Extruded polystyrene insulation, 1250 × 600 mm board, 20 mm thick

Moisture and pressure place different demands on insulation. Around foundations, beneath ground-bearing floors and on inverted roofs, a board faces quite different conditions from a dry internal wall. The dense closed cells of XPS limit water uptake and give the material compressive properties suited to lower parts of buildings and assemblies under load.

Water can impair the performance of unsuitable insulation. In XPS, the cells do not form an open network through which water can flow freely. This is useful on basement walls and plinths exposed to periodic moisture. Standard façade-grade EPS is specified for other conditions. In damp ground, low water uptake is valuable because repairs may involve excavating the foundations. The assembly still needs waterproofing. In an appropriate build-up the membrane keeps water out and the boards limit heat loss while shielding the membrane on the soil side.

The rigidity of XPS can be useful under floors, in garages and in suitable terrace assemblies. An appropriate grade works with the layers above it that distribute loads and provides support where a soft mat would be unsuitable. Check the compressive properties and performance under sustained loads. Matching the grade to the use of the floor helps maintain the insulating layer in more heavily loaded areas, for example where a living area meets the garage and entrance.

Once foundations are backfilled or a floor is laid, the insulation is difficult to reach. Resistance to moisture and suitability for the loads can matter more than a small difference in board price. XPS boards are light and easy to cut, with various edges for continuous insulation across plinths and flat surfaces. On an inverted roof, a suitable grade above the waterproofing can help protect it from exposure. Compare PIR for a dry assembly with very little depth. Where moisture, cold and pressure act together, XPS addresses that combination.

SteelProfil can help select insulation for a foundation, floor or another part of your building. Check stock, promotions and the grade available for the particular batch. Choosing the right board matters long after the insulation is covered.