Quick answer: there is no single fitting method for every PIR insulation job. The correct installation depends on whether the board is being used in a roof, framed wall, floor or another specified build-up.
The shared rules are simple: measure accurately, cut the board neatly, fit it without avoidable gaps, support or fix it correctly, and follow the moisture-control and ventilation details required by the construction.
Start with the complete roof, wall or floor build-up
Before fitting PIR, identify where the insulation sits in the construction and what the surrounding layers need to do. Membranes, cavities, ventilation zones, plasterboard and roof coverings can all change the installation method.
Do not copy a roof detail into a wall or floor simply because the same PIR board is being used.
Fit PIR tightly between timber where the design calls for it
When PIR is fitted between studs, joists or rafters, measure each opening rather than assuming every bay is identical. Cut the board so it fits neatly against the surrounding timber without large gaps.
Do not force a badly oversized board into position. A distorted board or damaged edge is not a substitute for an accurate cut.
Keep required ventilation and cavities clear
Some roof and wall constructions require a ventilation gap or cavity beside the insulation; others use different moisture-control details.
Follow the specified build-up and do not fill a required ventilation space simply because extra insulation will physically fit there. For flat roofs, read our warm roof vs cold roof guide.
Continuous PIR layers need the correct fixing method
Where PIR is installed as a continuous layer across framing or another substrate, use the fixing method specified for that construction. The board still needs to sit flat with close-fitting joints.
The fixings must suit the substrate, board thickness and layers being fixed through rather than being chosen from the insulation thickness alone.
Seal joints only as required by the air and vapour-control strategy
If the foil-faced PIR is being used as part of the air or vapour-control layer, joints need to be sealed with suitable insulation tape. Other constructions use a separate vapour control membrane or foil-backed plasterboard instead.
See Do PIR Insulation Joints Need Taping? for the difference.
Accurate cutting matters
PIR is easy to cut, but the cut needs to stay square through the board thickness so joints close properly. Use a suitable insulation saw or fine-toothed saw for thicker boards, or a sharp knife where score-and-snap is appropriate.
See How to Cut PIR Insulation Board for the full cutting method.
Choose the thickness from the project, not the fitting method
The fact that a certain thickness fits between the framing does not automatically mean it provides the thermal performance required by the project.
Choose thickness from the complete construction and thermal target. Our PIR U-value, lambda and thickness guide explains why.
Shop PIR insulation board
Our rigid PIR insulation board is supplied in 2400 × 1200mm sheets with 25mm, 50mm, 75mm, 100mm and 150mm thicknesses. You can also browse the full PIR insulation range.
