Phenolic Foam Insulation — Thin Build-Up, Low Lambda

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

Property	Typical specification
Standard	EN 13166
Thermal conductivity	Approximately 0.018 — 0.021 W/mK
Thickness	20mm — 100mm
Facing	Aluminium foil, or glass tissue
Fire class	Named EN 13501-1 class; chars rather than melts
Friability	Higher than PIR; handle with care

Thickness for a U-value, compared

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

Material	Approximate 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.