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Community and Q&A

Where to put the vapor barrier?

Mateo K. | Posted in General Questions on

I am in the Seattle area (climate Zone 4) and am going to be putting in an “addition” for a new utility room and I would like to double check the location of moisture control here’s the setup:

Slab above grade is the garage floor, which is 1-3″ below the top of the foundation walls. I plan on putting pressure treated scribed 2×12 floor joists down to 1) raise the level of the floor to the existing floor heigh in the rest of the house; and 2) allow for insulation to be put between the joist bays which would otherwise be open space.

I was thinking of putting 6 mil. barrier down on the concrete slab, then the PT joists, 1″ rigid foam (on the 6mil. barrier) between the joists and then unfaced fiberglass between the rigid foam and the subfloor (3/4″ plywood), over which a tile floor will be installed.

Does this sound Ok? I’ve seen a lot of contradictory advice when it comes to this!

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  1. David Meiland | | #1

    You might consider putting down poly followed by 2" or more of continuous XPS, then install 2x4 flat sleepers over that and finally your joists (now smaller than 2x12). Put XPS against the foundation also. I wouldn't put fiberglass in a floor system like this--if it ever has a chance to pick up moisture, the floor will be in trouble.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    I agree with David. Install several inches of dense rigid foam insulation (EPS or XPS), then sleepers. No fiberglass batts.

  3. User avatar
    Dana Dorsett | | #3

    Better yet, make it a step down- skip the sleepers, use t & g subfloor TapConned through the foam to the slab. 24" o.c.. With the foam fully supported by the slab and the subfloor fully supported by the foam (no span) you can even skimp on subfloor thickness, if you like, since it well flex FAR less than 3/4" ply that's only supported every 16".

    If you stick with the same-level/joist solution, use something fatter than 3/4" ply (or leave sufficient space to pour a thin slab on top of the ply) if you want the tile to hang in there for decades under the vibe of the laundry spin-cycle.

  4. David Meiland | | #4

    It sounds like the existing slab is not level and he wants the new floor to be level. I wouldn't pour anything over plywood, just screw/thinset CBU down and set tile on that, it will be fine if done correctly.

  5. Mateo K. | | #5

    Thanks for the feedback. I just want to make sure that the XPS won't collapse under the weight of the full washing machine about 8 inches away from a full 50 gal. water heater -- theoretically it shouldn't as the joists will be secured to the existing rim joist which is resting on the foundation wall.

    Also, with putting XPS on the concrete, then 2x4s on that -- I would not be securing the sleepers to the slab, or should I use tapcons to fasten them through the XPS?

    David is correct, the existing slab is not completely level and I want to new floor to be level with the rest of the house. Should I do 1/2 cement underlayment over 3/4 ply or use 1" ply as a subfloor?

    Thanks to everyone for their contributions.

  6. David Meiland | | #6

    You could use 2" + 1" XPS, 2x4 sleepers, and 6" tapcons.

    There are millions of floors with 3/4 subfloor over 16" O.C. joists, 1/4" cement board, and tile. 1-1/8" ply is designed for much larger spans, although I have used it on standard floors a few times. I definitely don't think you need it to keep the washing machine from breaking the tile off...

  7. User avatar
    Dana Dorsett | | #7

    I've used as much as 2.5-3"" of concrete (on the deep end) atop 3/4" plywood to level stuff up for tile. There's no down-side to it.

    Using sleepers concentrates the weight along the sleepers, more that quadrupling the psi on the foam. While the foam can certainly handle the dead-weighting of anything you put on it, that may not be the case from a dynamic weighting point of view, especially over years/decades. But you can drive a Caterpillar D9 over Type-II EPS with a 3" slab on top of it without cracking the slab or deforming the EPS. A 2x sleeper just isn't as rigid as a joist along the axis that matters. But concrete slabs are pretty good, (even relatively thin slabs.)

    The tile in my 1920s vintage bathroom is on a 3" slab poured on t & g planking supported by joists 16" o.c., and it'll be the last part of the house to come apart in an earthquake! :-) Tile on plywood on 16" o.c. joists may now be standard for new construction, but isn't really cutting it in 50 year time-frames, let alone the century mark. I've seen it crack in under 25 years, even without the vibe-factor of a spin cycle.

  8. David Meiland | | #8

    The down-side to it is the extra work and material involved. There are always ways to put more into something to make it beefier, but IMO it doesn't always make sense. I've been doing building and remodeling for close to 30 years and I'd have to say I've never repaired a tile floor where the washer sheared off tiles. Even if that problem were common, what's cheaper, resetting some loose tiles after a couple of decades, or pouring a slab to try to prevent it? And, what if the slab develops cracks and takes a few tiles with it?

    In a small area, the quickest thing for the OP *might* be to throw down 8" of XPS and then immediately pour a 4" slab over it. Cost of labor to make tapered joists vs. time saved by not.....

  9. Dave Hodges | | #9

    As far as the floor framing is concerned I think that sleepers and shims is a preferable way to go as opposed to scribing your joists. If you cut the PT then you would want to recoat the coat edge of the joist. Any opportunity to avoid using copper-green has go to be a benefit. IMHO ....

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