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Precast concrete basement walls

We are in the planning phase of a passive house near Louisville, Ky. The site is a South facing hill with a great view of fields, trees and a creek. We plan on a walkout basement under a 2300 square foot home. I am concerned about moisture through the wall of the basement. Some precast concrete wall providers guarantee water tight walls with insulation on the inside. If they are used what precautions should be taken relative to the floors and vapor pressure to the inside? I presume the vapor barrier would run under the wall and over the insulation of the floor directly under the concrete floor. Thanks PJN

Asked by Paul Nicholls
Posted Tue, 07/22/2014 - 08:15

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5 Answers

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Paul,
Q. "If they are used what precautions should be taken relative to the floors and vapor pressure to the inside?"

A. I'm not sure whether I understand your question. Concerning "precautions relative to the floors," the floor slab should be designed like any other floor slab. The best basement floor slabs include a continuous horizontal layer of rigid foam insulation under the slab, and also include a crushed stone layer connected to a passive radon mitigation system. If these concepts are new to you, you might want to read these articles:

Polyethylene Under Concrete Slabs

All About Radon

Concerning the second half of your question -- "What precautions should be taken relative to the vapor pressure to the inside?" -- I'm not sure what you mean. Are you worried about vapor diffusion from the damp soil to the interior of your home through your basement walls and basement slab?

If that's what you are worried about, the details to control vapor diffusion are simple. For your slab, you should include a layer of polyethylene directly under the concrete; that controls vapor diffusion.

For your walls, you should include a continuous layer of rigid foam insulation or closed-cell spray foam insulation. Either of these products will do a good job of controlling vapor diffusion. For more information, see How to Insulate a Basement Wall.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 07/22/2014 - 11:59

2.
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To clarify my question. A provider of precast concrete walls to be used in a basement claims the wall to be water tight. Does that mean I don't have to worry about capillary action? I don't have to have a vapor barrier for the wall? Dan I treat it like an insulated wall above ground with an air tight barrier like a Zip wall? Thanks PJN

Answered by Paul Nicholls
Posted Thu, 07/24/2014 - 12:08

3.
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Paul,
Even if the precast concrete wall is watertight, that doesn't mean that it is vapor-impermeable.

Every basement wall needs insulation. Your choices include rigid foam insulation and closed-cell spray foam insulation (either on the interior or the exterior of the wall).

If you include foam insulation, and you should, the foam is your vapor retarder.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 07/24/2014 - 12:16

4.
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I presume from your response that I would run the poly vapor barrier from the outside of the precast wall underneath it and up above the insulation under the floor and thus just beneath the concrete floor. Thanks for your thoughts. PJN

Answered by Paul Nicholls
Posted Thu, 07/24/2014 - 12:20

5.
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Paul,
Your precast concrete wall does not need a polyethylene vapor barrier.

I advise you instead to install rigid foam or spray foam insulation on either the interior or exterior side of your concrete wall; the foam acts both as insulation and a vapor retarder.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 07/25/2014 - 07:28

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