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Basement insulation retrofit - walls versus floors

Here and elsewhere its commonly advised to insulate basement walls with foam panels and permeable walls. My understanding is that this allows water vapor to diffuse through insulation and wall assembly without the occurrence of condensation and related problems. I've not seen where installation of a vapor barrier between wall and foam board, whether plastic sheeting or waterproofing agents is recommended, at least not unless liquid water was entering through the walls.

It seems like the installation of a vapor permeable floor without any type of vapor barrier would allow water to diffuse freely preventing sequestration of moisture or condensation (just like the walls).

Why is it then that installation of a vapor barrier on top of the slab seems to be widely recommended when foam boards and flooring are being installed as an insulating floor?

Perhaps its because many install a sleeper flooring deck consisting of wood panels that act as vapor barriers?

I have installed laminate over 1" XPS in the past with demonstrated (6 year) success. I am considering doing the same in my basement with a water-resistant, vinyl-based laminate product . In this situation would I want to install a vapor barrier between slab and foam? I'm assuming that the vinyl flooring product would act as a vapor barrier.

Asked by Andy Alden
Posted Jan 19, 2013 3:12 PM ET


1 Answer

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There is no benefit to allowing the diffusion of water vapor from a damp concrete wall or a damp concrete floor into your house. I have consistently maintained that it's a very good thing that most rigid foams are vapor barriers. You don't want to invite moisture into your home.

Joe Lstiburek used to advise builders to install vapor-permeable insulation on the interior side of basement walls, but he recently admitted that he was wrong. For more information on this topic, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.

For information on the best way to insulate a basement wall, see How to Insulate a Basement Wall.

Answered by Martin Holladay
Posted Jan 19, 2013 5:37 PM ET

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