Thermal insulation can be installed to the inside or outside of a basement structure – depending whether the project is a new-build or extension, and any constraints on the site. Any usable floor space below ground level is classed as a basement. The entire space can be underground or, due to a sloping site, part of the space may be at ground level and part may be below ground.

The cost of land continues to rise and space on sites is increasingly constrained, especially in densely populated areas. It’s little surprise, therefore, that designs for new buildings, and extensions in particular, are as likely to consider going down into the ground as they are to go ‘up’ or ‘out’.

Where should thermal insulation be positioned in a basement structure?

Choosing whether to position insulation on the inside or outside of the basement structure depends on the material’s capability to resist the static pressure of the soil, and its tolerance of wet environments.

Enclosing the basement structure within the thermal envelope helps to reduce linear thermal bridging at junctions between construction elements, and achieves a greater level of building performance, comfort and energy efficiency. The superior load-bearing performance of extruded polystyrene (XPS) means it is capable of resisting the significant loads placed on insulation below basement floor slabs and external to basement walls.

In situations where insulation can only be positioned internally, particular attention must be paid to achieving continuity with the insulation in the above-ground external walls, to avoid a significant thermal bridge.

What Polyfoam XPS products are suitable for external basement insulation?

Extruded polystyrene (XPS) is tolerant of damp or wet environments, meaning it can be installed externally of the basement structure and waterproofing without risk of its thermal performance being compromised.

Polyfoam Floorboard Extra is thermally efficient and offers a compressive strength capable of resisting the weight of soil against it. In a limited number of scenarios, it may be necessary to consider Polyfoam Floorboard Super, with an even higher compressive strength.

Polyfoam XPS Ltd is a leading manufacturer of closed cell, extruded polystyrene insulation. The Polyfoam range is lightweight, strong, moisture resistant and easy to cut and shape, providing thermal performance and strength for the lifetime of the building. Product benefits include:

  • High compressive strength and protects the basement waterproofing from damage.
  • Highly resistant to water absorption.
  • Robust and long term dimensional stability.
  • Environmental Product Declaration.
  • BES 6001: Responsible Sourcing of Construction Products.

Recommended Polyfoam XPS products for basement applications are: