Phenolic Foam Insulation — Thin Build-Up, Low Lambda

Phenolic foam insulation is a rigid closed-cell phenolic resin foam board, usually faced with aluminium foil or glass tissue, with the lowest thermal conductivity of the common rigid insulation boards. It meets a U-value in the thinnest build-up and chars rather than melting in fire, but it is not non-combustible and is more friable than PIR.

By Dhruv Agarwal · · 1 min read

What it is

A rigid closed-cell phenolic resin foam, usually faced with aluminium foil or a glass tissue. It has the lowest thermal conductivity of the common rigid insulation boards.

Specification at a glance

PropertyTypical specification
StandardEN 13166
Thermal conductivityApproximately 0.018 — 0.021 W/mK
Thickness20mm — 100mm
FacingAluminium foil, or glass tissue
Fire classNamed EN 13501-1 class; chars rather than melts
FriabilityHigher than PIR; handle with care

Thickness for a U-value, compared

Indicative, before thermal bridging, for approximately 0.25 W/m²K:

MaterialApproximate thickness
Phenolic~80mm
PIR~90mm
EPS~150mm
Rock wool~150mm

That difference is the entire argument for phenolic. Where it does not matter, it usually is not worth the premium.

Where it belongs

Refurbishments with fixed floor-to-ceiling heights · duct insulation in congested ceiling voids · roof build-ups where upstand heights are already set by existing thresholds · any envelope where the U-value target cannot be met in the available depth with a cheaper board.

Where projects go wrong

  • Used where depth is not constrained, paying a premium for nothing.
  • Handled roughly. Crumbled edges leave gaps in the insulation line.
  • Assumed non-combustible. It is not.
  • Facing damaged and not repaired, losing the vapour control function.
  • Cut undersize, leaving gaps that undo the thermal advantage the material was chosen for.

Standards referenced

Phenolic foam products under EN 13166. Reaction-to-fire classification to EN 13501-1. Opaque assembly U-value targets under ECBC 2017.

Standards referenced

  • EN 13166 — Thermal insulation products — factory made phenolic foam (CEN-CENELEC)
  • EN 13501-1 — Reaction to fire classification (CEN-CENELEC)
  • ECBC 2017 — Building envelope — opaque assembly U-value (Bureau of Energy Efficiency)

Frequently asked

Thermal performance per millimetre. Phenolic foam reaches roughly 0.018 to 0.021 W/mK against about 0.022 for PIR and 0.035 for rock wool, so it meets a given U-value in the thinnest build-up of the common rigid boards. That matters where depth is genuinely constrained — a refurbishment with fixed floor-to-ceiling heights, or a duct route through a congested void.

Better than PIR or EPS in that it chars rather than melting and produces relatively low smoke, and good grades achieve favourable EN 13501-1 classifications. It is still an organic foam and is not non-combustible. Where a fire strategy calls for non-combustible insulation — cavity barriers, ventilated facade cavities on tall buildings — rock wool remains the correct specification.

It is more friable than PIR, so edges and corners damage in handling and on site, and cut faces crumble if treated roughly. It also costs more. Where build-up depth is not constrained, PIR usually gives better value for the same U-value, and phenolic earns its premium specifically when millimetres matter.

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