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Community and Q&A

Protecting Subfloor over Crawlspace from Moisture

etheric42 | Posted in General Questions on

We have a 2-story addition over pier-and-beam that we have just built attached to an existing 1-story slab foundation home.

We’re nearing completion (foundation, framing, roof, drywall, rough electric and plumbing are all in) and I recently started asking about the crawl space (I’m the owner, I have an architect, engineer, and contractor on the job).

As far as I understand it, there’s going to be a concrete skirt with vents, a loose-laid moisture retarder on the ground, a continuous layer of Tyvek under the beams, batts between the beams, and then the wood subfloor with taped seams, leveling quick-set if any needed, silicone paper, cork flooring.

Some additional facts:
* We’re in Austin, Texas
* The expansion is over a flood zone (500 year per FEMA, 100 year per city)
* FFE is between 2-4 foot above grade depending on where you are measuring and at all points at least 3 feet over the floodplain
* Because this is over a flood zone we cannot fully encapsulate (we need leave inlets and outlets for floodwater)
* We’ve lived in this home for 15 years and during the worst flood the water was still about a foot in height and 8 feet and distance away from where, but 100-year floods happen sometimes

So what should we do to prevent this from turning into a mold factory?  Continuous sheath polyiso under the beams?

Thanks in advance!

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  1. Expert Member
    Akos | | #1
  2. etheric42 | | #2

    That figure doesn't show up for some reason. 403, no access. The rest of the article and most (but not all) of the other images show up.

  3. etheric42 | | #3

    Okay, very good article. I'll forward that to the contractor now. We'll need to especially take care where the slab foundation meets the floor framing as the concrete is permeable. In fact where the wooden beams meet the concrete they are attached via hangers bolted into the concrete... wouldn't they just wick up moisture from the slab and transmit it past any barrier I put up?

    I'm also starting to wonder if I should have worried about the walls previously.
    hardie->tyvek->plywood->studs and unfaced batts->drywall (and those are all finished).

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