PIR Insulation Board 120mm: Flat Roofs, Upgraded Floors, and the Joist Depth Problem

Order 100mm PIR for your flat roof and your building control officer will reject it. The Part L 2021 U-value target for flat roofs is 0.16 W/m2K, and 100mm PIR in a warm deck flat roof build-up achieves only approximately 0.20 W/m2K. That's a meaningful shortfall, and it won't pass an inspection. You'll need to strip the waterproofing membrane, lift the boards, relay with 120mm, then re-waterproof. The cost of that mistake on a 24m2 flat roof runs to several thousand pounds in wasted materials and labour. 120mm PIR is the thickness that gets flat roofs and upgraded floor specs over the line, and it comes with handling and installation quirks that thinner boards don't.

What it is and what it's for

PIR (polyisocyanurate) insulation at 120mm is the same rigid foam board as the thinner variants, just thicker and heavier. Same foil-faced sandwich, same closed-cell foam core, same lambda value of 0.022 W/mK. At 120mm, a single board delivers an R-value (thermal resistance, the measure of how well the board resists heat flow) of 5.45 m2K/W.

That R-value is what makes 120mm the go-to thickness for two specific applications where thinner boards fall short:

Flat roofs (warm deck construction). Part L 2021 sets the maximum U-value for new and replacement flat roofs at 0.16 W/m2K. In a warm deck build-up (where insulation sits above the structural deck), 120mm PIR achieves this target. 100mm doesn't quite get there. The Insulation Manufacturers Association confirms 120-130mm as the correct range for England and Wales compliance.

Floors where the building control officer upgrades the spec. The Part L floor U-value limit is 0.18 W/m2K for England and Wales. 100mm PIR achieves roughly 0.17 W/m2K on a solid concrete floor, which passes. But BCOs regularly ask for 120mm as a comfort margin, particularly if other elements of your thermal envelope are marginal. In Scotland, where the floor limit is a tighter 0.15 W/m2K, 120mm PIR (achieving 0.15 W/m2K) is the standard specification.

The standard board is 2400 x 1200mm, covering 2.88 m2. Each board weighs approximately 11.3 kg. That's noticeably heavier than a 50mm board (about 4.5 kg) and worth noting if you're carrying them up a ladder to a flat roof. Two people make the job safer and faster, especially in wind.

Types, sizes, and specifications

Every major PIR brand manufactures a 120mm board to BS EN 13165, and the thermal performance is identical across all of them. Lambda: 0.022 W/mK. R-value: 5.45 m2K/W. The differences that matter are compressive strength, price, and availability.

That Kingspan TR26 price is not a typo. At around 100 per board, it's 2.5 times the cost of a Celotex XR4120 that delivers identical thermal performance. The Kooltherm K107 at least offers a genuinely better lambda value (0.019 vs 0.022 W/mK), meaning you could achieve the same U-value in less thickness. But at 115-130 per board, the premium only makes sense where physical depth is severely constrained and those extra millimetres matter.

For most residential flat roofs and floor upgrades, buy Celotex XR4120 or Recticel GP 120mm. They perform identically. Get whichever is cheapest and in stock.

Compressive strength: the specification that matters for floors

This is the single most consequential technical detail most homeowner guides ignore.

PIR boards come in different compressive strength grades. Compressive strength measures how much weight the board can support without deforming, stated in kilopascals (kPa). Standard wall and roof grade boards have a minimum compressive strength of 100 kPa. Floor-grade boards need a minimum of 120 kPa, and products like the Celotex XR4120 exceed this at 140 kPa or above.

Why it matters: under a floor, the insulation carries the weight of the screed, the floor finish, the furniture, and everyone walking on it. A 100 kPa board used under a 65mm sand-and-cement screed will gradually compress over time, creating dips and unevenness in your finished floor. The screed cracks. The tiles crack. The repair bill runs into thousands.

The Celotex XR4120 is rated at 140 kPa or above, so it's suitable for both floor and roof applications. Recticel GP boards are rated at 120 kPa minimum. Both are fine for domestic floors. But if you're ordering from a less common brand or picking up seconds, check the spec sheet. Not all 120mm boards are floor-grade.

How to work with it

Handling and storage

At 11.3 kg per board, 120mm PIR is manageable for one person on the ground. Getting boards onto a flat roof is a different story. A 2400 x 1200mm sheet caught by a gust of wind acts like a sail. Work in pairs when lifting to height. Stack boards flat on a clean, level surface. Don't lean them against walls at an angle, as they'll bow over days.

