Quick answer: PIR insulation is moisture-resistant, not waterproof. It should be stored dry and protected from prolonged exposure to rain or standing water.
If PIR boards have been allowed to get wet before installation, do not simply install them as normal. Check the technical guidance for the board supplied and make sure the board is dry and fit for purpose before use.
PIR insulation resists moisture but is not waterproof
The rigid foam core in PIR does not readily absorb water, which makes PIR suitable for many roof, wall and floor applications where moisture resistance is useful.
That does not make the board a waterproofing layer. Roof coverings, membranes, damp-proof layers and other parts of the construction still need to keep water out.
Do not leave PIR exposed to rain during storage
PIR boards should be stored flat, clear of the ground and under suitable weatherproof cover. The plastic wrapping supplied with insulation is normally intended for short-term protection rather than permanent outdoor storage.
Keep boards protected from prolonged moisture and physical damage until they are installed.
What if PIR insulation gets wet?
If a board gets wet before installation, the correct action depends on the condition of the board and the technical guidance for the product supplied.
Do not install visibly saturated, damaged or deteriorated boards. Dry and assess the board where the product guidance allows it, or replace it where the manufacturer requires wet boards not to be used.
PIR is not a substitute for a waterproof membrane
PIR insulation can form part of a roof, wall or floor build-up, but the construction still needs the correct weatherproofing, damp-proofing and vapour-control layers for the job.
For flat roofs, the waterproof roof covering and the correct warm-roof or cold-roof arrangement remain essential. See our warm roof vs cold roof guide.
Can PIR be used in damp-prone constructions?
PIR can be used in suitable constructions where moisture risk has been dealt with correctly. The key is the complete build-up: drainage, ventilation, membranes, cavities and waterproofing all matter.
If the construction already has an unresolved leak or damp problem, fix that problem before adding insulation.
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Browse our PIR insulation range or shop rigid PIR insulation board in 25mm, 50mm, 75mm, 100mm and 150mm thicknesses.
