Limewash paint & vapor barriers
In CZ6 and currently installing interior smart vapor retarders. A combination of Membrain and Majrex.
Thinking about paint next. Have a strong desire to use something like JW Wall paints limewash mineral paint or similar. (https://www.jhwallpaints.com/products/lime-wall-paint).
From what I’ve read, limewash paint is very vapor open. If true, this would likely work well with the wall assembly, allowing moisture coming back through the smart vapor barrier to come back into the envelope. But I’m thinking with the ceilings, it makes sense to have a vapor barrier paint to prevent air from entering the floor system between floors 1+2, and from entering the attic space (there’s still a smart vapor barrier on the attic floor/ceiling.
Anyone have any thoughts or experience here? Am I understanding this correctly? Is this a concern/non concern?
Appreciate it–
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You are conflating vapor movement and air movement. They have next to nothing to do with each other. A vapor retardant coating will do nothing to stop air flow if there is a physical penetration of the surface, (a crack, a gap around a recessed light, etc.). And if you close all the gaps you'll stop air flow (your goal) while having no impact on vapor flow.
Limewash paint is vapor open. Correct. I'd go ahead and use it on both ceilings. I can see no reason to worry about either air or vapor movement into the floor 1 ceiling if floor 2 above is part of the same conditioned space. As far as vapor and air, focus on the exterior walls and the floor 2 ceiling.
Sorry for the delayed response. Didn't get an email update that anyone had answered this question.
Understood on closing all air gaps which is the goal at the moment. I guess I was just concerned reading the literature proclaiming the amazing qualities of the vapor open paint when so often I read here to use a vapor barrier paint. Seemed to go against everything advised.
Regular latex paint isn't much of a vapor barrier either so I don't see much of a difference.
We have Romabio limewash paint in our house. We get lots of compliments on the appearance. But it doesn't appeal to my practicality, I find it is easily damaged and is hard to match on touchups. It changes color slightly as it ages so new never matches the old.
Yeah... we've been digging into the rabbit hole of limewash and may come out the other side with a more traditional paint. Maintenance and touching up are a concern that most people continue to share
Sean,
I see what you are getting at in that MemBrain goes from 1 to 20 perms when wet, and two coats of regular latex paint are about 5, so you could argue they don't allow you to take full advantage of the variable-perm membrane.
However in a heating dominated climate like CZ6, the vapour-drive is to the outside. You aren't going to get much useful drying to the interior anyway - and 5 perms is still pretty vapour-open.
I can't remember - were you using a vented or unvented roof? If vented the vapour-permeance of the interior doesn't matter much, and if unvented, like the walls, 5 perms will allow adequate drying.
As JGSG said: Any air or vapour barrier needs to follow the outline of the enclosure, not divide parts of the house interior.
Hey Malcom,
As always, appreciate the insight. We have a vented attic assembly (soffit to gable) where this paint in question would be.
Sounds like there's no real cause for concern here -- just continue to detail everything tight as planned.