Keep boards covered and out of direct sunlight. UV degrades the foil facing. A few days won't cause problems, but boards left uncovered on a roof for weeks will have compromised foil surfaces. On flat roof jobs, your roofer should install the waterproofing membrane on the same day the PIR goes down. If boards are left exposed overnight, cover them with tarpaulin.

Cutting

At 120mm thick, score-and-snap still works but takes more effort than with thinner boards. Score the foil on one side deeply with a sharp utility knife (don't try to cut all the way through), then snap over a straight edge. The break won't be as clean as with 50mm boards. A better approach at this thickness is a fine-toothed handsaw or panel saw. Mark your cut line on the foil with a pencil, then saw through. The foam cuts easily, but keep the saw straight to avoid a wavy edge.

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Cut 5mm oversize and trim to fit. A friction fit between joists or against a deck edge is better than a gap. Gaps get filled with expanding foam, which has inferior thermal performance to PIR and is a bodge that building control will question.

Flat roof installation (warm deck)

The warm deck method is the standard approach for flat roof insulation in UK new builds and extensions. The PIR sits above the structural deck, with waterproofing on top of the insulation. This keeps the deck warm and eliminates condensation risk within the roof structure.

The build-up from bottom to top:

1. Plasterboard ceiling finish
2. Structural deck (typically 18mm OSB or plywood on timber joists)
3. Vapour control layer (VCL), overlaps taped and sealed, minimum 100mm overlap
4. 120mm PIR boards, staggered joints, all joints sealed with aluminium foil tape
5. Waterproofing membrane (EPDM rubber, GRP fibreglass, or torch-on felt)

Before laying the VCL, check that the deck's moisture content is below 20% using a pin-type moisture meter. Wet timber under a VCL will rot, and you won't see it until the ceiling stains.

Stagger the board joints like brickwork. Never line up the joints in a straight line across the roof. Tape every single joint with aluminium foil tape. This isn't optional. Untaped joints leak warm air through the gaps, reducing the effective thermal performance by 20-30% and creating condensation paths.

Floor installation

For solid concrete ground floors (the most common in extensions), the build-up is: oversite concrete or slab, DPM (damp-proof membrane), 120mm PIR, polythene separating layer, then screed.

That polythene layer between the PIR and the screed is critical. Wet cement screed is alkaline and attacks the aluminium foil facing, causing it to delaminate over time. The polythene prevents this chemical reaction and acts as a slip layer during screed curing.

Stagger board joints. Leave 5-10mm expansion gaps at perimeter walls (covered by skirting boards later). The boards carry the full weight of the screed and everything above it, which is why compressive strength grade matters.

The joist depth trap

This is the most common practical problem with 120mm PIR, and it catches people on every forum thread about this product.

If you're fitting 120mm PIR between floor joists in a suspended timber floor, your joists need to be at least 170mm deep. Standard joists in most UK housing are 150mm (6 inches). 120mm of insulation plus a 25mm support batten plus some clearance for air movement means 150mm joists simply don't have enough depth.

The result: you order 30 boards of 120mm PIR, go to install them, and discover they physically don't fit. The solutions are all compromises:

• Use 100mm PIR instead (fits 150mm joists with room for battens). This achieves 0.17 W/m2K for a solid floor, which passes in England. For a suspended timber floor, the U-value will be slightly worse due to air movement beneath the joists.
• Install above the joists instead of between them. This raises the floor level by 120mm plus the thickness of the new deck, which causes problems at door thresholds, stair landings, and where the extension meets the existing house.
• Install below the joists in the void beneath. Possible in some constructions but creates vapour management problems and makes the boards harder to support long-term.

For new-build extensions where you're specifying the joists from scratch, specify 170mm or 195mm joists to accommodate 120mm insulation. The cost difference for deeper joists is minimal compared to the insulation headache of trying to work around shallow ones.

How much do you need

Each board covers 2.88 m2. The calculation:

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

Worked example: flat roof for a 4m x 6m extension

Roof area: 24 m2. Divide by 2.88 = 8.3 boards. Add 10% waste = 9.2 boards. Order 10 boards.

At 33-42 per board from an online specialist (ex VAT), that's 10 boards for roughly 330-420 ex VAT, or 396-504 inc VAT.

Worked example: solid ground floor for the same extension

Floor area: 24 m2. Same calculation: 10 boards. Same cost range. For an extension with both a flat roof and a concrete floor insulated at 120mm, you'd need approximately 20 boards.

Cost and where to buy

The online specialist market for PIR insulation is well-established and consistently cheaper than traditional builders' merchants.

