My installer put polyethylene between hardwood and subfloor. Is that a problem?

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    Depends on what the subfloor is, what level of the home we're talking about, what your climate is, what the flooring is.... Please revise with more detail. – isherwood May 27 '18 at 14:25
  • sub floor is plywood - main level with framed basement- southern on ca home has an erv this is a brand new energy star home – Jim Drake May 27 '18 at 16:16
  • Please revise to add more detail. – isherwood May 27 '18 at 16:36
  • Is it a nail down or floating installation? – Jack May 28 '18 at 3:31

It is a moisture barrier. See Moisture Barrier and Underlayments: Don’t Leave It Out! for a little more explanation. However, in your case that appears to have been an unnecessary expense. I doubt it would hurt anything except your wallet.

  • Given the new information in the comment above, from where are we barricading moisture, exactly? – isherwood May 27 '18 at 16:38
  • @isherwood You are correct. Which means either (a) the installer is an idiot or (b) the installer wanted to be able to charge for installation of the moisture barrier despite the moisture barrier being effectively useless. I vote for (b), but I have become skeptical in my old age. – manassehkatz-Reinstate Monica May 27 '18 at 16:44
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    To be fair, it's standard practice to put some sort of sheet between the layers of wood for noise abatement. Maybe that's the intention. Typically it's rosin paper, but who knows? – isherwood May 27 '18 at 16:46
  • @isherwood Noise abatement?? What STC rating is one layer of paper? – Lee Sam May 27 '18 at 19:44
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    I would actually ask the installer why - there could be many good reasons dreamt up. But it does sound like a waste. Did they write it up as polyethelyne on the ticket? They did the job, so they may have a solid idea. – noybman May 28 '18 at 1:09

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