PIR Board Calculator — Thickness, Boards & U-Value

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.

The default target U-values used here are: walls 0.18 W/m²K, roofs 0.16 W/m²K, floors 0.18 W/m²K. These align with Approved Document L 2021 (Volume 1) limiting values — the wall 0.18 matches Table 4.2 (new elements in existing dwellings/extensions), while the new-build limiting wall value in Table 4.1 is 0.26 W/m²K. The roof 0.16 and floor 0.18 are the Table 4.1 new-build limits (the extension roof limit in Table 4.2 is tighter at 0.15). For reference, Part L 2021 limiting U-values for NEW dwellings (Table 4.1) are: walls 0.26, pitched roof 0.16, floor 0.18 W/m²K. To meet the wall 0.18 target over a typical existing construction you need roughly 100-125mm of PIR. PIR boards come in standard thicknesses including 25, 50, 75, 100, 120, and 150mm. Use the Custom U-value option in Advanced Options for a tighter notional target (e.g. 0.13).

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.

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.

The main UK PIR brands are Kingspan (Thermawall, Thermapitch, Thermafloor), Celotex (GA4000, RS5000), and Recticel (Eurothane, Powerdeck). These standard PIR boards have lambda values around 0.022 W/mK — the figure this calculator uses. Kingspan Kooltherm is a phenolic (not PIR) board with a lower lambda of about 0.018-0.020 W/mK, so it achieves the same U-value in a thinner board; if you use Kooltherm, set a Custom calculation or follow the Kooltherm datasheet. 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.