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.

Goal:
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 subfloorover
–¾” Advantech subfloor (two layers, screwed together, staggered seams)over
–1” – 1 ½” rigid foam (NGX is readily available)over
–Vapor (air?) barrierover
–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 anexcellentchance 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?