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I have spray foam under the subfloor with vapor barrier. The foam is sprayed to the underside of the subfloor deck, and vapor barrier is attached to the joists bottoms between the joists and the soil in the crawlspace (converting crawlspace is not an option).

I'm going to be installing engineered wood flooring.

This install calls for an underlayment (such as floor muffler). It seems that most hardwood underlayment also includes moisture barrier.

If I use such an underlayment will that create the dreaded moisture sandwich in my subfloor deck?

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  • Sorry about the lack of response here. If you arrived at a solution, please post and accept an answer.
    – isherwood
    Jun 9, 2020 at 13:08

3 Answers 3

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Not at all, you are just getting extra moisture protection for a narrower seasonal expansion range, this is an advantage. Your underlayment vapor barrier just has less work to do, as moisture transfer is reduced to just the exposed joist surface area. Assuming you have controlled excessive moisture in the crawl space, which needs to be done in any case. That is let's say you have a high water table and a "wet" crawl space, even if you didn't have the spray foam installed, you would still need to make sure you have a sump pump as well as adequate ventilation no matter the flooring type.

You will still want the vapor barrier so the moisture transference is consistent across your flooring. Not installing with a moisture barrier underlayment would permit disproportionate transference to the more permeable fiberboard plank backing where it is located above the joist ends. Not a big deal in most circumstances, but omitting this underlayment is not best practice and may void warranty.

You will find increased risk in the event of flooding (appliance failure or otherwise) penetrating beneath the flooring underlayment, but again that problem is just a feature of spray foam effectively sealing the underside of your subfloor, reducing the surface area available for atmospheric drying... that's the cost of the benefit of the improved thermal performance. If this happened, you'd just have to pull up your click lock floor and underlayment until it temporarily while it dries. We're talking flooding on the order of "homeowner's insurance involvement" magnitude. Your click lock is actually advantageous in a scenario like this because it can be carefully pulled without damage due to no fastening.

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Real hardwood flooring does not have a vapor barrier. It's just wood.

So your product is "click-flooring" type laminate ? In such a case, Yes you should check with the manufacturer (call their 800 number perhaps)

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  • Sorry for lack of clarity. They're engineered wood planks. You install an underlayment between it and the subfloor for a floating or double stick install.
    – Matthew
    Jun 4, 2018 at 6:40
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    I don't think the underlayment would qualify as a vapor barrier. The underlayment I'm thinking of is 1/16-1/18" foam. Jun 6, 2018 at 4:15
  • It's advertised as "moisture barrier" but not sure if that's accurate....
    – Matthew
    Jun 7, 2018 at 5:35
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In a similar situation, we floated the hardwood, removing the polythene sheet off the underlay material to avoid the double vapor barrier. Been good for 5+ years. Engineered wood flooring is the right choice in this circumstance.

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