I had a prefab shed installed, which I'm finishing as a detached office. Since the floor got covered in drywall dust, it often looks like there's moisture seeping up between the floorboards. I'm planning to lay down a vapor barrier and laminate flooring. Should I be worried about this moisture?

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The floor is built on metal joists set on a concrete slab. I'm concerned that rain is flowing onto the slab. Is there a product I can use to seal around the bottom of the joists? I assume I shouldn't completely plug those rectangular holes, but I was thinking of covering them with wire mesh or something to keep critters out.

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2 Answers 2


I don't believe that's rain, just condensation. An uninsulated concrete pad whicks up moisture. Can't be stopped without alot of work.

Your best bet is to surround the outside perimeter with styrofoam insulation board, the thicker the better, up to 2". Then you can try sealing it as you stated. Make sure to leave a couple of those holes open for ventilation, or it'll get sweaty under there.

Back to the pad condensation, a good "college try" could be to dig down a foot or two vertically around the pad and insert 2" foam vertically, flush to ground level, the backfill. That won't stop condensation, but it will reduce it by breaking the frostline, which causes extreme temperature variations between the pad and the ground, and thus condensates.

Like I said, alot of work.

  • Is it even a problem that I need to worry about? Possibly relevant: I live at 2500' in southern Arizona, so there's no frostline. Commented Aug 9, 2017 at 2:43
  • Arizona? Well you should have started with that. No worries man, keep building. I live in Virginia, where moisture can be a real issue, so I approached from that angle.
    – NPM
    Commented Aug 10, 2017 at 1:08
  • Can you suggest anything to seal between the structure and the slab? Exterior caulk, or perhaps is there something like injectable mortar? Commented Sep 1, 2017 at 22:34
  • Short of bending a large z-shaped piece of flashing, just try some silicone caulking. Mortar caulks may not keep up with the expansion and contraction of the steel.
    – NPM
    Commented Sep 1, 2017 at 23:07

Yes, moisture coming up from under the slab is bad, but I doubt that darkened edge along the joint is from moisture coming up from below the slab.

You can test this with a moisture meter. Most flooring companies have them , which you may borrow.

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