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Vapor barrier in porch floor with mineral wool insulation

jacques_nj | Posted in General Questions on

I have a screened porch with an exposed wood floor (8′ above ground) that I have started to transform into a 3 season porch (location: New Jersey ). Tilt windows were first installed to replace the original screens. Original floor is 1×6 pressure treated boards (with small space between boards to let air through) on 2×12 joists. I started by installing rockwool comfortbatt (R23 – for 2X6) between the joists and covering underneath with painted and caulked SmartSide 38 panels. On the interior side, I am planning to cover the 1×6 boards with RevolutionPly plywood and install on top of the plywood laminate flooring (Pergo Outlast+) . I understand I need a vapor barrier underlayment between the plywood and the laminate floor. Should I also consider CertainTeed MemBrain in case there is a different flow of humidity between winter and summer? Should I also assume I do not need a vapor barrier between the 1″x 6″ boards and the plywood?



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  1. Expert Member
    Dana Dorsett | | #1

    A plywood subfloor is already a powerful Class-II vapor retarder. But installing 6 mil polyethylene between the p.t. 2x6 and subfloor will protect the subfloor from taking on moisture from the outdoor air during the air conditioning season, when the floor temp can often dwell for days or weeks below the average outdoor dew point temp. In winter it won't matter- the subfloor won't be cold enough to take on wintertime moisture. MemBrain won't buy you anything in this stackup, due to the low low vapor retardency of the plywood itself.

    In the zone 5A parts of NJ (the cooler NW counties) R23 won't quite meet IRC code min (=R30 for zone 5), but is fine for zone 4A (= most of NJ) where the code minimum us R19.

    Is the rock wool snugged up to the 2x6 decking, or is it resting on the SmartSide? If the latter, hopefully the band joists have been air sealed and also insulated to R23(?).

  2. jacques_nj | | #2


    Thank you for the reply.
    I am in Zone 4A. The rock wool is pushed/snugged up to the 2x6 decking.
    From your answer I conclude that the best choice is to install "only" 6 mil polyethylene between the p.t. 1x6 and the plywood subfloor. Do I read you correctly?

    Thanks again,


    1. Expert Member
      Dana Dorsett | | #3

      >"I conclude that the best choice is to install "only" 6 mil polyethylene between the p.t. 1x6 and the plywood subfloor. Do I read you correctly?"

      That would be sufficient protection for the flooring, both summer and winter. You might even be able to skip it, but it's cheap insurance.

      Any p.t. planking can handle intermittent (or even substantial) cooling season condensation from the outdoor air on the underside of the 6 mil polyethylene, whereas a plywood subfloor might not. The planking will dry toward the exterior, the subfloor will dry toward the interior, and stays warm enough in winter to not accumulate moisture from the indoor air.

  3. jacques_nj | | #4

    I will follow your advice.



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