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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Where a vapor retarder is in an assembly matters. In cold climates, you want to maintain humidity/temperature of air in the assembly above the dew point. The temperature in the assembly is going to drop as you move inside to out, so you need to drop humidity of the air prior to the temp dropping.
Paint on the inside of the sheathing will retard vapor movement into the sheathing, but it will also encourage condensation to form at that surface. That condensation can lead to mold growth inside the wall.
I expect that with the time frames involved, the plywood behind hi-perm paint would be about as wet as without the paint - this isn't like brief rain that is deflected from painted wood. Probably better off applying a fungicide to the plywood.
> the walls could dry out easily without additional vapor retarders
Except that you need an additional, interior side vapor retarder to keep the wall insulation dry.
If you look at the permeability curves, smart vapor barriers outperform vapor barrier paint. And could provide additional air sealing. Enough of either to be worth the cost? That's a valid question.
It has some downsides, but the benefits of exterior foam over wood sheathing are considerable. For example, here in Michigan, adding R5 outside a 2x4 wall has about 1/5 as much moisture condensing/sorbing in the wall (but depending on exterior perms, this may not accumulate).
Additionally, Membrain is only about 2x the cost of 6 mil poly last time I looked. Being that poly is not very expensive, it only costs $100-$200 more to use for most houses. You only need to tape/seal the Membrain if it is being used as an air barrier. You can skip that if it is only being used as a vapor retarder.
Robert,
Precluding drying to the outside, especially with double-walls, leaves the cavities vulnerable to moisture accumulation. Relying on drying to the inside makes sense if there is exterior foam and the whole wall assembly will remain above the dew p0int, but without it the permeability of the exterior sheathing is important - the worry isn't just moisture accumulation in the sheathing, it's the whole wall, where drying needs to exceed wetting.
Exterior sheathing accumulation is a problem best solved by good air-sealing, an interior vapour-retarder, and a rain-screen cavity.
What Malcolm said!
Putting a low-perm paint on the interior side of the sheathing could result in liquid moisture (or frost) accumulating on the painted surface over the winter, putting the structural framing at risk. Plywood & OSB sheathing behave a bit like smart vapor retarders, and can pass a good fraction of wintertime moisture drives from the interior into the dry wintertime outdoor air, as long as the vapor permeance on the warm side of the assembly is as low or lower than the sheathing.
Vapor retarder paint on wallboard is effective, but only if the wall is meticulously air sealed on the interior side, which is always a good idea, fixing the first 90% of the problem even without the vapor retardent paint. Membrane type smart vapor retarders offer a much bigger drying capacity toward the interior when needed, and can/should also be detailed as an air barrier.
Malcolm I should have mentioned the whole assembly. I would use a rainscreen, target 0.6ACH50 or better airtight shell, among other objectives. Matt, for various reasons I like exterior polyiso with 2x6@24” stud wall and ComfortBatt insulation. 3” polyiso would minimize drying to the exterior but keep the sheathing above freezing.
Previously I only liked the idea of a smart vapor barrier inside of the double stud wall where its protected from so many punctures. But with Malcolm, Jon, Matt and Dana all providing good reasons for paying up, it seems justified to pay for an additional smart vapor retarder sheet product. Thanks all, for contributing those ideas. Jon thanks for pointing out permeability data that justifies the smart product over just vapor retarding paint on drywall.
Dana my primary air barrier would be taped plywood sheathing, but I make some attempts to make drywall a secondary or interior air barrier. Dana and Matt, now I’d make more of an effort to pay attention to airtight drywall detailing, including using a smart vapor retarder sheet product.
You guys have a future in sales! :-)
In one of his videos, Matt Risinger mentioned the combination fungicide/insecticide product Bora-Care. Its relatively cheap and easy to spray on the inside of wall framing. Seems a good addition to wall assemblies.