# Extruded Polystyrene (XPS) Insulation Board

> Extruded polystyrene (XPS) is a rigid closed-cell insulation board made as a continuous extrusion, so it absorbs very little water. That moisture resistance suits it to inverted roofs, below-grade walls and under slabs, where cheaper EPS degrades; it is combustible, so it does not belong in ventilated facade cavities.

## What it is

**Rigid closed-cell extruded polystyrene board**, manufactured as a continuous
extrusion so the cell structure is uniform and almost impermeable to water.

That moisture resistance is the reason it exists alongside cheaper EPS.

## Specification at a glance

| Property | Typical specification |
| --- | --- |
| Standard | EN 13164 |
| Thermal conductivity | Approximately 0.029 — 0.035 W/mK |
| Compressive strength | 200 — 700 kPa by grade |
| Water absorption | Very low; closed cell |
| Edge | Square, shiplap or tongue and groove |
| Fire class | Combustible; class stated |

## Where it belongs

| Application | Why |
| --- | --- |
| Inverted / protected-membrane roofs | Survives above the waterproofing |
| Below-grade walls and rafts | Resists ground moisture |
| Under floor slabs | Takes load and stays dry |
| Perimeter insulation | Handles backfill contact |
| Cold store floors | Combines strength and moisture resistance |

## Where it does not belong

Ventilated facade cavities on buildings of any height, escape routes, and
anywhere fire performance governs. It is combustible, and in a cavity that is
the wrong material regardless of its thermal numbers.

## Where projects go wrong

- **Compressive grade unspecified**, then surface depressions appear under
  paving or traffic.
- **Used in facade cavities** where non-combustible insulation is required.
- **Left exposed to sunlight** on site; UV degrades the surface.
- **Solvent-based adhesives** used with it, which dissolve polystyrene.
- **Board joints left open** below grade, creating thermal bypass paths through
  the insulation layer.

## Standards referenced

XPS product requirements under **EN 13164**. Opaque assembly U-value under
**ECBC 2017**. Reaction-to-fire classification to **EN 13501-1**.

## Frequently asked questions

### Why is XPS used on inverted roofs?

Because its closed-cell structure absorbs very little water, so it keeps most of its thermal performance even when it sits above the waterproofing membrane and gets wet. On an inverted or protected-membrane roof the insulation is deliberately outside the waterproofing, shielding the membrane from UV and temperature swings, and only a material that tolerates being wet can do that job.

### What compressive strength should be specified?

It depends on what sits on top. Roughly 200 to 300 kPa suits normal roof traffic and ballast, while trafficked areas, podium decks and parking above insulation need considerably higher grades. Compressive strength is the property that gets ignored and then fails visibly as depressions in the finished surface, so it should be stated explicitly in kPa rather than by brand name.

### Is XPS suitable for facade cavity insulation?

Rarely the best choice. It is combustible, and in a ventilated facade cavity on a building of any height that is the assembly to avoid. XPS belongs below grade, under slabs and on inverted roofs where its moisture resistance is the deciding property. Where insulation sits in a facade cavity, non-combustible rock wool is the right specification.

## Sources

- [EN 13164](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Thermal insulation products — factory made XPS
- [ECBC 2017](https://beeindia.gov.in/) — Building envelope — opaque assembly U-value
- [EN 13501-1](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Reaction to fire classification

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Source: https://hagerstone.com/materials/extruded-polystyrene-xps