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I intend to place a hearth on to the plywood floor of my mobile home. Just a basic question really. Do I need a plastic barrier between the plywood and the concrete? My worry is that it may condensate under the concrete if I put plastic. Any thoughts?

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I can't think of a strong reason to put polyethylene sheeting down, but if you want to preserve the plywood surface for the future, or you're afraid that water will leak through the seams, you could. I'd be a bit concerned that poly would act as a lubricant and allow more movement than is desired.

Condensation isn't really a concern with such a small, interior area unless you expect temperatures above and below to be dramatically different.

  • “A small interior area...” has NOTHING to do with condensation. There’s a reason the code does not allow wood to be in contact with concrete or masonry without it being pressure treated or protected (separated). Concrete and especially masonry is porous and will absorb moisture from the air. Then, without ventilation, poof: dryrot . – Lee Sam Oct 19 '17 at 5:15
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    So... a person can't install cementboard or tile mortar (masonry products, both) over wood subfloors? Methinks your horror scenario is overstated. – isherwood Oct 19 '17 at 15:47
  • Cementboard is not “masonry” and yes, mortar needs a barrier. – Lee Sam Jul 21 '18 at 6:30
  • In this example the masonry is not exposed to any rain, and masonry materials don’t just magically absorb water from the air in any amount in excess of what’s just in the air itself. I see no reason for a moisture barrier... it’ll be fine. – paul Jul 21 '18 at 13:42
  • A hearth is usually warmer then the surrounding area so how would it absorb moisture? Cement board not masonry? Hummm. – Ed Beal Sep 19 '18 at 13:39

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