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Insulation and vapor management for walls against earth

Background: I have cement walls for a walkout basement and concrete slab on grade floor and I am trying to insulate without trapping moisture. I would like vapor permeable insulation and am trying to figure out how to both trap vapor from the outside earth outside, and prevent heat of the inside conditioned space from meeting the cold exterior surfaces. There is exterior drainage and some rigid foam outside the house protecting it but I once say wetness at the base of one wall. The floors are dry.

Floor: I don't want to have a vapor impermeable strate that, while protecting the flooring could create a greenhouse effect for rising vapor. Im am considering 1/4 inch rigid insulation with parallel slits cut into it to allow breathability, and no plastic sheeting. Then flooring on that, or boards and then flooring on them.

Walls: a contractor wants to put rigid against the concrete wall against the earth and seal with spray foam to protect from having warm conditioned air touching the cold cement. I don't want to trap moisture that could possibly seep in from the foundation into the walls. So Im thinking of build the framing out from the walls a few inches and sealing the back of it with rigid and using batts between framing 2x4s.

For both walls and floor im considering using a sealant but I understand these decay.

Please give me your best formula to address both problems of need to keep warm air from hitting cold exterior surfaces, and needing to allow inward vapor to breath without being trapped.

Also is there a hard flooring that is more breathable than ceramic tile?


Asked by Chris Campbell
Posted Mon, 02/03/2014 - 20:51
Edited Tue, 02/04/2014 - 08:10


1 Answer

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Before I provide recommendations for methods of insulating your floors and walls, we need to address a few fundamental misconceptions.

You write that you are worried about "trapping moisture," and that this worry is leading you to a preference for vapor-permeable building materials. You need to re-think these issues.

The soil under your basement slab is damp, and always will be. It's dirt. The soil on the exterior side of your basement walls is also damp, and always will be.

It's perfectly OK to install materials that are vapor-impermeable between the damp soil outside your basement and the interior. The whole point is to keep that dampness where it belongs -- on the exterior side of your slab or walls. If you made the building materials vapor-permeable, you would be encouraging moisture to enter your house. That's nuts. You need a barrier -- a barrier to separate the damp exterior from the dry interior.

If the soil is damp, it's not because the moisture is "trapped." It's because that's the way that God made dirt.

For more information on these issues, I suggest that you read How to Insulate a Basement Wall.

For information on how to install insulation on top of an existing basement slab, see Fixing a Wet Basement.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 02/04/2014 - 08:24

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