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?
Best Answer
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.