# 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.

## 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**.

## Frequently asked questions

### Why choose phenolic over PIR?

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.

### How does it behave in fire?

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.

### What are its practical drawbacks?

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.

## Sources

- [EN 13166](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Thermal insulation products — factory made phenolic foam
- [EN 13501-1](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Reaction to fire classification
- [ECBC 2017](https://beeindia.gov.in/) — Building envelope — opaque assembly U-value

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Source: https://hagerstone.com/materials/phenolic-foam-insulation