PIR Insulation Board 100mm: The Standard Thickness for Extension Floors and How to Get It Right

Order 75mm PIR boards for your extension floor because they were cheaper, and you've just guaranteed a building control failure. The floor U-value comes in at 0.22 W/m2K against a limit of 0.18. Your builder will need to rip them out and replace them with 100mm, wasting both the original boards and a day's labour. The 100mm board is the single most commonly specified PIR thickness for extension floors in England and Wales because it's the thinnest that reliably passes Part L 2021. Get the thickness right and the installation details right, and this is one of the simpler tasks in an extension build.

What it is and what it's for

PIR (polyisocyanurate) insulation is a rigid foam board with aluminium foil laminated to both faces. The foam core contains gas trapped in millions of closed cells, giving PIR the best thermal performance per millimetre of any mainstream insulation you can buy. At 100mm thick, a standard PIR board delivers a lambda value (thermal conductivity) of 0.022 W/mK and an R-value (thermal resistance) of 4.55 m2K/W.

Why 100mm specifically? Because of the maths. In a standard solid floor build-up (concrete slab, DPM, PIR, screed), 100mm of PIR at 0.022 W/mK achieves a U-value of approximately 0.17 W/m2K. The Part L 2021 limit for extension floors in England and Wales is 0.18 W/m2K. So 100mm passes with a small margin. Drop to 75mm and the U-value jumps to around 0.22, which fails. That's why 100mm is the default on virtually every extension floor specification in the country.

The standard board size is 2400 x 1200mm, covering 2.88 m2 per board. A Celotex GA4100 100mm board weighs approximately 12 kg; Kingspan boards in the same thickness run 9–10 kg depending on grade. Either way, one person can handle them but you won't be carrying a stack overhead.

All PIR boards comply with BS EN 13165 (the European standard for PIR insulation products). The foil facing serves two purposes beyond protecting the foam during handling: it acts as a low-emissivity radiant barrier (reflecting heat back into the building), and it provides high vapour resistance, making each board part of your vapour control strategy. But only if you tape every joint. That detail matters more than the brand on the wrapper.

Types, sizes, and specifications

Every major PIR manufacturer sells a 100mm board with identical core performance: lambda 0.022 W/mK, R-value 4.55 m2K/W. The differences between brands are price, availability, and application-specific product naming.

The Kingspan naming trap

Kingspan sells what is essentially the same 100mm PIR board under four different product codes: TF70 (floors), TW55 (walls), TP10 (pitched roofs), TR26 (flat roofs). The lambda value is identical across all four. The compressive strength varies slightly, and the BBA certification covers different applications. If you search "Kingspan 100mm" online, you'll get whichever variant the retailer stocks, which might not be the right one for your floor.

For floor applications, you need the TF70 (ThermaFloor). It's rated at 140 kPa compressive strength, compared to 120 kPa for the general-purpose boards. Under a 65mm screed with domestic foot traffic, the 120 kPa boards are technically adequate (120 kPa translates to roughly 12 tonnes per square metre), but building control inspectors in some areas will question non-floor-grade Kingspan boards under screeds. Celotex GA4100 avoids this problem entirely because it's certified for all applications at 120 kPa.

Are brands really interchangeable?

Yes. Forum after forum confirms it. One BuildHub user ordered Celotex and received Recticel instead because the merchant was out of stock. "Same performance, different wrapper." Builders' merchants routinely treat Celotex, Kingspan Therma, Recticel, and Ecotherm as interchangeable. At 0.022 W/mK, they are.

The only exception is Kingspan Kooltherm, which is a phenolic board (not PIR) achieving 0.018-0.020 W/mK. It's thinner for the same performance but costs 50-100% more. Unless your architect has specifically called for phenolic insulation to save build-up depth, standard PIR is the right choice.

How to work with it

Cutting

100mm PIR is too thick for a utility knife. Use a handsaw. Any panel saw or general-purpose wood saw cuts through PIR cleanly. The foam is far softer than timber. Mark your cut line on the foil face with a pencil, saw through, done. For curved cuts around pipes, a jab saw or padsaw works well.

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Cut boards 5-10mm narrower than the gap, fit them, then fill any remaining gaps with low-expansion polyurethane foam from a foam gun. Don't use high-expansion foam, which can bow the boards. Let the foam cure, trim it flush, then tape over the junction with aluminium foil tape.

Floor installation: the full build-up

This is the most common use of 100mm PIR in extensions. The sequence matters and building control will inspect it before the screed goes down.

