Vapor barrier under shed floor

Question

I'm purchasing a shed that I intend to use as a home gym. On delivery, the shed will be uninsulated, aside from the roof which is metal with a bubble underlayment. The floor is a single layer of plywood, so I will likely beef that up with an additional course of plywood/OSB to accommodate the weight of equipment.

I'm going to doing some site prep this week, and plan on putting down a bed of clear gravel. The shed will sit on part of an asphalt drive, and part lawn, so I play on making a level bed of gravel. We're in Ontario, so climate sees a lot of snow and has relatively warm summer. I will probably end up insulating the rest of the shed in the future. It will not be continually heated or AC'ed, but probably will have a bit of heating/cooling it's in use and we're exercising in it during certain seasons.

My question is about vapor barrier/house wrap for the floor. The layers of the shed from added floor to ground will be:

• A. added floor (plywood or OSB).
• B. "factory" floor (SPF plywood, assuming pressure treated).
• C. sleepers the shed sits on, this will be a 4-6" air gap.
• D. gravel
• E. bare ground/pavement

If I was going to add vapour barrier (if advisable), between which layers would I situate it? Getting under the shed before deliver to put a vapour barrier been B&C is unlikely.

Is a moisture transmitting house wrap thing (like Tyvek) more appropriate for this use case?

Answer 1

This is a subjective question since it's outside the realm of standard building envelope scenarios. I've built two such sheds with similar structure spanning over 20 years. Both had house wrap between the floor framing and the untreated plywood with the intention of minimizing moisture coming up from underneath. They did fine, with the only degradation being where wind-driven rain slipped past the roll-up door.

Your situation is a bit different, however. My advice is to not bother. House wrap is intended as a breathable liquid water barrier. It doesn't do much at all sandwiched between sheets of subfloor, and your treated plywood layer does at least as much to stifle moisture transfer anyway.

I suggest a couple coats of polyurethane on your floor, assuming you don't have plans to install finish flooring. This will obviously prevent stains, but will also provide stability to the strands in the OSB if you go that route.

Answer 2

I'm in the exact same situation. I put some Kimberly Clark Block It wrap between the factory floor and the 3/4 inch tongue and groove subfloor. I set the printed side facing down toward the elements. However the original floor was sagging between the sleepers so that had to be addressed before the wrap went down. I think installing it might be overkill but can do no harm.