the purpose of rigid insulation.

Fitting between timber studs

This is where 75mm boards earn their place. Standard 75mm (3-inch) timber studs in internal walls and some timber frame constructions give you a cavity that's exactly the right depth for a 75mm PIR board. The board sits flush with the stud faces on both sides.

But fitting between studs only leaves a problem. Timber studs conduct heat roughly five to six times faster than PIR insulation. Every stud is a thermal bridge, a stripe of heat loss running floor to ceiling. The fix is a continuous 25-50mm PIR layer fixed over the stud faces before the plasterboard goes on. This layer crosses every stud, eliminating the thermal bridging. Stagger the joints between layers (like brickwork bonding, never align joints in a straight line).

Counter-battens (timber strips screwed through the over-stud layer into the studs) give you a fixing surface for the plasterboard and create a service void for running cables without piercing the insulation.

Fitting in roof builds

When 75mm PIR serves as the under-rafter continuous layer (below the between-rafter insulation), it's fixed to the rafter faces using counter-battens screwed through the board into the rafters. The plasterboard fixes to those counter-battens.

Remember the ventilation gap above the between-rafter insulation. With a breathable membrane (Tyvek or similar), you need 25mm. With traditional bituminous felt, you need 50mm. Check your membrane before ordering boards, because this gap determines how much rafter depth is available for insulation.

Joints: the non-negotiable

Every joint between boards must be sealed with aluminium foil tape. Board-to-board joints, board-to-timber junctions, board-to-masonry edges. Untaped joints allow warm moist air to pass through, causing condensation behind the insulation and reducing thermal performance by 20-30%. A roll of proper foil tape costs under £10. There is no argument for skipping it.

Use genuine aluminium foil tape (Celotex, Kingspan, or any builders' merchant equivalent). Gaffer tape, duct tape, and general-purpose polymer tapes degrade over time and don't provide vapour resistance.

How much do you need

Each 2400 x 1200mm board covers 2.88 m2. The calculation is the same as for any PIR thickness:

1. Measure the total area to insulate (wall, floor, or roof) in square metres
2. Divide by 2.88
3. Add 10% for waste, cuts around openings, and fitting tolerances
4. Round up to whole boards

Worked example: timber frame internal walls for a 4m x 6m extension

Two side walls at 4m x 2.4m = 19.2 m2, one rear wall at 6m x 2.4m = 14.4 m2. Total = 33.6 m2. Subtract openings (bifold doors and windows, roughly 6 m2) = 27.6 m2 net.

For the between-stud layer: 27.6 / 2.88 = 9.6 boards. Add 10% = 10.6. Order 11 boards.

If adding a 25mm over-stud layer, the area is the same (27.6 m2) but the boards are thinner and cheaper. You could use 25mm PIR or insulated plasterboard for this layer.

At £22-28 per board from an online specialist, 11 boards of 75mm PIR costs roughly £240-310 for the primary insulation layer.

For combination roof builds (100mm between rafters + 75mm under rafters):

A 20 m2 roof area needs roughly 7 boards of 100mm between rafters and 7 boards of 75mm under rafters (both at 2.88 m2 coverage, plus 10% waste). The 75mm under-rafter layer for this example runs approximately £155-195 from an online specialist.

Cost and where to buy

The price difference between buying channels is stark. The same Celotex GA4075 board that costs £22-28 from an online insulation specialist can cost £45+ from Wickes (where they stock the premium Kingspan TP10 instead of the general-purpose GA4075) and potentially more from a walk-in counter at a general builders' merchant.

A typical extension needs 15-25 boards of 75mm PIR across different applications. At £22-28 versus £45 per board, the total difference is £255-575. Order from an online specialist. Delivery is typically free above a modest threshold, and the boards arrive on a pallet direct to site.

Don't forget accessories. Budget £30-50 for foil tape (you'll use 2-3 rolls at £7-10 each for a full extension), mechanical fixings if fitting to masonry, and any counter-battens for over-stud or under-rafter layers.

Factory seconds with cosmetic damage (dented corners, scuffed foil) appear regularly from specialist outlets and on eBay at 50-70% of full price. The thermal performance is identical. Behind plasterboard, nobody sees the dent.

Alternatives

PIR 50mm is enough for cavity walls (achieves 0.22 W/m2K against the 0.26 limit). If your build is a standard masonry cavity wall extension and you don't need extra margin, 50mm saves £5-7 per board. See the 50mm PIR page for full details.

PIR 100mm is the standard choice for ground floors (achieves ~0.18 W/m2K, meeting the notional target). If your floor specification calls for insulation, start at 100mm unless height is genuinely constrained. For roof builds, 100mm between rafters is the standard first layer.

Mineral wool 100mm provides roughly the same thermal resistance as 40mm of PIR. It's bulkier and less space-efficient but non-combustible (Euroclass A1), better for acoustic insulation, and easier to fit around irregular shapes and pipes. Mineral wool is the go-to between joists in lofts where space isn't limited and fire performance matters.

Insulated plasterboard (Celotex PL4000, Kingspan Kooltherm K118) bonds a PIR layer to plasterboard in a single product. Useful as the over-stud or under-rafter finishing layer, saving a separate operation. More expensive per m2 than buying the two components separately, and you're locked into the manufacturer's thickness combinations.

EPS (expanded polystyrene) costs 30-40% less than PIR but needs much thicker boards for equivalent thermal performance (lambda 0.032-0.038 W/mK versus PIR's 0.022). EPS has its place in floor insulation where depth isn't constrained, but where cavity depth or rafter depth is limited, PIR wins.

Where you'll need this

• Insulation - wall, floor, and roof insulation specification and installation

These applications appear across all stages of any extension or renovation project where thermal performance must meet Part L requirements. PIR boards are used in every project type, not just kitchen extensions.

Common mistakes

Assuming 75mm is enough for everything. The most common error. 75mm PIR comfortably passes the wall U-value requirement (0.17 against a 0.26 limit). It does not pass for floors (0.23 against 0.18) or roofs (0.25 against 0.16) as a standalone layer. If your specification says "75mm PIR" for a floor or roof without describing additional layers in the build-up, question it before the boards go in. Failing a building control inspection after the screed is poured means demolition.

Over-specifying walls. The flip side. Using 100mm PIR in cavity walls that only need 75mm wastes money and reduces your cavity width unnecessarily. The residual cavity (the air gap between insulation and outer leaf) should be at least 50mm in severe exposure zones to prevent rain tracking to the inner leaf. Thicker insulation in the same cavity leaves a thinner residual gap.

Ignoring thermal bridging through studs. Fitting 75mm PIR between 75mm studs and calling it done leaves serious heat loss through the timber. Every stud is a cold stripe. Add a continuous 25-50mm PIR layer over the stud faces. Building control inspectors increasingly look for evidence of thermal bridging mitigation, and your SAP calculation probably assumes you've dealt with it.

Using roof-specific boards for walls. Kingspan TP10 is a pitched-roof product with enhanced specifications and premium pricing (around £45 per board at Wickes). For wall insulation, the general-purpose GA4075 or TW55 at £22-28 per board delivers identical thermal performance. Don't pay the roof premium for a wall application.

Skipping the foil tape. Covered in detail on the 50mm PIR page, and addressed in the Joints section above: untaped joints reduce effective performance and create condensation pathways that cause damp, mould, and timber rot. Aluminium foil tape. Every joint. No exceptions.

Storing boards in direct sunlight. UV degrades the foil facing over weeks of exposure. Keep boards covered or indoors until installation. A few days of site exposure won't cause problems, but boards left unwrapped on a sunny scaffold for a fortnight will have compromised foil surfaces.