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Community and Q&A

Should I install a vapor barrier under a LVT (Luxury Vinyl Tile) flooring material?

user-4341253 | Posted in GBA Pro Help on

Existing home has an inordinate amount of moisture/water under the raised wood floor (two-story SFD structure).

Realizing that the moisture should be mitigated by possibly at least installing an impermeable moisture barrier at dirt, I am wondering (assuming owners do not elect to install mitigation in crawl space) if the floor assembly should be as follows?

Existing Plywood Subfloor
Vapor Barrier (6 mil poly)
Plywood Underlayment
Floating Click system LVT

Our flooring contractor mistakenly installed a particle board underlayment and used the Glue Down version of the LVT which is made by Moduelo. The PB appears to have swelled at the seams projecting through the finished flooring material. In my research Modeulo does not refer to any type of vapor barrier material as used in other products (foam roll) and refers us to Resilient Floor Covering Institute.

Link to product:

I would like to verify if the GBA Pros here agree some type of vapor barrier should be used within the floor system. Help…Bill

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  1. GBA Editor
    Martin Holladay | | #1

    You describe this house as having a "raised wood floor." What does that mean? What type of foundation is this? Is it a crawl space foundation? Or is it a house with a post foundation? Or perhaps is this a house with floor joists above a slab on grade?

  2. user-4341253 | | #2

    Martin it is a perimeter concrete foundation with crawlspace below. Floor system is typical joists supported by girders and pkywoodnsubfloor. 1970's era tract . We are in SF Bay Area Climate so very mild. This home is practically built on top of the Bay. Moisture permestes up through the ground. This is a common problem in this development. Thanks much for your replies.

  3. charlie_sullivan | | #3

    It sounds like a bad situation either way. You have excessive moisture in the crawlspace. With only vapor permeable flooring materials, the moisture can get into the flooring materials, but at least it can dry to the inside of the house. That increases the humidity inside the house, which is bad, but it's sort of tolerable for the floor. With a vapor barrier in the floor, everything below it can get damp from crawlspace moisture, but things above are protected.

    So as it stands, everything below the tile can get damp. That's causing trouble with the particle board. If you had a vapor barrier as listed above, only the plywood below it could get damp. That's perhaps better, because fewer things are damp, and the plywood tolerates moisture better than the particle board. But it doesn't seem like a very good solution. So that brings us to what you already knew: that fixing the crawlspace is the right solution.

  4. GBA Editor
    Martin Holladay | | #4

    You have to follow the flooring manufacturer's instructions regarding acceptable substrates and installation methods, of course.

    The usual way to separate a damp crawl space from a wood-framed floor assembly is to install a continuous layer of foil-faced polyisocyanurate on the underside of the floor joists. All seams should be taped with a compatible tape, and the perimeter should be sealed with canned spray foam or high-quality tape. Of course, all penetrations should also be sealed to prevent air infiltration.

    In addition, you will want to install a polyethylene vapor barrier on the dirt floor, held in place with bricks or rocks.

  5. user-4341253 | | #5

    Thanks all for the replies. After working the flooring MFR and your great feedback I have heavily recommended to our client to have the water issue mitigated and that installing a vapor barrier under the flooring material is in consequential and may exasperate the build up of moisture under the sub-floor. Thanks again and looking FWD to participating. Cheers, Bill

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