Latex paint as class III vapor retarder
Hi –
My building inspector is questioning my intention to use latex paint as my class 3 vapor retarder in the following zone 5 assembly – vinyl siding, zip sheathing, 2×6 wall w dense pack cellulose, blueboard & plaster, 1 coat primer, 2 coats latex paint. This assembly should be fairly straightforward IRC R702.7 code application for class 3 vapor retarder.
The inspector wants me to install poly (class 1 vapor barrier) on the walls over the cellulose. In addition to the added cost, I think it’s a bad idea. He also seems to want something on the frame (like kraft-faced insulation) rather than on the blueboard. Thus he is questioning the paint as the class 3 vapor retarder.
1) Is there any clarity on where the latex paint is supposed to go? As in, it’s ok to put the latex paint on the drywall as part of the assembly rather than on the cellulose netting.
2) How do we know latex paint has a class 3 permeance? Ben Moore only gives permeance for their class 1 products. Malcolm Taylor gave the following information to a question from 202o
Drywall has a permeance of 50
Ordinary primer takes it to 5
Vapour-Retarder primer is 0.5 to 0.9
Two coats of latex paint 5
But I’d like to be able to cite a source rather than ‘Malcom said’.
Thanks!
Ted
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Replies
Ted,
A more reliable source than Malcolm's advice:
https://codes.iccsafe.org/s/IRC2021P3/chapter-7-wall-covering/IRC2021P3-Pt03-Ch07-SecR702.7
The entire purpose of the code language is to allow painted drywall (or plaster) to be enough of a vapor retarder when the right conditions are in place, which you have. In fact your proposed assembly is safer; when air conditioning is running, having poly on the wall interior is a liability.
Hi all - thanks for your feedback. When researching this prior to posting, I couldn't find any info to suggest that two coats of (any) latex paint over regular primer has perm rating between 1 and 10 other than Malcom's post. The manufacturer's don't do perm ratings for their products unless it's for Class 1 vapor retarder product. Does Malcolm have a source for his perm ratings?
Similarly, while it seems obvious that "latex paint" in the code means latex paint on drywall, is there a source for what exactly "latex paint" means? In one of my conversations with the BCO, he implied that paint on drywall was in the wrong spot and the vapor barrier needed to be on studs behind the drywall.
I've already quoted R702.7 chapter and verse to him and that wasn't good enough. Then I got him some manufacturer literature on the cellulose (they don't want any vapor barrier in the northeast) and that wasn't good enough. If I can't figure this out soon I will just apply intello or Membrain and call it a day. Sorry this last paragraph was just sharing my tale of woe.
Ted,
The perm rating I quoted is from one of Martin's blogs with no attribution, so not much help.
https://www.greenbuildingadvisor.com/article/all-about-vapor-diffusion
The table I linked to from the IRC explicitly says that latex paint is a class 3 vapor retarder. I don't know what more an inspector could require than code language allowing it? The suggestion the vapor retarder has to be behind the drywall betrays a worrying lack of understanding of the code he is enforcing. Not that that helps you deal with him.
I imagine his concern might be inward vapor drive being blocked by the paint and making the drywall mushy. That's why I specify "standard acrylic latex paint" and not a vapor-barrier paint or any type of wallpaper.
For data, perhaps he would trust Building Science Corp, by way of the US Department of Energy?
https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/building_materials_property_table.pdf
Attached is some data from the ASHRAE Handbook − Fundamentals, 2021 edition.
As to location, the IRC just says "on the interior side of frame walls."
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