The results of a Canadian study that examined the long-term performance of expanded polystyrene (EPS) in ground contact led EPS manufacturers in North America and Canada to claim the insulation material’s suitability for ground use, similar to the properties of extruded polystyrene (XPS). 

Subsequently, based on evidence of in-service failures with roof decks, industry-sponsored research claimed that XPS’ performance in practical application did not match laboratory tests, which made EPS the superior material. While the XPS industry has countered these research findings with studies of their own, it’s interesting that XPS manufacturers have shifted focus from low moisture uptake, as observed with the laboratory immersion and moist air testing regimes, to the moisture vapour diffusion properties of XPS.

Instances of XPS failure are mostly related to complicated roof deck situations and the material’s use in conjunction with inferior waterproof membranes. Evidence points to the superiority of XPS in offering greater resistance to moisture uptake in ground contact when there is no deliberate provision for drainage around and under the insulation.

EPS perimeter insulation was traditionally installed in conjunction with drainage material backfill, polyethylene to protect the foam and a drain below the insulation. However, XPS is installed only with the polyethylene membrane.

The make-up of EPS and XPS insulation materials has evolved with time; blowing agents, for instance, have changed for both materials. In North America and Canada, XPS is now made without ozone-depleting blowing agents but this is not the case elsewhere. Some of the XPS products being imported into New Zealand appear to be made from slicing down of bulk stock rather than extruding at thickness with skins. The skins on XPS sheets slow down the ageing and are a major contributor to moisture properties.

BRANZ has tested an XPS product with a thermal conductivity of 0.036 W/mK. In contrast, the carbon-infused EPS can have a conductivity significantly lower than that value. A significant part of the EPS manufactured in New Zealand includes recycled content and the texture can sometimes be more porous.

So that moisture is allowed to diffuse out into the soil, the foam should ideally not be fully encapsulated in waterproof protection. During winter any moisture in the wall footing will be driven outward into the perimeter insulation so it’s best to use vapour-permeable protection for the outside of the insulation. If that is not possible, the foam should extend below ground, only leaving the above-ground component with the moisture-impermeable protection layer.

In general, the footing should not have very high moisture content; therefore, the main risk in a retrofit situation comes from capillary action carrying water up between the insulation and concrete. This can be prevented by using a capillary break such as a butyl strip at the bottom edge of the insulation.