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Community and Q&A

Drying of under slab insulation

DanSilver | Posted in Green Building Techniques on

I am trying to understand moisture absorption of closed cell foam insulations.  I am currently starting to build a house  in Ontario and plan on insulating the under slab with an R20.  I can see that xps, eps and even spray foam have low moisture absorption qualities. What confuses me is drying.  EPS boosts that it dries more readily then XPS,  I see this over and over in many articles.  My question is how does the insulation dry under the ground? I would think that the only reason any of these insulations would get wet is due to hydraulic pressure forcing water into the products, with lack of air or any drying pressure how do they or especially EPS dry?  Can some one clarify this for me?

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  1. Tyler Keniston | | #1

    The 'drying' of EPS might be relevant to walls. Probably not under a slab. EPS is usually more permeable to vapor than XPS. I imagine that's all that's being said.

    Did any of the sources that mention EPS's ability to dry specifically mention sub-slab installations?

    Note that it will be a good idea to have a 'free draining' material (e.g. crushed stone) beneath your insulation, and poly on top of the insulation, prior to the pour.

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