Vapor barrier for existing ext. house wall (to become 4th wall for new adjacent sunroom addition with hot tub & indoor pool)
We’re planning to add a stick built addition to our home which will contain a hot tub and small therapy swimming pool. We’re going to include negative pressure exhaust fans and a dehumidifier system.
The three new walls will be built with Zip-R sheathing and 3″ of closed cell spray foam and will therefore be very tight. The fourth wall will be the existing exterior wall of our house (currently 2*4 wood studs with fiberglass batt insulation, OSB Sheathing, tyvek house wrap and vinyl siding).
Our plan is to remove the siding and house wrap and install a vapor barrier and drywall over it. I’m trying to decide what would be the best way to accomplish this.
This will probably be there least airtight of the four walls and if the room becomes pressurized or if there’s vapor drive because of high humidity, I don’t want it going into my house through this existing wall which has fiberglass Batts and isn’t built to deal with moisture.
The good news is that the addition will be conditioned space, so this will no longer be an “exterior” wall and there shouldn’t be huge temp differences on the two sides which should mitigate condensation risk, but there could still be vapor drive with greatly differing relative humidity levels on both sides so I want to be careful.
I’ve considered applying a peel and stick vapor barrier membrane on my existing sheathing and drywalling over that, but I’m concerned about using a total barrier that wouldn’t ever let the wall dry to the sunroom side (in case there was ever a situation where my interior humidity was higher than the sunroom and that wall needed to dry to the exterior) and about having a flat enough surface for a good drywall install.
Another possibility would be to put a layer of the Zip-R Sheathing over my existing sheathing (foam side facing existing house sheathing) and drywalling over that. It would create a good surface to apply drywall over, the foam would act as a vapor retarder and it would just add another buffering layer between the rooms to help restrict airflow across that wall assembly.
I’d greatly appreciate any thoughts on the best way to handle the detail for this existing house wall. I’m in Maryland. Thanks in advance!
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