This pertains to a townhouse (built-in 2005) in central Florida, US (climate zone 2). Ever since living in this townhouse, almost 10 years now, I always noticed a difference in air quality (perhaps odor) *only* in the living space directly above its patio. The patio ceiling assembly consists of 2 layers of gypsum board (at least ½” thickness for each board) and kraft-faced fiberglass batts (unknown orientation of up or down) above it. The exposed face of the ceiling has an unknown coating, besides being the color white.

If the exposed face of this ceiling isn’t coated with a vapor retarder having a low water vapor perm rating, wouldn’t ambient humidity rise and seep through the gypsum boards at a greater rate than desired? And could this be the reason for the difference in air quality above, in the living space?

If a fluid-applied vapor retarder having a low water vapor permanence for gypsum is recommended, please provide a product name. I have researched and followed a few papers by Joseph Lstiburek but have a difficult time finding products that are even given an E96 rating. And the few products which I personally contacted the manufacturer about, provided misleading or incorrect figures in their TDS (Techincal Data Sheet).

Also, considering climate zone 2, what harm would it be to have a less than or equal to .1 perm coating on the exposed ceiling face? Because humidity rises in the air of the same temperature. Why would there be a need to have this ceiling vapor-permeable and not “vapor-proof”?