Talk:Insulation and Condensation

Water retention in insulation.

Extruded polystyrene (XPS) is waterproof, it has minimal water retention like 0.001% or something. Expanded polystyrene (EPS) and the popular PIR sheets will retain moisture as described in the main article.

But XPS and EPS do not insulate as well as PIR, they need to be 20-30% thicker sheets to get the same u-value.

And XPS and EPS don't come with a silver vapor barrier glued to each side, so it's a cheaper item to make. That means the cost is similar between XPS and PIR for the same level of insulation.

Without prejudice, some designs may be better implemented in XPS rather than PIR, for example where there is a lot of penetration by services that would make the vapour barrier difficult to seal. Or a very wet outside wall that allows a lot of rain penetration and you don't want to render it.