Supporting · Insulation materials

PIR insulation explained: what it is, how it performs, and when to use it

What PIR insulation is, how it performs, and when to use it in your UK home.

Last updated: 10 May 2026·4 min read

In short

PIR (polyisocyanurate) is the most common insulation board in UK retrofits. It's a rigid foam board with foil facings, offering good thermal performance (0.022 to 0.023 W/mK) at a competitive price (£40 to £60 per m² installed). Brands: Celotex, Kingspan, Ecotherm.¹

What is PIR insulation?

PIR stands for polyisocyanurate. It's a closed-cell rigid foam board, usually with reflective foil facings on both sides. The foil improves thermal performance and protects the foam from moisture.

Thermal performance

PIR has a thermal conductivity of 0.022 to 0.023 W/mK. This means:

100mm PIR achieves U-value of 0.22 W/m²K (good for walls and floors).
140mm PIR achieves U-value of 0.16 W/m²K (building regs for roof insulation).

This is better than mineral wool (0.032 to 0.044 W/mK) but not as good as phenolic foam (0.018 to 0.020 W/mK).¹

Cost

PIR boards cost £40 to £60 per m² installed (including labor). Materials only: £20 to £30 per m².

Where to use PIR

Roof insulation: Between and under rafters (loft conversions, warm roofs).
Wall insulation: Internal wall insulation, external wall insulation (though EPS is cheaper for EWI).
Floor insulation: Suspended timber floors (between joists), solid floors (on top of slab).

Pros and cons

Pros:

Good thermal performance.
Easy to cut and fit (rigid, stable).
Widely available (every builders' merchant stocks it).
Competitive price.

Cons:

Combustible (needs fire barriers on tall buildings over 11m).
Loses performance if it gets wet (foil can delaminate).
Not breathable (not ideal for heritage solid walls).

PIR vs other materials

PIR vs EPS: PIR is thinner (better thermal performance) but more expensive. EPS is cheaper and better for floors (higher compressive strength).
PIR vs phenolic: Phenolic is thinner (best thermal performance) but more expensive. Use phenolic when tight on space.
PIR vs mineral wool: PIR is thinner but combustible. Mineral wool is non-combustible and breathable, but thicker and lower performance.
PIR vs wood fibre: Wood fibre is breathable and natural, but much thicker and more expensive. Use wood fibre for heritage buildings.

Brands

The main PIR board brands in the UK:

Celotex: Most common. GA4000 (general application), TB4000 (roof), PL4000 (floor).
Kingspan: TP10 (roof), TW55 (wall), TF70 (floor).
Ecotherm: Eco-Versal (general), Eco-Deck (roof).

Performance is similar across brands. Choose based on availability and price.