What is polystyrene?
Polystyrene (PS) is a thermoplastic made from the monomer styrene. In solid form it is hard, clear and brittle. In foamed form it is the best-known insulation material in construction: EPS and XPS are both made from it.
So the polymer itself is not the insulation — the foam is. By volume, the overwhelming majority of the foam is air or gas; the polystyrene only forms the cell wall around it.
The three forms used in building
- EPS — expanded polystyrene. Small beads are blown up with steam and sintered together. Recognisable by the bead structure. Used as cavity insulation, floor insulation, roof insulation and as void formers in lightweight floors. Thermal conductivity is broadly between 0.031 and 0.038 W/m·K; the grey graphite grades sit at the favourable end because the graphite reflects radiant heat.
- XPS — extruded polystyrene. Extruded into a closed-cell, solid-coloured board. Higher compressive strength and far lower water absorption than EPS. Hence the standard for perimeter insulation against basement walls, under loaded floors and in the inverted roof, where the insulation sits above the waterproofing. Thermal conductivity broadly between 0.029 and 0.036 W/m·K.
- Solid polystyrene and HIPS for sheet material, trims and internal window boards.
Points to watch
- Fire. Polystyrene is combustible: it melts, drips and gives dense smoke. Products contain a flame retardant, but that does not make them non-combustible. In almost every application the foam has to be covered by a non-combustible or protective layer. Check the reaction-to-fire class of the specific product and — more importantly — of the build-up it was tested in.
- Temperature. PS softens well below 100 °C. Keep it clear of flues, chimneys and downlights.
- Solvents. Styrene foam is attacked by solvents: some bitumen products, solvent-based PU foam and many thinners literally dissolve it. Use only adhesives and sealants the supplier has declared polystyrene-compatible.
- UV. Exposed to sunlight the surface yellows and chalks. Board left open for long periods during construction must be covered.
- Moisture. EPS is not fully vapour-tight and can take up water under sustained wetting. Beneath a loaded or wet construction, XPS is the safer choice.
Related terms
- EPS
- XPS
- Plastic
- Polymer
- Thermal conductivity
- Perimeter insulation
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