Paint the interior side of sheathing to minimize possible rot or mold inside walls?
Question 1: To avoid condensation on the interior side of sheathing, why not just paint the interior side of the ply or USB sheathing? Cheap, easy, relatively quick, and would reduce the moisture that gets into the sheathing from any condensed water, frost or ice buildup that does occur inside the wall during winter. (After all, we use paint on wood products on the exterior to protect against water and sun damage.) It wouldn’t necessarily stop mold growth, but mold won’t grow in the cold walls in winter, and the walls could dry out easily without additional vapor retarders to the interior of the sheathing paint.
Exterior insulation would reduce the chances of wintertime water vapor condensation on the interior side of sheathing, but it adds to wall complexity, cost, and details can be fussy and difficult to do well. Maybe risk introducing bulk water around windows especially. Easier to build a thicker wall, double wall, Mooney wall etc.
A smart vapor retarder and tape is expensive in comparison to paint.
Granted its more important to reduce air infiltration/exfiltration than worry about minimizing holes for water vapor transmission, but still better to find more effective vapor retarders that are cheaper, easier, and faster ways to avoid sheathing rot and mold.
Poly sheeting is cheap, but its a vapor BARRIER that would prevent drying to the interior. Not good to trap moisture in the wall during warm summers when mold likes to grow.
Vapor retarder paint on interior drywall is similar, but you have the holes in the wall from electrical outlets, kitchen or bathroom sinks, kitchen cabinets, wainscot, base trim, pictures, TVs, etc.
The advantage of vapor retarder paint on the interior drywall is that its even cheaper (you have to paint the drywall anyway). But some places may not have drywall to paint on the interior side of walls (e.g., unfinished rooms, behind kneewalls, bathroom tiled walls, wood paneling on ceilings).
Question 2: If vapor retarder paint on drywall is so effective, why would anyone want to use an expensive extra labor step of using a smart vapor barrier inside the wall?
Question 3: Could painting the interior side of sheathing provides more protection for the sheathing from rot, even if vapor retarder paint is used on the drywall? Okay, its extra cost and work, likely not worth the effort unless there’s a defect somewhere that results in rot or mold growth. Some of us put in multiple air barriers since we know the primary barrier is not going to be perfect everywhere. Similar for vapor management, which is difficult to model?
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