Expanded Polystyrene (EPS) Insulation and Core

What it is

Expanded polystyrene — beads expanded with steam and fused into blocks, then cut into boards or moulded into shapes. The cheapest common insulation and the most widely misused.

Specification at a glance

Property	Typical specification
Standard	IS 4671 or EN 13163
Density	15 — 35 kg/m³, stated
Thermal conductivity	Approximately 0.033 — 0.040 W/mK
Water absorption	Moderate; higher than XPS
Fire class	Combustible; FR grades available
Forms	Board, block, moulded shapes, panel core

Where it is appropriate

Dry above-ground thermal insulation · cold room and chiller panels where fire strategy permits · void formers and lightweight fill in civil work · under-screed insulation in dry conditions · packaging and protection during fit-out.

Where it is not

Ventilated facade cavities. EPS in a cavity behind a combustible panel is the assembly behind the most serious cladding fires. This is not a marginal judgement.

Wet or below-grade positions, where its water absorption degrades performance — XPS is the material there.

Anywhere solvents are present. Solvent-based adhesives and sealants dissolve polystyrene, and this catches people out on site.

Where projects go wrong

• Density unspecified, so the lightest grade arrives.
• Used in facade cavities on buildings where fire governs.
• Assumed interchangeable with XPS in wet or loaded positions.
• Solvent adhesives applied directly to the board.
• Left exposed to sunlight, which degrades the surface quickly.

Standards referenced

EPS for thermal insulation under IS 4671 and EN 13163. Reaction-to-fire classification to EN 13501-1.