A Canadian study investigating the long-term performance of expanded polystyrene (EPS) in contact with the ground prompted EPS manufacturers in North America and Canada to claim that the insulation is suitable for use in the ground, similar to the performance of extruded polystyrene (XPS).
Subsequently, based on evidence of shingle failures, industry-sponsored studies have shown that the performance of XPS in real-world applications does not match laboratory testing, making EPS a premium material. While the XPS industry has refuted these results with their own research, it is interesting that the XPS manufacturers have shifted their focus from the low hygroscopicity observed in laboratory immersion and humid air conditions to the moisture diffusion properties of XPS.
Most XPS failures are due to difficult shingle installation conditions and the use of materials in combination with poor quality waterproofing membranes. There is evidence that XPS is better when there is no deliberate drainage around and under the insulation, providing more moisture resistance when in contact with the ground.
EPS perimeter insulation is traditionally installed with a backfill of drain material, polyethylene for foam protection, and drain pipes under the insulation. However, XPS is only installed using polyethylene membranes.
The composition of EPS and XPS insulation has changed over time, for example, the blowing agents of both materials have changed. In North America and Canada, XPS is currently manufactured without ozone depleting blowing agents, but this is not the case elsewhere. Some XPS products imported to New Zealand appear to have been made by cutting loose material rather than extruding it from the thickness of the leather. The cuticle on the XPS sheet slows down aging and is a major contributor to moisture retention.
BRANZ has tested an XPS product with a thermal conductivity of 0.036 W/mK. In contrast, the conductivity of carbon-filled polystyrene foam is significantly lower than this value. Most of the styrofoam produced in New Zealand contains recycled materials and can sometimes have a more porous structure.
To allow moisture to diffuse into the soil, ideally the foam should not be completely covered by a waterproof barrier. In winter, any moisture at the base of the wall will be forced out into the perimeter insulation, so it is best to use a steam barrier on the outside of the insulation. If this is not possible, the foam should penetrate into the ground, leaving only an impervious protective layer for above ground components.
As a general rule, the water content of the foundation should not be too high, so the main risk in retrofitting comes from the capillary effect when water enters between the insulation and the concrete. This can be avoided by using a capillary break (eg butyl tape) on the bottom edge of the insulator.
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Post time: Jul-25-2023