Air Barrier and Vapor Retarder for Drywall Ceiling
What should I do to the drywall at the attic floor to make it a better air barrier, and to assure if moisture is introduced, it can dry out? I assume the previous owners’ painters probably used latex primer/paint everywhere, so wall and ceiling assembly permeances are around 5. If I want to get drywall permeance to .5, I should repaint the ceilings with a vapor retardant primer? I read Martin’s “All About Vapor Diffusion” here: https://www.greenbuildingadvisor.com/article/all-about-vapor-diffusion, and am left wondering if my drywall is sufficiently impermeable, and what steps I should take to lower its permeance. (Wouldn’t it be nice if there was a gadget to obtain a reading of an assembly’s permeance in situ?)
It seems a far easier task to paint the ceiling with a vapor retardant primer, but would a sprayed or brushed liquid vapor retardant applied to the attic side of the ceiling drywall be tighter? Assume I’ve properly sealed all the larger holes & gaps: removing or enclosing light cans; using sprayfoam and caulk around fans and electric boxes; install wind-washing dams at soffits; and other measures – following Matt Bath’s nicely detailed air sealing efforts described here: https://www.greenbuildingadvisor.com/article/saving-sustainably-air-sealing-and-insulation; and Martin Holladay’s “How to Insulate an Attic Floor” here: https://www.greenbuildingadvisor.com/article/how-to-insulate-an-attic-floor#0.
It makes sense to me to get attic floor/ceiling drywall air-leak secure before I have the insulation contractor add loose fill insulation into the attic space – to R49. Original attic fiberglass loose-fill insulation has subsided.
This house is in a small 55+ community in a western suburb of Philadelphia. Philly is in a mixed humid 4a Zone with large swings in temp and humidity: muggy late summer periods of highs above 90, summer nights around 80; January days can stay in 20s for a week.
The semi detached house built in 2003 has a poured concrete basement walls, unfinished; typical timber-framed construction with vinyl siding & asphalt roof. All but 1 room is on the first floor; a “loft” room with dormer: rest of house has peaked roof with one gable.
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