PIR Insulation
/ piː aɪ ɑː ˌɪnsəˈleɪʃ(ə)n /
Also known as: polyisocyanurate insulation, PIR board, rigid foam insulation
Definition
PIR insulation (polyisocyanurate) is a rigid closed-cell foam insulation board with a very low thermal conductivity (lambda value) of approximately 0.022-0.023 W/mK. Faced on both sides with aluminium foil, PIR boards achieve very good U-values in a thin profile, making them the preferred choice where depth is limited - flat roofs, floor builds, and rafter spaces in pitched roofs. Brand names include Celotex, Kingspan, and Recticel.
In practice
PIR boards come in standard thicknesses from 25mm to 200mm and are cut with a hand saw or utility knife. In flat roofs they are laid above the deck in a warm roof build-up, tapered to provide falls, and covered with the waterproofing membrane. In pitched roofs they are cut between rafters and combined with a second layer below to eliminate thermal bridging at the rafter. In ground floors they are laid between or over the structural slab, under a screed or chipboard deck.
The foil face acts as a vapour control layer when joints are taped, but in most applications a separate vapour barrier is still specified above or below the insulation. PIR is not suitable for applications where it will be exposed to sustained moisture or direct sunlight without cover, as UV degrades the foil facing. It should not be used as the sole insulation in applications where fire resistance is critical without additional protection.
Building Regulations
Approved Document L sets target U-values for new dwellings. PIR boards are often the product of choice for achieving these targets in thin construction profiles. Approved Document B (Fire Safety) requires that PIR insulation is not left exposed - in habitable spaces it must be covered by at least 12.5mm plasterboard. Lambda values are certified under BBA or LABC certification schemes; always use the certified design lambda, not the declared lambda, in U-value calculations.
Full Building Regulations guidance