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Cutting PIR Insulation to Size: From Measuring to Airtight Joints
To get PIR insulation cut to size, measure accurately first, mark the line and then cut or saw it. Boards up to about 5 cm thick can be cut with a wide insulation knife. For thicker boards, use an insulation saw, a multi-tool with a serrated knife blade, or a reciprocating saw with a long, fine blade.
Cut the board to fit as closely as possible, fill small gaps with PU foam and tape the joints with aluminium foil tape. Only then will the board insulate as it should.
What will you learn in this article?
What makes PIR different from other insulation?
PIR (polyisocyanurate) is a rigid foam board, usually with an aluminium foil or glass fibre facing on both sides. It insulates well at a fairly small thickness. Because the board is rigid, it has to fit exactly: unlike mineral wool, it will not spring into a gap. Every gap is a leak that lets heat and moisture through.
Unlike EPS or XPS, PIR does not melt when friction heats it up. A saw blade that runs hot can still cut less cleanly and fray the facing, though. A sharp blade and a steady pace matter more than stopping to let things cool down.
Which tool suits which thickness?
| PIR board thickness | Tool | Note |
|---|---|---|
| Up to about 5 cm | Wide insulation knife or snap-off knife with a long blade | Score both sides and snap |
| Thicker than about 5 cm | Insulation saw (hand saw with fine serrations) or multi-tool with a serrated knife blade | Keep it straight with a batten |
| Very thick boards | Reciprocating saw with a long, fine wood blade, or a long insulation saw | The blade must be longer than the board is thick |
| Lots of boards or angled cuts | Circular saw or plunge saw on a rail, with dust extraction | The cutting depth must be large enough |
With a multi-tool or reciprocating saw from the Maxxpack 18V range, you can work in the loft or on the roof without a cable. When you cut with a power tool, do watch out for the fine dust (see below).
Step by step: cutting and fitting PIR to size
Measure the gap accurately
Measure the width between the joists or rafters in at least two places: top and bottom, or front and back. Timbers are rarely perfectly parallel. Work from the smallest measurement.
Decide on the clearance
A PIR board should fit snugly, but you still need to be able to fit it without forcing it. Cut the board a few millimetres smaller than the measured size. That way you can slide it in and are left with a narrow gap that you fill with PU foam. If the gap is more than a centimetre, cut a new board or fill the space with a strip of PIR.
Mark the line
Lay the board on a flat surface, such as a workbench or two trestles with a sheet on top. Mark the size with a pencil or marker and a long straight edge. For angled cuts, for example in a roof slope, copy the angle with a sliding bevel.
Cut or saw the board
- With a knife: using the straight edge as a guide, cut through the facing and as deep as you can into the foam. Turn the board over, cut along the same line and snap the board.
- With an insulation saw or multi-tool: hold the straight edge along the line and saw steadily and straight, keeping the blade at right angles to the board.
- With a reciprocating saw: choose a blade that is longer than the board is thick, clamp the board in place and saw at an even speed.
Fit the board
Slide the board into the gap with the correct side facing in or out (see the printing on the board or the manufacturer's instructions). Fix the board as the manufacturer specifies: wedged between battens, with insulation screws or fixings, or with adhesive.
Seal gaps and joints
Fill the gaps around the edges with PU foam. Once the foam has cured, trim it flush with the board. Then tape all the joints and junctions with aluminium foil tape so the layer is airtight and acts as a vapour control layer.
Finishing and clearing up
After cutting, remove any frayed facing with a knife or sanding block. Clear up the dust with a vacuum cleaner, not a broom. Keep usable offcuts for small gaps and take the rest to your household waste recycling centre. Check with your local council how they accept insulation waste.
Is PIR dust dangerous?
PIR dust is fine and irritates the eyes and airways. When cutting, wear a dust mask (FFP2 at least), safety glasses and gloves. Use power saws outdoors or with dust extraction where you can. Cutting with a knife or sawing by hand produces far less dust than a fast-spinning saw.
Common mistakes
- Measuring in only one place, when the timbers are not parallel
- Cutting the board too tight, so it breaks when you fit it
- Filling large gaps with foam alone instead of a fitted strip
- Not taping the joints with aluminium foil tape
- Cutting with a blunt or coarse blade, which tears the facing
Frequently asked questions about cutting PIR insulation
Does PIR melt when you cut it?
No. PIR is a thermoset foam and does not melt from the heat of sawing, unlike EPS and XPS. A hot, blunt blade can still leave ragged edges.
How much clearance should you leave with PIR boards?
A few millimetres is enough to be able to fit the board. Fill that narrow gap with PU foam. Fill a larger gap with a strip of PIR rather than foam alone.
Can you cut PIR with a utility knife?
Boards up to about 5 cm can be scored on both sides with a long-bladed knife and then snapped. For thicker boards, an insulation saw, multi-tool or reciprocating saw is faster and gives a straighter cut.
Which tape should you use on PIR joints?
Use aluminium foil tape that is suitable for insulation boards. Apply it to a dry, dust-free surface and rub it down firmly so it bonds everywhere.
In short: cutting PIR insulation to size
Measure each gap in several places, cut the board a few millimetres smaller than the smallest measurement and choose your tool by thickness: a knife up to about 5 cm, an insulation saw or multi-tool above that, and a reciprocating saw for thick boards. Fill gaps with PU foam, tape the joints with aluminium foil tape and wear a dust mask when cutting with power tools.