1. Confirm the slab is dry and level. The concrete slab must have cured for a minimum of 4-6 weeks and achieved below 75% relative humidity before insulation goes on. Laying PIR on a wet slab traps moisture under the foil facing, causing interstitial condensation and bond failure. The slab surface should be level within 5mm tolerance. High spots prevent boards from sitting flat; low spots create voids.

2. Lay the DPM. A 300-micron (minimum) polythene damp-proof membrane goes directly onto the slab. Overlap all joints by at least 150mm and seal them with DPM jointing tape. Turn the DPM up at the edges to meet the wall DPC. This membrane stops ground moisture from reaching the insulation and the rooms above. Without it, you'll get damp floors within a few years.

3. Lay the PIR boards. Lay boards in a staggered bond pattern, like brickwork, with joints offset by at least 300mm. Press boards together firmly. Aim for tight joints. Where gaps are unavoidable, fill with low-expansion foam, cure, trim, and tape.

4. Tape every joint. Use aluminium foil tape (50-75mm wide) across every board-to-board joint. This creates a continuous vapour control layer across the entire floor. Untaped joints allow warm moist air to track downwards, reducing the effective R-value by an estimated 20-30% and creating condensation risk in the build-up below.

5. Install perimeter edge strips. Fix compressible foam strip (10-20mm thick) around the full perimeter where the insulation meets the walls. This strip runs from the top of the insulation to the full depth of the screed. It serves two purposes: it accommodates thermal expansion of the screed (preventing cracking), and it breaks the thermal bridge between the floor and the cold external walls.

6. Screed. Minimum 65mm unbonded sand-cement screed on top. The screed sits on the insulation, not bonded to it, which is why the perimeter strip and level base are critical. If you're having liquid screed (anhydrite/calcium sulphate), it typically goes down at 50mm minimum, but check with your screeder.

The foil tape rule

This is worth repeating because it's the single most common installation failure. Every joint must be taped. Board-to-board joints. Board-to-perimeter junctions. Any foam-filled gaps after the foam is trimmed flush.

The foil facing on PIR acts as your vapour control layer. Break the continuity of that layer and warm moist air from the room above penetrates into the cold zone below the insulation, condensing as it goes. The IMA (Insulation Manufacturers Association) guidance is explicit: tape all joints with aluminium foil tape. No specific Building Regulation mandates it, but the IMA design guidance does, and building control inspectors increasingly check for it.

A 45-metre roll of foil tape costs under £10. For a 20 m2 floor, you'll use 2-3 rolls. There's no excuse to skip it.

Wall and roof applications

100mm PIR works in walls and roofs too, but the compliance picture is different from floors.

For cavity walls, 100mm PIR as full-fill cavity insulation achieves approximately 0.16-0.17 W/m2K, comfortably meeting the 0.18 wall limit. But full-fill cavity insulation removes the drainage cavity, so it's only suitable in sheltered exposure zones. In most extension builds, 50mm partial-fill is the standard wall specification, with 100mm reserved for cases where the architect needs to hit a tighter target.

For pitched roofs, 100mm PIR between rafters alone gives a U-value of roughly 0.19-0.21 W/m2K. That doesn't meet the 0.15 roof limit. You need either a thicker board (120-150mm) or a combined system: 100mm between the rafters plus a 50mm continuous layer below the rafters. Building control officers routinely ask builders to upgrade from 100mm to 150mm in roofs. Budget for this possibility.

For flat roofs, 120-150mm PIR is the minimum to hit the 0.15-0.16 W/m2K target. 100mm alone won't pass.

How much do you need

Each board covers 2.88 m2 (2400 x 1200mm). The calculation:

1. Measure the total floor area in square metres
2. Divide by 2.88
3. Add 5-10% for cuts, waste, and fitting around pipes and ducts
4. Round up to whole boards

Worked example: 20 m2 extension floor

20 / 2.88 = 6.94 boards. Add 10% waste = 7.64 boards. Order 8 boards.

At £27-34 ex VAT per board from an online specialist (Celotex GA4100), that's 8 boards at £216–272 ex VAT. Including VAT, you're looking at roughly £260–330 for the insulation material for a 20 m2 floor.

Worked example: 30 m2 extension floor (typical 5m x 6m)

30 / 2.88 = 10.42 boards. Add 10% = 11.46 boards. Order 12 boards.

12 boards at £27-34 ex VAT = £324-408 ex VAT, or roughly £390-490 inc VAT.

Add accessories: aluminium foil tape (3-4 rolls at £7-10 each = £21-40), DPM if not already included in your groundwork price (£30-50 for a 20-30 m2 area), perimeter edge strip (£15-25), and low-expansion foam for gap filling (£8-12 per can, allow 2 cans). Total accessory cost: £60-97 on top of the board price.

