I have enclosed a screen porch with airflow underneath into a 3-season room. I have installed OSB over the existing pressure-treated decking, with the intention of installing ceramic tile over the OSB. I have been advised that I should have installed a vapor barrier over the pressure-treated decking and under the OSB, which I did not do. Do I need a vapor barrier, and if so, can I attach 6 mil plastic to the underside of the joists to provide a moisture barrier?

  • My opinion, is OSB is not a suitable subfloor for tile. Remove the OSB, put down the vapor barrier then a 3/4 plywood with a 1/4 inch cement board. then you can tile.
    – RMDman
    Jan 26, 2023 at 12:14
  • You're telling me, @RMDman, that my contractor who just installed 3/4" T&G OSB subflooring in my bathroom addition shouldn't have done that if I was putting down tile?
    – FreeMan
    Jan 26, 2023 at 12:30
  • 3
    Nothing wrong with OSB as a subfloor for tile as long as it has a suitable support structure (joists, blocking), and as long as you use an appropriate base for your tile (cementboard, hardieboard, etc.).
    – Huesmann
    Jan 26, 2023 at 12:43
  • If we are talking ceramic tile, unless there is going to be at least 1/2" of some type of cement board over it, then NO. I would never tile over OSB. (Frankly I never use OSB for anything.) From the question..." I have installed OSB over the existing pressure-treated decking, with the intention of installing ceramic tile over the OSB. "...
    – RMDman
    Jan 26, 2023 at 12:43
  • We still need an Answer, preferably to the vapor barrier question... Pondering reworking my own enclosed porch, so interested in general opinions about what to do with this kind of floor.
    – keshlam
    Jan 26, 2023 at 14:53

1 Answer 1


No, I would not install a vapor barrier under the floor joists. IMO, doing so would tend to trap moisture on the inside of the vapor barrier. And if that happened it would be almost impossible for it to dry out inwards toward the house with a ceramic floor.

Note that this not a whole lot different from having a room above an unconditioned crawl space, except that in your case you don't have a big source of moisture (no bare floor of a crawl space to contend with), and you have plenty of air circulation.

I would leave it open.

If you really feel the need to add something, consider a semi-permeable house wrap.

  • 2
    Joists hold up floors and are generally horizontal. Rafters hold up ceilings and are generally angled away from horizontal. ;)
    – FreeMan
    Jan 26, 2023 at 15:35
  • I increasingly lean to cellulose insulation which simplifies the vapor barrier question down to "none works fine" rather than the delightful question of "which side is really the warm side, here?" But then, I don't see any insulation mentioned, so it might be more of an "extended summer" porch than a real 3 season - or it will be an expensive-to-operate 3 season with cold floors at the beginning and end of its use..
    – Ecnerwal
    Jan 26, 2023 at 15:43
  • I didn't see anything about insulation mentioned either. Which is why I limited my answer just to the vapor barrier aspect.
    – SteveSh
    Jan 26, 2023 at 17:07
  • I originated the question regarding the vapor barrier issue below the 3-season room. To be more specific, the room will have no heat or A/C,, or insulation under the floor. The open space underneath the porch is approx 2' deep with a dirt base. I'm considering adding a vapor barrier under the joists, leaving a 10" - 12" gap between the vapor barrier and the pressure treated (PT) decking above. The OSB is sitting directly over the PT decking, which has gaps between boards of between 3/16ths and 1/4." The OSB is not painted on the bottom side. Any additional thoughts given this additional info? Jan 27, 2023 at 2:31
  • I still would not add the vapor barrier the way you're suggesting, for the reasons I gave in my answer. I think this is one of those situations where no vapor barrier is better than one in the wrong place.
    – SteveSh
    Jan 27, 2023 at 2:39

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