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Community and Q&A

Where to Put the Vapor Retarder in a Concrete Slab

Renovate101 | Posted in General Questions on

I have an existing basement floor with a 2 car garage under the house. The floor appears solid except for 5 feet in from the garage doors where cracks have developed due to water. In this location, It also sounds hollow when tapped with a metal pry bar in this area. The rest of the basement floor sound solid with no cracks. I’ve received a quote to pour a new 4” concrete slab above the existing floor about 20 feet in from the garage doors with a vapor barrier between the slabs and a perimeter drain. My first question is should a vapor barrier be placed between the old slab and new slab? Should we dig up the damaged area first (5’ in from doors) before pouring a new slab? I do plan to prevent any water from outside coming into basement by installing gutters and using exterior foundation perimeter channel drains. I don’t plan on refinishing the basement to a living space. Thanks!

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  1. Expert Member
    Michael Maines | | #1

    Renovate101, it would be a good idea to remove the concrete where there is likely subsidence below the slab, so you don't have the same problem with the new pour. That would also give you a chance to see if there is a vapor retarder under the existing slab. There probably is not, in which case you should add one under the new slab.

    Alternatively, there is a technique of injecting spray foam under the slab to fill voids. I would rather see for myself what's going on by removing the slab, but structurally there should be no problem with closed cell foam under a slab, and it might save you money. Including a vapor retarder is a good idea but few older homes have them, and they are still often omitted from garages, though they should always be included in new construction.

    Subsidence under slabs is sometimes due to poorly compacted soil, but more often it's from water running under the slab, so prioritize fixing water problems before attempting a repair.

    1. Renovate101 | | #3

      Thank you Mike for the sound advice and feedback! I spoke with the contractor today and they agreed it would be best to dig up and replace the 5’ Section that appears to have voids underneath. We will then add a vapor barrier below and perimeter drains before pouring the new 4” thick slabs. Thank you again!

  2. GBA Editor
    Kiley Jacques | | #2

    Martin Holladay provides pertinent information in this article:
    Polyethylene Under Concrete Slabs

  3. Renovate101 | | #4

    Hi Kiley,
    Great resource, thank you for sharing!

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