How to install PIR insulation boards

15 October 2026
Aaron Davis

PIR boards are light, cut easily and don't need special tools. But they're rigid, so the result depends on accurate cutting and sealed joints. This guide covers the three ways to install them and the details that make the difference.

Before you start

Check your drawings. For consented work, follow the insulation specified by your designer. Our free detail drawings cover common roof, wall and floor situations.
Store boards dry, flat and off the ground. Don't drop packs on their corners or edges.
Protect your hands. The foil edges can be sharp; don't run bare hands along them.
Tools: tape measure, straightedge, a sharp knife and a fine-toothed handsaw, Eurothane tape, and adhesive if you're gluing. Wear a dust mask and eye protection when sawing.

Cutting boards

Measure each gap rather than assuming stud spacings are exact. For thinner boards, score both foil faces with a knife along a straightedge and snap. For thicker boards, cut with a fine-toothed saw, keeping the blade square to the face so edges butt tightly. Aim for a snug fit with no gaps; gaps let air, and heat, straight past the insulation.

Method 1: between the framing

This is the traditional approach, with PIR in place of batts.

Cut boards to fit tightly between studs, rafters or joists.
Press them in so the face sits flush or at the depth your detail shows.
Where you need a service gap, use a board thinner than the framing; for example, 70mm board in a 90mm stud leaves 20mm for cables.
Seal any small gaps at the edges.

This gives a higher R-value than batts in the same depth, but the framing still bypasses the insulation. Under the new Building Code rules, walls are assumed to be at least 38% framing, so see the next method.

Method 2: over the framing

Fixing boards in a continuous layer across the face of the framing wraps the building in unbroken insulation. It's the most effective way to deal with heat loss through studs, plates and rafters.

Fix boards across the framing, staggering the joints like brickwork.
Butt boards tightly together.
Tape every joint with Eurothane tape. This stops air leaking through, completes the vapour control layer and gets the most from the board.
Fix the cladding, battens or lining as shown in the relevant drawing. The fixing method depends on the cladding type; see the cavity fix and direct fix wall details.

Method 3: between and over

For the best result, combine the two. Fit boards between the framing, leaving a service gap if needed, then add a continuous layer over the top. This is how you get high wall R-values without making walls much thicker.

Floors

Timber floors: fit boards between the joists, supported so they can't drop, or follow our existing timber floor detail. If you're laying a floating timber floor over boards, put a vapour control layer under the floor finish.

Under a concrete slab or screed: lay the damp-proof membrane (normally 1200-gauge polythene), then the boards, then a 500-gauge polythene vapour control layer over the boards before pouring. The top layer protects the foil from the alkaline concrete and stops wet concrete opening the joints. Hydronic underfloor heating pipes can be clipped through this layer without harming it.

Gluing boards

To fix boards to plasterboard or metal, use an adhesive such as Selleys Liquid Nails or PVA, and hold the boards in place until the glue sets.

Electrical cables and services

Cables can pass through PIR without being wrapped; the board doesn't affect cable sheathing. You can cut channels for cables, but this reduces performance, so a service gap is better. Seal where boards meet other materials with a low-modulus silicone sealant.

Common mistakes

Gaps between boards or at the framing, especially around windows
Untaped joints on a continuous layer
Boards left out in the weather before they're covered
No vapour control layer over boards under a slab

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