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Vapor Control for Tile Floor on Sleepers

canada_deck | Posted in General Questions on

I’m in the Pacific Northwest.  Previous owner had converted a garage to a living space.  I’ve only done some poking around but they appear to have done a fairly reasonable job.  6 mil poly on the concrete, sill plate foam, untreated 2*4s (standing 4″ tall), and then 1/2″ plywood.  On top of that is underlay and carpet.  The 4″ space underneath the floor is not ventilated or vented in any way.   I need to do some more investigation but so far, it looks nice and dry.

We are going to be taking out the carpet, adding some reinforcement to the floor, then Ditra uncoupling membrane, and then tile.

My concern is that perhaps the plywood/carpet is currently allowing a way out for any moisture in that subfloor area and I might trap it in if I go with tile.  That space will be trapped between two vapor barriers.

1) Does that sound like a reasonable concern?  Does the plywood/underlay/carpet actually provide much of a path for vapor?

2) If yes, what would you propose as a solution?  Ideally something that doesn’t involve tearing out the current plywood because that’s basically a full rebuild.  It’s a little tricky because the sleepers means that the floor is actually a large collection of smaller compartments.

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  1. Jon_R | | #1

    Very little moisture will move up through the 6 mil poly. Admittedly, even less moves up through the Ditra. So I'd measure the cavity % RH and if/when there is an accumulation problem, provide a little bit of forced ventilation. Pay attention to the dew points (ie, make sure the ventilation supply air is actually drying).

    > floor is actually a large collection of smaller compartments

    Yes, but given the tiny amount of moisture involved, the sleepers aren't air/vapor tight enough to prevent adequate cross drying.

    1. canada_deck | | #2

      Thanks. Where would you recommend the forced air originates from and goes to? Should I be looking to provide cross ventilation across the sub-floor area (e.g. an inlet and exhaust vent on opposite sides of the building with a fan running in one of them - but circulating outside air?) Or should I be looking to push some air from the conditioned space of the house through that subfloor (and then back into the house or out to the world)? There are periods here in the winter when the air is extremely moist (the lawn grows mushrooms, nothing dries out, etc.) so I'm thinking the second option is probably best.

      I wonder if I have to start thinking about radon as well.

      1. Jon_R | | #3

        It depends on dew points. But typically, interior air (as a source) is dry enough to work if the space isn't too cool. Returning interior air to the interior is more energy efficient.

        Similar concepts apply to sealed crawlspaces - sometimes (eg, when the windows in the floor above are open) the exhaust fan put in to dry the crawlspace makes it wetter. Dehumidifiers don't suffer from this.

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