Cost and where to buy

The price gap between online insulation specialists and high-street merchants is significant for PIR boards. The same product can vary by 30-50% depending on where you buy it.

Kingspan TF70 (the floor-grade board) runs about 30% more than Celotex GA4100 at online specialists: £35-36 ex VAT versus £27-34. The extra cost buys you higher compressive strength (140 kPa vs 120 kPa), which is technically better under screed but not required by building regulations for domestic floors. Unless your structural engineer has specified 140 kPa minimum, save the money and use Celotex GA4100.

Recticel Eurothane GP is the budget-conscious choice at £27-33 ex VAT per board, often 5-10% cheaper than the equivalent Celotex. Less widely stocked at merchants but readily available online.

Don't forget the accessories

Budget £60–97 on top of the board cost for foil tape, DPM, perimeter edge strip, and gap-filling foam. See the full breakdown in the How much do you need section above.

Alternatives

EPS (expanded polystyrene) is the main budget alternative. EPS has a lambda value of 0.032-0.038 W/mK, compared to PIR's 0.022. To hit the same 0.18 W/m2K floor U-value target, you need approximately 150-160mm of EPS where 100mm of PIR would do. That's 50-60% more build-up depth. EPS costs 40-60% less per square metre than PIR.

The trade-off is straightforward: if you're digging out for a new extension slab and you control the dig depth, EPS saves money at the cost of a deeper dig. If floor-to-ceiling height is constrained (lowering the finished floor isn't an option), PIR wins because 100mm is thinner than 150mm of EPS. As one BuildHub forum member put it: "You only get one chance, then it's all buried for ever." Don't scrimp on thickness to save a few hundred pounds.

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Mineral wool (100mm) is cheaper still and non-combustible (Euroclass A1), but it's only suitable between joists in suspended timber floors, not under screed on solid slabs. Mineral wool has no compressive strength and absorbs moisture. Different tool for a different job.

PIR 120mm or 150mm is what your building control officer may request for roofs. For floors, 100mm is the sweet spot, but if you're building in Scotland (where the floor U-value target is 0.15 W/m2K, not 0.18), you'll need 120mm PIR to comply.

Insulated plasterboard (Celotex PL4000, Kingspan Kooltherm K118) is a PIR layer pre-bonded to plasterboard. Used for internal wall insulation, not floors. More convenient but more expensive per square metre than buying PIR and plasterboard separately.

Where you'll need this

• Insulation - primary floor and roof insulation specification and installation

100mm PIR appears in the floor build-up of virtually every extension project in England and Wales. It's not specific to kitchen extensions. Any single-storey rear extension, side return, garage conversion with a new slab, or ground-floor renovation that needs to meet current building regulations will use this product.

Common mistakes

Buying roof-grade boards for the floor. Kingspan TP10 (roof) and TF70 (floor) look identical and share the same lambda value. But TP10 has a lower compressive strength rating and a different BBA certification. Some building control officers will reject non-floor-grade Kingspan boards under screed. Celotex GA4100 avoids this issue because it's BBA-certified for all applications. If you're buying Kingspan, make sure it says TF70 on the label.

Skipping the DPM. The damp-proof membrane below the PIR isn't optional. Without it, ground moisture wicks up through the concrete slab and into the insulation layer, degrading thermal performance and eventually causing damp in the rooms above. DPM goes on first, lapped 150mm at joints, sealed with jointing tape, turned up at perimeter walls to meet the wall DPC.

Leaving gaps at the perimeter without edge strip. The compressible foam strip around the perimeter isn't decoration. When screed heats and cools (especially with underfloor heating), it expands and contracts. Without the perimeter strip to absorb that movement, the screed cracks. The strip also prevents a thermal bridge where the cold external wall meets the warm floor slab.

Pouring screed onto uncured foam. If you've filled gaps between boards with expanding foam, wait for it to fully cure (24 hours minimum) before screeding. Uncured foam will continue to expand under the screed, creating humps and uneven surfaces that are invisible until the floor covering goes down.

Laying boards on a wet slab. Four to six weeks minimum cure time for the concrete slab before PIR goes down. A hygrometer reading below 75% relative humidity confirms the slab is dry enough. Rushing this step traps moisture under foil-faced boards with nowhere to go. You won't see the damage until damp patches appear on your finished floor months later.

Confusing new-build targets with extension targets. Part L 2021 sets different U-value limits for new-build dwellings (floors: 0.13 W/m2K) and extensions to existing dwellings (floors: 0.18 W/m2K). The extension targets are less demanding. 100mm PIR achieves 0.17 W/m2K, which passes the extension target comfortably but wouldn't meet the new-build target without additional measures. Don't over-specify (and over-spend) based on the wrong table.