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Community and Q&A

Uninsulated slab and hardwood flooring

BG9999 | Posted in Energy Efficiency and Durability on

I’m renovating a small outbuilding on a farm (15′ x 15′), which has slab on grade foundation. The slab is sitting on crushed stone but has no vapor barrier underneath. I have put a 10 mil poly sheet on top of the slab, and 2×4 joists (sleepers) on top of the poly sheet.

My plan was to now spray foam insulate (about 2″ – 3″) on top of the poly sheet, then install subfloor and then hardwood.

However, before spray foaming, I can see significant water pooling under the poly sheet. Probably to be expected, but somewhat surprised at how visible. With these details, is the original plan (spray foam over poly) sensible — (with the goal being to thwart the entry of all water/vapor)? Or, is another vapor permeable approach a better way to go? Or dimple plastic? Any advice welcome.

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  1. GBA Editor
    Martin Holladay | | #1

    Considering the details you have shared, it is not true that it is "to be expected" that you "can see significant water pooling under the polyethylene sheet." Something is unusual.

    The first question to ask is, "Where is the water coming from?" My guess is that this building isn't well drained -- perhaps the grade around the perimeter of the building is too high, or perhaps the grade isn't properly sloped away from the building. But this is a guess -- a site visit would be necessary to figure out what's going on.

  2. BG9999 | | #2

    Many thanks. The area uphill from the structure (about 30' away) has always been quite wet--there must be an underground spring somewhere nearby. So a couple of weeks ago I installed French drains around the building in anticipation of a possible water problem. The structure is also next to a large barn with a 60 square roof, directing rain water toward it. To deal with this, next week I am scheduled to install a gutter system for the barn as well as the new structure, (the underground drains are already installed). This should alleviate the water issue to some extent. But I am not confident it will fully mitigate the problem.

    If it doesn't, is it reasonable to spray foam insulate over the poly, creating an insulated, impermeable barrier, with the expectation that the water being soaked up by the slab will slowly vent through the edges of the slab, which are above grade and untreated?

  3. GBA Editor
    Martin Holladay | | #3

    If I were you, I would implement all of the drainage projects you listed, and then I would remove the sleepers and polyethylene to see of the slab dries out. I wouldn't go forward with the project if you can see pools of water under the polyethylene.

  4. BG9999 | | #4

    Agreed. Will follow this approach. Many thanks.

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