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Felt paper on subfloor before installing hardwood floor?

rockinroger | Posted in General Questions on

I’m getting ready to install a solid 3/4″ t&g red oak floor on a raised house on the gulf coast.
It is not insulated underneath, although the owner would like to get spray foam under there sometime down the road.
I was wondering if 15 lb.or 30 lb. felt paper, or underlayment available at a place like Lumber Liquidators would have any effect on moisture being trapped in the 3/4″plywood sub floor?

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  1. Dana1 | | #1

    Asphalted felt is not a vapor barrier, but has variable permeance. Most commercial floor underlayments are fairly vapor permeable.

    Rigid foam on the underside of the joists is in several ways better than spray foam on the subfloor, starting with the fact that it keeps all of the structural wood inside the conditioned space. An air conditioned house with joists that penetrate a foam layer can have issues with the joist being colder than the outdoor dew point in summer, and take on excessive moisture where it exits the foam. A layer of 1.5- 2" rigid polyiso with the seams taped cap-nailed to the bottom side of the joists works well. The band joists need to be air-sealed as part of that project.

  2. davidmeiland | | #2

    Sounds like you are thinking of putting the felt over the subfloor before installing the flooring. That's what I would do. Before you install, it is critical to make sure the subfloor and flooring are within a couple percent moisture content. You need to get ahold of a good moisture meter to do this. I'll hazard a guess that the subfloor will have a higher MC than the flooring, and you'll need to let the flooring acclimate before installing it.

    I agree with Dana about rigid foam under the joists, but few crawl spaces have 4' openings that allow full sheets through, so you end up slicing the foam into 2 or 3 pieces and then reassembling it in place. If there is ductwork in the crawl you may want to considering sealing and insulating the perimeter instead of the floor.

  3. GBA Editor
    Martin Holladay | | #3

    I don't advise installing red oak flooring over an uninsulated floor assembly exposed on the bottom to Gulf Coast weather conditions. The oak flooring will be subjected to big swings in humidity if you do this, and the possible consequences include buckling during the summer or wide cracks during the winter.

    A layer of polyisocyanurate under the floor joists, as Dana suggests, will help keep the moisture content of your flooring more stable. Don't forget to tape the joints of the polyiso, and to seal the perimeter with canned spray foam or a high-quality tape.

  4. rockinroger | | #4

    Thank you all.
    I would rather deal with polyiso sheets than messy spray foam.
    I only have about 2'-6" under the house.

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