EPS (Expanded Polystyrene) Roof Insulation

Expanded polystyrene (EPS) is rigid foam insulation made from polystyrene beads containing a blowing agent, usually pentane. The beads are pre-expanded with steam, then fused in moulds into large blocks that are cut into boards, including tapered boards for creating slope.

Properties

• Density: commonly about 11-32 kg/m3 (0.7-2.0 lb/ft3); higher densities give higher compressive strength.
• Thermal conductivity: about 0.032-0.038 W/(m.K), depending on density. Grey EPS with graphite or similar additives lowers this to roughly 0.029-0.034 W/(m.K).
• Vapour resistance factor: about 30-70.
• Stability: EPS uses air-filled cells once the pentane escapes, so its thermal value is relatively stable over time.

Uses on roofs

• insulation under single-ply membranes, often with a cover board or separation layer
• tapered insulation schemes
• recovery boards and lightweight fill
• composite boards laminated to polyiso or other facings

Fire

Polystyrene foams are combustible. Unmodified EPS has an oxygen index near 18%, so flame retardant is added to meet North American requirements such as ASTM C578. In assemblies, fire performance depends on the deck, cover board, membrane and any thermal barrier.

Compatibility

Solvent-based adhesives and some membranes, particularly plasticised PVC, can attack polystyrene, so manufacturers specify separation sheets or compatible adhesives. EPS is permeable to water through the gaps between beads, so it is not normally used above the membrane in inverted roofs, where XPS is preferred.

Water and freezing

Water can collect in the gaps between bonded beads, and repeated freezing can loosen beads. EPS used on roofs is therefore kept dry beneath the membrane.