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Community and Q&A

Insulating an Existing Slab

Zins | Posted in General Questions on

Looking to insulate the slab of an 1950s home as part of large scale energy retrofit. Zone 5, decently drained lot but with clay soil that retains moisture. The rest of the home will be well sealed and insulated with new windows.

The floors aren’t coming up, so foam or barrier under isn’t an option. There appears to be some thin plastic barrier in place under the sub floor.

All that said:

1.) After digging away from the foundation I’ll apply a membrane to the foundation wall to keep moisture out.
Recommendations on a product? I’ll only be able to get the wall so clean, I worry about proper adhesion. 

2.) Should I aim for 2′ or 4′ down? 4′ is a lot more work, is the extra gain worth it?

3.) Planning to put 2.5″ EPS on. What’s the best method? Furring strips sunk into the slab? Direct anchor into the concrete/block? Adhesive? I’ve read of people using all methods.

I’ll seal the seam between the footer and the slab well, before any of this. Will cover any exposed EPS with metal.


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  1. Expert Member
    Michael Maines | | #1

    Hydrostatic pressure increases with depth, so if you're trying to reduce water infiltration, you should dig all the way down to the footings. Then you can also install good perimeter drains. If you've gone that far, you might as well get the foundation clean enough to install a fluid-applied membrane, which may be rolled but is usually sprayed on. Then backfill with a freely-draining material such as sand or crushed stone.

    If you want to use a self-adhered membrane, Grace, Henry, Delta and others all have products made for that purpose. I don't have direct experience but would trust any of those three brands.

    I'm jumping the gun a bit--it's not clear from your description but I assume you have a full basement? If you're trying to reduce water infiltration, you should dig all the way down to the footings. If you don't want to do that, I wouldn't bother doing anything on the exterior, except perhaps ground gutters.

    I don't understand where you are proposing to place EPS. On the interior of the walls? On the basement or crawlspace slab?

    You mention plastic under the subfloor--do you mean at the main floor level, or do you mean under an existing slab at the basement or crawl space?

  2. Zins | | #2

    Thanks, Michael. I should have been more clear, there's no basement. It's a slab home. The goal is to limit cold and moisture infiltration into the slab. There area a few portions of the foundation I won't be able to dig out around, but 90% of it should be possible. And I intend to install drainage and backfill with crushed stone. Should have said that too.

    The intention of the EPS is to insulate the slab.

    Any further thoughts based on that?

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