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Using polystyrene insulation under concrete slabs

Suitable polystyrene to provide insulation under concrete slabs on ground is a minimum of S grade expanded polystyrene (EPS) board (with a nominal density of 16 kg per cubic metre) with a minimum thickness of 50 mm.

The insulation should:

  • be laid over the top of the concrete underlay or damp-proof membrane

  • be close butted

  • have the reinforcing steel installed immediately following the polystyrene so that the insulation is held in place (unsecured polystyrene will become airborne) – place mesh over the polystyrene on mesh chairs.

Do not lay the polystyrene under footings or slab thickenings.

Where the R-value is required to be at least R1.9 for a heated slab, edge insulation will also need to be provided.

Summarised from BRANZ Guideline newsletter, March 2012