What is the lambda value of PIR insulation?

PIR (Polyisocyanurate) insulation boards have a lambda value of 0.022 W/mK, making them one of the most thermally efficient rigid board insulations available. This is better than mineral wool (0.032-0.044), EPS (0.032-0.038), and XPS (0.029-0.036). The low lambda means thinner boards achieve the same U-value, which is valuable where space is limited.

How thick does PIR insulation need to be?

The default design targets used here (walls U = 0.16, roofs U = 0.13, floors U = 0.13) are typical specification values that exceed Part L 2021 minimum requirements. Part L 2021 limiting U-values are: walls 0.26 W/m²K, pitched roof 0.16 W/m²K, flat roof 0.18 W/m²K, ground floor 0.18 W/m²K (new-build). The 0.13 roof/floor values are closer to Part L 2021 notional values used for compliance modelling. To achieve these design targets: walls need about 100-120mm PIR, roofs 120-150mm, floors 100-120mm. PIR boards come in standard thicknesses: 25, 50, 75, 100, 120, and 150mm.

Should PIR board joints be staggered?

Yes — staggering joints between PIR boards is essential to minimise thermal bridging. Where two layers are used, the second layer should be offset by at least 300mm from the first. All joints should be taped with aluminium foil tape to maintain the vapour barrier continuity. Boards should be tightly butted with no gaps — even small gaps significantly reduce thermal performance.

Can PIR insulation get wet?

PIR boards have a closed-cell structure and foil facings that make them highly moisture-resistant. They absorb very little water and maintain their thermal performance even in damp conditions. However, the cut edges are vulnerable — always tape cut edges to maintain the vapour barrier. In below-ground applications, use XPS instead as it has better long-term water resistance.

What brands of PIR are available in the UK?

The main UK PIR brands are Kingspan (Kooltherm, Thermawall, Thermapitch), Celotex (GA4000, RS5000), and Recticel (Eurothane, Powerwall). All have similar lambda values around 0.022 W/mK. Prices vary by supplier — trade merchants like Travis Perkins and Jewson typically stock Kingspan and Celotex. Online suppliers may offer better prices for larger quantities.

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PIR Board Calculator — Thickness, Boards & U-Value

Calculate PIR insulation board thickness for target U-value (lambda 0.022). Works out board count, staggered joint tape, and achieved U-value.

Area to Insulate

Wall, roof, or floor area

Application

Board Size2400×1200 is most common

Existing R-Value

From existing construction

Price Per Board (optional)

How We Calculate This

This calculator determines the PIR insulation board thickness needed to achieve a target U-value, then calculates the number of boards and joint tape required.

The formula

Required R = (1 ÷ Target U) - Existing R - Surface resistances

PIR thickness = R × Lambda (0.022) × 1000 mm

Boards = Ceiling(Area ÷ Board area × Wastage)

For example, 50 m² wall at target U = 0.16: Total R = 6.25. Foam R needed = 6.25 - 0.5 - 0.17 = 5.58. Thickness = 5.58 × 0.022 × 1000 = 123mm → use 125mm or 2 × 75mm. Boards = 50 ÷ 2.88 × 1.10 = 19 boards (2400 × 1200).