I'm renovating a slab on grade half bath. I'm concerned about moisture/condensation forming under the joists. Should I seal/paint the slab? If so, what product should I use? Most of the products I've seen are for sealing basement walls. The house was built 50 yrs. ago in Maryland.

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  • Are you replacing the floor framing? It's done ok for 50 years. Why the concern now? Do you see rot? – isherwood Dec 30 '15 at 18:18
  • @isherwood Thanks...I'm leaving the floor framing. I'll sister them when I install the new subfloor. The issue is that the bathroom is cold in winter/hot in summer because the walls aren't insulated well. For some reason, they didn't put plywood sheathing and tar paper between the studs and the clapboards. I can see the backside of the clapboards. So, the structure is not very tight. I'm going to air seal and add insulation. I think that it lasted for 50 yrs in decent shape b/c there was a lot of air flow to dry things out. If I tighten things up, I'm afraid that condensation will get trapped. – Laxmidi Dec 31 '15 at 17:31
  • You're right to have the concern. What's on either side, can you make underfloor vents that exit to adjoining spaces? – Bryce Jan 1 '16 at 5:57
  • @Bryce Thank you for your comment. The ductwork ends in the wall opposite the plumbing fixtures. It serves the vent in the bathroom. The ductwork doesn't extend past this room. Is your idea to punch a hole in the side of the duct and have it heat the slab? – Laxmidi Jan 21 '16 at 17:09
  • @Laxmidi possibly leaking some air from the ductwork would help. Just ensure that any space has a way to vent, or a least one permeable side so it can slowly dry out. – Bryce Jan 22 '16 at 6:04

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