INSULATING BASEMENTS WITH EXTRUDED POLYSTYRENE (XPS)

Extruded polystyrene (XPS) can be used to insulate basement structures both externally and internally. The material’s unique combination of performance characteristics means it is particularly well-suited to insulating on the outside of below-ground structures. Choosing to insulate internally or externally depends whether a project is new-build or retrofit, and how continuity will be achieved with above-ground wall insulation.

Specifying thermal insulation should be done while also considering the recommendations of BS 8102:2022 Protection of below ground structures against water ingress. Code of practice. Extruded polystyrene is moisture tolerant, but it is not waterproof and should not be considered as contributing to any of the three protection types (barrier, structurally integral, and drained protection) outlined in the standard.

What are basements and below ground structures?

Any part of a building that is partially or wholly below ground must be protected so as to avoid the ingress of water. For the purposes of this blog post, we are focusing on basements – rooms, or whole storeys, below ground which require thermal insulation in order to comply with building regulations.

Other below ground structures include swimming pools, which we have separate posts about looking at regulatory requirements for swimming pools and typical swimming pool insulation thicknesses.

Civil engineering is a sector where below-ground construction features heavily. In some cases, insulation might be used, though not necessarily for reasons of thermal comfort. For example, expanded polystyrene (EPS) is sometimes used as a fill material. XPS is not typically used in civil engineering and is not part of the scope of this post.

Insulating a basement externally with XPS

For new-build basements, XPS can be installed on the external side of the structure. This arrangement takes advantage of extruded polystyrene’s robustness and low water absorption.

From a robustness perspective, the ground can be backfilled against the insulation with no detriment to its performance (it has no foil facings that can be damaged, for example).

And because XPS absorbs little water, even when the insulation is exposed to ground water it won’t absorb enough moisture to significantly alter its fundamental thermal performance. This certainty of performance is essential in an area where the insulation becomes inaccessible.

BS 8102:2022 highlights how installing insulation in this way can also help to act as protection for waterproofing measures such as liquid-applied membranes.

External insulation to basement walls may also continue around the basement footings and under the basement floor slab, providing a complete thermal envelope around the outside of the basement structure. From a thermal efficiency perspective, a continuous insulation envelope is essential. Externally insulating a basement arguably makes it easier to maintain that continuity to the above-ground structure.

The technical support page of the Polyfoam XPS website offers standard details to download. Our ‘typical’ basement detail shows external insulation to the basement walls, but internal insulation to the basement floor. This is because the structural loads transferred through basement structures into the ground can be substantial, and a structural engineer should be consulted accordingly.

XPS is capable of offering higher compressive strengths than many other insulation types, but care must be taken to specify the right grade for the anticipated loadings. For that reason, we have taken a cautious approach with our detail. Our technical helpdesk is happy to provide compressive strength data that structural engineers can use to make informed judgements about project-specific solutions.

Insulating a basement internally with XPS

Installing insulation to the inside of a basement structure places far fewer constraints on the specification.

The material doesn’t have to be moisture tolerant with low water absorption, because it is protected by the waterproofing. And it doesn’t have to bear the pressure exerted by the ground or structural loads – the insulation in the floor simply needs to be capable of bearing the loads imposed by the use of the room.

In many cases, therefore, insulation used in the interior of basements doesn’t need to be XPS. In fact, depending on the floor space available – especially when retrofitting existing structures – it may be preferable to select a material with a lower thermal conductivity so the insulation layer is thinner.

Again, however, decisions must be made taking into account how continuity of insulation will be achieved with the above-ground structure, among other good basement design practice.

About Polyfoam XPS

Compared to other commonly used rigid insulation boards, XPS offers a higher compressive strength, lower water absorption and better overall robustness, making it the ideal choice for basements, ground floors and inverted flat roofs in particular.

To discuss how XPS could be used in your project, contact us. If you’d like to know more about the insulation produced by Polyfoam XPS, learn how XPS is made or read about the steps Polyfoam XPS is taking to reduce waste from the factory.