# perm rating



## joefromcal (Apr 8, 2008)

Hello, where can I get the perm (breathability or vapor permeability) rating of certain paints? I've read that sometime the claims of the paint manufacturers are not accurate and I was wondering if there was an independent, subjective, source? Also, If a paint has a perm rating of let's say 7 and I put 2 coats on the stucco does that now mean my perm rating has dropped by 50% or some amount? Thank you.


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## user1007 (Sep 23, 2009)

joefromcal said:


> Hello, where can I get the perm (breathability or vapor permeability) rating of certain paints? I've read that sometime the claims of the paint manufacturers are not accurate and I was wondering if there was an independent, subjective, source? Also, If a paint has a perm rating of let's say 7 and I put 2 coats on the stucco does that now mean my perm rating has dropped by 50% or some amount? Thank you.


I think it is more important to know the breathability of the bags your dry cleaning comes in than worry about your paint. Especially if stuck to your exterior.

Why your concern about whether your paint can breath, outgass or whatever by the way. 

Worrying about the vapor perm rating of paint may only be answered by one man I know on this site. Something tells me, if he has time, Ric is going to past a long post if he chooses to get involved in this one.


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## Will22 (Feb 1, 2011)

Perm rating measurements are conducted primarily for a couple of specific products: Interior vapor barrier primers (they have to be < 1 .0 perms), and elastomeric coatings. This property is not tested for most architectural paint products. Testing is conducted with ASTM specifications; it is not a marketing gimmick.

Vapor barrier primers for interior walls help prevent humidity (water vapor) from passing through walls into rafters, which can contribute to mold growth. Exterior architectural paint products do not provide an external vapor barrier.


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## joefromcal (Apr 8, 2008)

The reason I ask is I want to paint my outside stucco and it's important that the stucco can breathe. From what I've found out, it seems that there are pro's and con's whether I go with elastomeric paint or a hi-performing acrylic paint. I'm leaning towards acrylic but the sources I've found said the perm rating should be at least a 7 or higher. thank you


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## ccarlisle (Jul 2, 2008)

Well, you should only consider elastomerics if you have significant cracking already that can't really be repaired by hand. And only certain elastomerics are breathable ie have perm rating of about 7 or more. Exterior acrylics are, in general, breathable for this purpose and have perm ratings much higher than this. 

But don't let all this confuse you too much; the purpose of all this talk about perm rating is so that you don't create a double vapour barrier that moisture can collect inside of. That's not the best English but I trust you catch the gist.

You can proceed with 2 or even 3 coats of acrylics before you even come close to the perm rating of an elastomeric, which require 2 coats almost all the time.


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## Will22 (Feb 1, 2011)

Exterior paints are breathable. Elastomerics, though, have a perm rating of 13-30+, depending on which one you choose, and the makeup of the coating.


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## Brushjockey (Mar 8, 2011)

A product to consider is Loxon from SW. Made for cement/stucco etc. 
It is not an elastomeric.


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## joefromcal (Apr 8, 2008)

Thank you all for the replies.


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## concretemasonry (Oct 10, 2006)

The thicker the coats and the more of them gets you closer to non-breathing coating, since permeability is variable and not absolute.

Dick


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## ric knows paint (Oct 26, 2011)

joefromcal said:


> Hello, where can I get the perm (breathability or vapor permeability) rating of certain paints? I've read that sometime the claims of the paint manufacturers are not accurate and I was wondering if there was an independent, subjective, source? Also, If a paint has a perm rating of let's say 7 and I put 2 coats on the stucco does that now mean my perm rating has dropped by 50% or some amount? Thank you.


Hiya Joe,

I can't really add much to the great info you've rec'd so far ('specially from Will & Ccarlisle), other than when considering non-conventional coatings, or coating systems, strict adherence to the mfr's. printed instructions regarding surface prep, means of application (including application restrictions), min-max mil thickness limits, # of coats, dry times, etc. etc. etc., are extremely crucial. 

Generally speaking, a perm level of 6 or above is acceptable for typical stucco application. To achieve a level of 6 (or whatever the specific mfr has specified) requires that the product be applied to a specific mil thickness - either in a one coat app, or multiple coats (as recommended by mfr). If the mfr recommends 20 mils of dry film thickness (DFT) to achieve a perm rating of 6 (or 7, whatever), that is irrespective of the number of coats applied (generally speaking). For example, if it were possible - and allowed by the mfr - to spray apply, then back roll a single application to achieve a 20 mil DFT (@ 50% solids would require a 40 mil wet application), then add'l coats would, at best, would be futile but more likely to cause many of the adhesion, or blistering, problems often times associated with elastomeric finishes - when in fact those problems would actually have been caused by an over zealous applicator. 

Specialty products are designed to address specific problems. These are not your typical, run-of-the-mill, DIY coatings or applications. If the product directions say "Do Not Thin" - do not thin...Problems can, and will, occur as a result. Same with number of coats, means of application (example: if directions say "Do not apply by airless application" - then...you know)


Elastomerics can be great problem solvers or preventers. They can also be a paint job from hell. The difference, absolutely and definitively, is whether this system was applied in strict accordance to the mfr's explicit instructions.


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## berwin (Jun 16, 2017)

Joseph Lstiburek recommends a perm rating greater than 10 (highly permeable) for stucco.

buildingscience.com/documents/building-science-insights/bsi-061-inward-drive-outward-drying

Which paint would be best for that?

Thanks.


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