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Community and Q&A

Floor Assembly for Garage Conversion

paulmagnuscalabro | Posted in General Questions on

I have a poorly-built garage + shop building on the property that I’m trying to turn into a comfortable home gym with bathroom, office space above, with a wood shop in the garage bay (will be completely isolated from the rest of the interior space, maybe one exterior door from wood shop to gym area).

The overall challenge is to make this space nice enough that I will spend time there, while spending as little money as possible (I know… harder to do than say. I want to do things right, without going overboard. Doing everything myself). There are a million challenges to unpack, but one I’m grappling with now is how to build up the first floor in the home gym / bathroom area.

I’d like to prevent water from below the slab from migrating up into the living space. I am also concerned that once I air seal and insulate the building, radon could become a problem. I plan to sink a PVC pipe into a gravel bed, pour around it, and run the PVC out the roof. Can add a fan later if needed.

The assembly I’m thinking of is:
–          Finish floor (TBD – might go with ¾” thick horse stall mats as gym flooring, hardi + tile for bathroom) over
–          ¾” Advantech subfloor over
–          ¾” Advantech subfloor (so, two layers total, screwed together, staggered seams) over
–          1” – 1 ½” rigid foam (NGX is readily available) over
–          Vapor (air?) barrier over
–          Concrete slab (variable thickness, mostly 4-5”) over
–          Dirt & gravel, roughly 6”, over
–          Another concrete slab, unknown thickness
(I can guarantee that there is no vapor barrier or insulation anywhere under any of the slabs. Why there are slabs on slabs, I can’t answer. The “why” regarding the decisions made when this thing was built are a total mystery box. In some areas there are three slabs, each separated by about 6” of gravel).

The uppermost slab has some cracks. It’s mostly in good shape other than that. It is filthy and has had various finishes applied to it / worn off over the years. Space will be conditioned (eventually). Climate Zone 6B, just outside Bozeman, Montana. No water problems in the garage currently (though wind moves right through the building, so any water that made its way through the slab would evaporate before I noticed).

Option 1: Put down a layer of poly
I like this idea because it is stupid-simple and inexpensive. Am I correct in thinking I’d have to tape the seams, and would also need to tape the perimeter (maybe wrap the poly up to the bottom plate of the walls, and tape it there)? 10mm? Thicker?
Downsides are that there’s an excellent chance I’ll inadvertently put some holes in the poly.
This would be my preference, if it’s not a dumb idea.

Option 2: Use something liquid-applied over the concrete
Probably a more robust solution than putting down poly. Likely more idiot-proof too, especially at the perimeter? Not sure what product would be best. Epoxy or RedGard come to mind.
Downsides are that I really don’t want to do much/any surface prep on the existing slab. Also, this would be way more expensive than poly.

The slab(s) is not level, but I’m not sure I’m bothered enough by it to try leveling it. If I do level it, I’ll put down 2x sleepers on the flat and level those. Rigid 1 ½” insulation between the sleepers, double layer of subfloor on top. If I tapcon the sleepers to the slab, obviously I’ll be putting a bunch of holes in whatever membrane I’ve put down. Not sure if that’s a big deal?

Thoughts? Am I way overthinking this?

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  1. Expert Member
    Michael Maines | | #1

    I would use a heavy-duty membrane such as 10-mil Stego Vapor Wrap. But fluid-applied or cheap 6-mil poly would probably work as well. I would take the time to level the floor using sleepers. Screw holes through the membrane won't hurt anything.

    For what it's worth, despite marketing claims, NGX is still one of the worst materials you can use in construction. EPS has significantly lower carbon emissions.

    1. paulmagnuscalabro | | #2

      Noted, thanks Michael. I'm sure I can source EPS somewhere out here. I'm trying to make product decisions based on what's least-environmentally damaging, so that's good to know.

      Honestly, I just had some NGX sitting around from old jobs, and the price at the big box store was reasonable when I checked last week. The first Google result was this FineHomebuilding article:
      Which made it seem like a decent alternative - but then, the article doesn't offer a direct comparison between foam products, so I guess without context it didn't tell me much.

  2. Expert Member
    KOHTA UENO | | #3

    Hi Paul! Just checking you're the same one I email with for the day job, right?

    Don't know if you have a Pro subscription, but this column from Martin goes into this assembly in great detail:

    Installing Rigid Foam Above a Concrete Slab
    You can sandwich rigid foam between an existing concrete slab and new plywood or OSB subflooring

    As clarification--you're talking two layers of Advantech, not three, correct? The way you wrote your callout was a bit ambiguous.

    Also, I would put the polyethylene *above* the rigid foam--analogous to how you set up a slab on grade.

    1. maine_tyler | | #4

      Hi Kohta,
      Can you elaborate on the reason to put the poly above the foam? There seems to be a wide variety of opinions on this. I understand the order when beneath a slab (tight to the slab) but have a harder time understanding what advantages/disatvantages there are with the two options in this type of assembly.

      Is the poly above to keep interior moisture from condensing on the concrete? What about taping the foam seems?

      1. Expert Member
        KOHTA UENO | | #5

        Hi Tyler!

        First--just so we're not talking past each other, swapping those two components was assuming a polyethylene plastic vapor barrier is part of the layup. If a vapor barrier is being included, it should be inboard of the foam. This is both consistent with previous discussions on concrete slab floors, BSI-001, and traditional guidance of "keeping the vapor barrier on the warm side of the assembly in winter."

        BSI-001: The Perfect Wall

        The next question would be: am I okay omitting a polyethylene plastic vapor barrier? The answer is a qualified yes: *if* you can make a decent air barrier out of taping the foam seams, and the foam is vapor closed enough (e.g., 2" XPS ~ 0.5 perms = Class II vapor retarder), yeah, I can live with skipping the layer of plastic. Take a look at the example assemblies I posted in the comments in Martin's column--they were examples where I skipped a separate vapor barrier.

        1. maine_tyler | | #7

          Thanks for the reply Kohta.

          When I referred to taped foam, I actually just meant as redundancy to keep interior moisture from contacting the poly if placed beneath the foam. My take on what you are saying is that it is indeed interior moisture that we should be concerned with and what dictates the placement of the poly in this assembly. (?)

          I struggle with that a little given the specifics of a slab in direct contact with a moisture reservoir (such as dense soil, no poly) vs a roof or wall that is open to ambient air conditions (the perfect walls). Can we safely assume the vapor drive is seldom enough inwards vs outwards? Does contact with the soil not change the equation? If the concrete reaches 100RH, will the temperature profile still keep the vapor drive from in to out?

          I also get confused by the 'walk the plank' issue of adhered vapor barriers.

          It sounds to me like Joe has a concern with moisture laden air pockets existing beneath a vapor barrier in a floor assembly such as this. If the poly goes above the foam, it seems the air pocket is effectively even larger. And then any screws that poke out through the double layer of subfloor would puncture the poly which-- while not a vapor diffusion concern-- could present an air leakage concern it would seem.

          I am sure I am overthinking this or perhaps not approaching the issue correctly. I imagine most assemblies would survive either stack-up. I have just found this issue particularly hard to make sense of and be confident in a given layering.

    2. paulmagnuscalabro | | #6

      Hi Kohta! Yep, same Paul.
      I don't have a Pro subscription, but it's probably about time I got one - thanks for linking the article.

      Correct, two layers of Advantech total (original post edited for clarity).

      And thanks for clarifying on Tyler's question below. I'll definitely be using a poly vapor barrier (above the foam) - just seems easier to detail than taping foam seams.

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