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Vapor-permeable flooring materials?

tannerc | Posted in General Questions on

Hi all,
I would like to bother you with yet another question regarding the floor assembly for a tiny house on wheels. Hopefully the answers will be relevant for others as well.

( Skip to question below if uninterested in THOW)
My existing subfloor construction from bottom (of trailer) to top: large sheets of galvanized steel sealed together with urethane to form a continuous pan (should be an air and vapor barrier unless I cut holes in it), 1.5 in polyiso, rockwool filling 2 by 6 joist bays, 3/4 inch ply. The steel trailer (iron eagle) cross members are below the steel pan and the floor framing sits on top of the polyiso though is supported by a rim joist. My insulation, not accounting for joist R, should be about R32.

Assuming that the base of my floor assembly is a vapor barrier, I would think that it’s a good idea to install a vapor permeable flooring over top my plywood to allow inward drying if necessary.

I like the look and durability of faux wood adhered vinyl flooring but I’m guessing that it would form a second vapor barrier. Does anyone have suggestions for a vapor permeable flooring option? I would prefer not to carpet.

Thanks again!

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  1. tannerc | | #1

    Here's a mid-construction photo to show some of the assembly

  2. GBA Editor
    Martin Holladay | | #2

    You're right about vinyl flooring. It's a vapor barrier.

    Ordinary 1x4 or 1x6 pine boards (tongue-and-groove) are vapor-permeable. Leave them unfinished for the farmhouse look, or use a simple oil finish (not too thick) if vapor permeance matters.

    By the way, why does vapor permeance matter?

    1. tannerc | | #3

      Thanks Martin. My thinking, or rather my interpretation of what I've read on the site, is that there should not be a double vapor barrier. I think that it is very unlikely that water will get into the subfloor assembly in the first place but if it's easy to choose a vapor permeable flooring then maybe it's the right choice? I am not set on any particular solution and recognize my hunch could be completely off. Would love to hear your thoughts on the matter!

  3. GBA Editor
    Martin Holladay | | #4

    Moisture is unlikely to enter this assembly from the exterior. The most likely way for moisture to enter the assembly is when you spill a drink or break a bottle of wine. If you go with vinyl flooring, it will limit moisture entry from that type of spill.

    1. tannerc | | #5

      Thanks Martin I was thinking of that exact conundrum. I will go with the vinyl and report back after the house raising cocktail party. Stop by if you find yourself in Northern California!

      1. severaltypesofnerd | | #6

        Got cabinets?
        Don't extend the vinyl under the cabinets, and you can expect some limited slow vapor exchange there.

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