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.

By Dhruv Agarwal · · 1 min read

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

PropertyTypical specification
StandardEN 13164
Thermal conductivityApproximately 0.029 — 0.035 W/mK
Compressive strength200 — 700 kPa by grade
Water absorptionVery low; closed cell
EdgeSquare, shiplap or tongue and groove
Fire classCombustible; class stated

Where it belongs

ApplicationWhy
Inverted / protected-membrane roofsSurvives above the waterproofing
Below-grade walls and raftsResists ground moisture
Under floor slabsTakes load and stays dry
Perimeter insulationHandles backfill contact
Cold store floorsCombines 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.

Standards referenced

  • EN 13164 — Thermal insulation products — factory made XPS (CEN-CENELEC)
  • ECBC 2017 — Building envelope — opaque assembly U-value (Bureau of Energy Efficiency)
  • EN 13501-1 — Reaction to fire classification (CEN-CENELEC)

Frequently asked

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.

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.

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.

Related

More in Materials

All Materials →

Planning an office or commercial project?

Tell us what you are planning. Message our team on WhatsApp, call us, or leave your number and we will call you back.

Chat on WhatsAppCall +91 88829 10969

Request a callback