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Community and Q&A

Choosing a Vapor Barrier for Slab-on-Grade Foundation

deerefan | Posted in General Questions on

Good afternoon all,

I am looking for advice on what vapor barrier to use under a subfloor. My house has a slab on grade foundation. In half, I will be putting hardwood floors from fallen pecan trees on our property. I plan on using 3/4″ advantech subfloor. I considered using Delta Fl, however, it would increase the height of my floor excessively, messing with transitions. The other option would be DMX, but I am a little concerned about how well this material seals around penetrations due to fasteners anchoring the Advantech to the concrete. I read some about Tremco Vulkem 350, which is a roll on product that would seem to seal around fasteners. Henry tech support recommended using Moistop 15, which is really for underslab use.  Now, I have no idea on what would be best. Any advice from experts on this forum would be greatly recommended. Thank you.

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  1. user-2310254 | | #1

    I'll give your post a bump.

  2. Expert Member
    PETER Engle | | #2

    Does your slab have a vapor barrier under it? If so, you don't need another VB on top of the slab. If there is no VB under the slab, you should put one on top. It really doesn't matter much which one. Fastener holes won't affect its performance much, as vapor emission is related to surface area. The surface area of the fastener holes is very small compared to the total slab area.

    Is the slab heated (radiant slab)? And, is there insulation under the slab? With any permeable floor covering installed over a slab, there is a risk of condensation when the room air is warm & humid and the slab is cool. The subslab soil temperatures are generally close to the mean outdoor temperatures. In GA, dewpoints above the annual mean temperature are common, making slab insulation a good idea for condensation control in addition to better comfort and energy savings. Energy savings might be a bit of a wash, as a cool slab decreases AC use but increases heat use.

  3. deerefan | | #3


    Thank you for your reply. Yes, we installed Stego wrap underneath the slab, 2 layers in most places to avoid rebar puncturs and I painstakingly taped off all the penetrations.

    I am in Texas and that is the first time that I hear someone not recommending any underlayment underneath the concrete floor. The slab is elevated in many areas as the house is built on sloped terrain. What about the potential of moisture/vapor migrating from the sides of the slab through it and into the flooring? I know this may only be an issue near the edges but still. Thank you again.

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