PIR Insulation Panel — Lambda, Thickness and Fire

A PIR insulation panel is a self-supporting sandwich panel with a rigid closed-cell polyisocyanurate foam core bonded between two metal skins, used for roofs and walls. It gives among the best thermal performance per millimetre of the common industrial insulations, but the core is combustible, so its EN 13501-1 fire class must be named.

By Dhruv Agarwal · · 1 min read

What it is

A rigid polyisocyanurate foam core bonded between two metal skins, supplied as a self-supporting sandwich panel for roofs and walls. The foam is closed-cell, which is why it insulates so well for its thickness.

Specification at a glance

PropertyTypical specification
CorePolyisocyanurate, closed cell
Thermal conductivityApproximately 0.022 W/mK
Core density38 — 45 kg/m³ typical
Panel thickness30mm — 150mm
SkinsPre-painted Galvalume, 0.45mm — 0.60mm
StandardEN 14509 for the panel system
Fire classNamed EN 13501-1 class; core is combustible

Thickness against U-value

Indicative, before thermal bridging at fixings and joints:

Core thicknessApproximate U-value
40mm~0.52 W/m²K
50mm~0.42 W/m²K
80mm~0.27 W/m²K
100mm~0.21 W/m²K
150mm~0.14 W/m²K

The ECBC 2017 target for the climate zone sets which row applies. On a single-storey industrial building the roof usually governs, because it takes the highest solar incidence of any surface.

Why it gets specified

Best thermal performance per millimetre of the common industrial insulations, so it wins wherever build-up depth is constrained or the U-value target is demanding.

Single-fix envelope. The panel is structure, insulation, weather line and finish in one operation, which compresses the programme against a built-up system.

Where projects go wrong

  • Thickness chosen by habit rather than from the ECBC target for the climate zone.
  • Fire class unstated. "PIR" is not a fire specification.
  • Combustible core behind a combustible outer panel in a ventilated cavity — the assembly behind the worst facade fires.
  • Joints and fixings ignored in the U-value. The installed assembly underperforms the datasheet, sometimes substantially.
  • Panels stacked and handled carelessly. A crushed edge cannot be made weathertight afterwards.

Standards referenced

Opaque assembly U-value targets under ECBC 2017. Panel system requirements under EN 14509. Reaction-to-fire classification to EN 13501-1.

Standards referenced

  • ECBC 2017 — Building envelope — opaque assembly U-value (Bureau of Energy Efficiency)
  • EN 13501-1 — Reaction to fire classification (CEN-CENELEC)
  • EN 14509 — Self-supporting double skin metal faced insulating panels (CEN-CENELEC)

Frequently asked

Divide the core thermal conductivity by the target U-value, then allow for the metal skins and fixings. With a lambda near 0.022 W/mK, roughly 50mm gives about 0.42 W/m²K and 100mm about 0.21 W/m²K before thermal bridging. Fixings and joints degrade this, so the installed assembly performs worse than the panel datasheet and the calculation should account for it.

They are related but not identical. PUF is polyurethane foam; PIR is polyisocyanurate, a modification with a higher proportion of isocyanate that gives better fire behaviour and slightly better thermal performance at the same thickness. Indian industry uses the terms loosely, so the specification should name PIR explicitly with its fire classification rather than relying on the trade name.

Wherever fire performance governs the envelope: high-occupancy buildings, compartment lines, cold stores holding high-value stock, and any facade cavity where an insurer takes a view. The core is combustible even in PIR grades. Rock wool is the answer in those zones, and many industrial envelopes use PIR in general areas with rock wool at fire-critical lines.

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