Under-slab insulation

Under-slab insulation is rigid foam, specifically high-density EPS or XPS chosen for its compressive strength, laid under a concrete slab in cool-climate housing to reduce heat loss from the slab to the ground. Standard low-density foam is not suitable: it crushes under the slab load and loses its R-value.

In a cool climate a slab-on-ground can lose useful heat down and out to the cooler earth, especially at the perimeter. Under-slab (and slab-edge) insulation puts a thermal break between the slab and the ground so the slab’s thermal mass works for comfort rather than bleeding heat away. It is a standard move in southern-Australian and alpine builds and in any home chasing a high energy rating; in hot-humid climates it is usually unnecessary or even counterproductive.

The critical specification point is compressive strength. The foam carries the slab and everything on it, so it must be a high-density grade (high-density EPS to the right grade, or XPS) rated for the load without crushing. Use a standard low-density insulation board and it compresses, the slab settles, and the R-value you specified is gone. Slab-edge insulation (a vertical board around the perimeter) is often as important as the horizontal layer, because the edge is the biggest loss path.

For a builder the practical points are to specify the correct high-density board for the slab load (not whatever foam is on the truck), to insulate the slab edge as well as under it where the design calls for it, and to coordinate with the waterproofing and termite management, because a continuous foam layer under and around a slab can create a concealed termite path if it is not detailed with the right barriers.

Also known as: Slab insulation, sub-slab insulation.

Category: Insulation / Slabs.

Related

• Rigid foam
• NCC energy efficiency

See also

• Continuous insulation

References

• Rigid foam (Chalkline) (verified 2026-06-01)

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Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.