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.

XPS Insulation Board Specification

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.

Extruded Polystyrene (XPS) Insulation Board

What it is

Specification at a glance

Where it belongs

Where it does not belong

Where projects go wrong

Standards referenced

Standards referenced

Frequently asked

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