PIR prices have risen in recent years and continue to fluctuate. The raw material base is oil-derived, so PIR tracks petrochemical markets. Prices can move meaningfully between when you get quotes and when you place an order, particularly in periods of broader construction cost inflation. Get quotes close to your order date rather than planning your budget on prices from several months earlier.

If your builder is ordering insulation through their trade account, ask what price they're getting per board. On a 20-board order (roof plus floor), the difference between 33 per board online and 50+ per board at a walk-in merchant counter is over 340. Order the insulation yourself from a specialist and have it delivered to site.

Don't forget the aluminium foil tape. Budget 10-15 per roll. A 24 m2 roof will use at least two rolls. Also budget for the VCL membrane (around 30-50 for a roll covering 50 m2) and mechanical fixings if required by the waterproofing manufacturer's specification.

Alternatives

PIR 100mm is the more common thickness. It handles walls, most floor applications in England (0.17 W/m2K against the 0.18 limit), and pitched roofs as part of a layered system. If you don't have a flat roof and your BCO hasn't asked for an upgrade, 100mm is probably what you need. It's lighter (about 8.5 kg per board), cheaper (30-35 per board), and fits between standard 150mm joists.

PIR 150mm is the step up for Scotland's tighter flat roof U-value requirement (0.13 W/m2K) or where you want extra headroom above the Part L target. The price premium over 120mm is moderate, but availability at specialist retailers can be inconsistent. Check stock before specifying.

Mineral wool (100-150mm) is much cheaper and non-combustible, but you need nearly double the thickness for equivalent thermal performance (mineral wool's lambda is typically 0.032-0.044 W/mK vs PIR's 0.022). For flat roofs, mineral wool isn't practical because it absorbs water and compresses under load. For suspended floors between joists, mineral wool with a breathable membrane is a valid alternative, particularly in older buildings where PIR's low vapour permeability can trap moisture.

EPS (expanded polystyrene) is budget insulation at roughly 60% of PIR's cost, but with a lambda of 0.032-0.038 W/mK, you'd need approximately 180-200mm of EPS to match what 120mm PIR achieves. Where physical depth is available (ground floors with deep voids), EPS works fine. Where depth is constrained (flat roofs, between joists), PIR is the practical choice.

Where you'll need this

• Insulation - flat roof and floor insulation where Part L 2021 demands more than 100mm

These tasks appear across all stages of any extension or renovation project involving flat roofs or upgraded floor specifications. 120mm PIR is not specific to any one project type.

Common mistakes

Using wall-grade boards under a floor screed. This is the one that costs real money. Wall-grade PIR has a compressive strength of 100 kPa. Floor-grade needs 120 kPa minimum. A wall-grade board under a 65mm screed will slowly compress. The screed cracks. The tiles lift. The fix involves stripping the entire floor back to the insulation layer. Always check the datasheet for compressive strength before using any PIR board in a floor application.

Ordering 120mm for between-joist installation without measuring the joists. The single most reported problem on building forums. If your joists are the standard 150mm deep, 120mm PIR won't fit between them with the necessary support battens and air gap. Measure the joist depth first. If they're 150mm or less, you need 100mm boards or a different installation method.

Skipping the polythene layer between PIR and wet screed. As covered in the floor installation section, alkaline cement chemically attacks the foil facing. A simple polythene sheet over the PIR before screeding prevents this. Costs almost nothing. Saves the integrity of your vapour control layer.

Leaving boards exposed on a flat roof overnight without protection. Rain soaks into cut edges and untaped joints, and prolonged exposure damages the foil facing (see handling notes above). The IMA guidance is clear: waterproof on the same day you install the PIR. If weather forces a delay, cover the boards with weighted tarpaulin.

Not taping the joints. Same issue as thinner PIR boards, but arguably more critical at 120mm because these boards are used in flat roofs and floors where moisture management is everything. Every joint needs aluminium foil tape. Every board-to-board junction, every perimeter. The foil facing is your VCL. Gaps in it mean condensation, and condensation in a flat roof means timber rot that you won't discover until the ceiling stains.

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Confusing Kingspan product lines and overpaying. Kingspan TR26 at approximately 100 per board delivers the same lambda (0.022 W/mK) as Celotex XR4120 at 33-42 per board. The Kingspan premium buys you their brand name and technical support, not better thermal performance. For domestic work, the cheaper board does the same job. The exception is Kingspan Kooltherm, which genuinely has a lower lambda (0.019 W/mK) and justifies a premium only where depth is severely constrained.