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