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Vapor retarder latex paint

Kevin McGuire | Posted in Energy Efficiency and Durability on

Code here in 4c says you need a vapor retarder class I or II on the interior side of framed walls.

I have a rainscreen system – 1/2″ plywood sheathing with taped joints, tyvek homewrap, 1/2″ furring, then hardie panels. No rigid foam. Inside I have 2×4 studs, un-faced mineral wool insulation (ultrabatt by owens corning), 1/2″ sheetrock installed 6 months ago, kilz pva primer, benjamin moore interior latex paint eggshell. Vented attic, vented crawl.

I did the best I could to air seal the exterior (and also could have done a better job with an airtight drywall approach as I’m just now learning about that stuff) but it was not a new house being built, just a remodel which posed some issues getting to everything. We will not have a perfect air seal… We do not have a whole house fan or anything like that either. Electric heat in bedrooms, and a natural gas fireplace in living room.  No A/C.

What the inspector said to me when i asked about it was basically, as long as you have pva primer, latex paint, and drywall you will be fine. I don’t know if I buy that based on what code says, and that my wall doesn’t come close to 1.0 perm.. however it is not necessarily my goal to merely ‘pass code’, I do want a healthy wall assembly as well of course.

So should I just primer over the paint with a latex vapor barrier primer sealer then paint again?
Or tear down and put kraft faced insulation?
And/Or any other recommendations. 

and going forward…
What is actually going to be my best (and affordable) option for the inside of an exterior wall given my outside wall assembly/climate since I will be doing the entire perimeter of the house in time.

Thanks again.

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Replies

  1. Trevor Lambert | | #1

    It's debatable whether you need a vapour retarder at all. I suspect you are fine.

    https://www.greenbuildingadvisor.com/article/do-i-need-a-vapor-retarder

    1. doreendoreendoreen | | #7

      Read this link -Would a latex vapor barrier Be painted on thin 1/4” non treated plywood on the wall interior ? Or the side of the wall facing the insulation ?

  2. Josh Durston | | #2

    I'm no expert on this, but I've read enough threads to suspect you got lucky with your inspector. He seems more up to date than some. Many are old school and insist on poly where latex paint will do.

  3. Expert Member
    Dana Dorsett | | #3

    With the rainscreen (= "Vented cladding over wood structural panels" ) the IRC allows a class-III vapor retarder in zone 4C (= Marine 4). See Table R7o2.7.1 :

    https://codes.iccsafe.org/content/IRC2018/chapter-7-wall-covering?site_type=public

    That doesn't just barely meet code- it's the standard exception for vinyl sided houses, and your rainscreen assembly is better vented than vinyl siding.

  4. Jon R | | #4
  5. Carlos Moreno | | #5

    I had a similar conversation with my city's building official, here in Shoreline, WA (Zone 4c). I have a bathroom addition with one exterior wall, with 1/2" drywall, 2x6 studs, unfaced Rockwool R23 batts, then 7/16" OSB and 1/2" drywall on the exterior side (it's an unheated shop/utility room). A while back I saw that the Washington State Energy Code requires a Class I or II vapor retarder (I don't have the vented cladding exception like you do), so I asked the building official if I should install a Membrain smart vapor retarder before hanging the bathroom drywall, and he said "Nah, you just need a PVA primer and you'll be fine." (maybe it was the same guy you spoke with!?)

    I ended up using Kilz Premium Primer/Sealer/Stainblocker (because when I was at the store I couldn't remember what the guy had said!) and I put one coat of finish paint. Reading more about the perm ratings, it became clear to me that something was wrong with this picture. This link: http://www.energy.wsu.edu/Documents/BFG%20Chapter%205-Jan2011.pdf shows that Latex Primer/Sealer is 6.28 perms, while PVA Primer is 5.5 perms. So, what I used is a Class III vapor retarder, but so is PVA Primer. I called the guy back and pointed this out, and he put me on hold for a couple of minutes (presumably looking something up), and came back and said, "Yeah, it needs to be Class I or Class II."

    There's a great list of vapor retarder paints here: https://www.greenbuildingadvisor.com/question/answers-moisture-vapor-retarder-primer-paints . I'm going to run out today and buy something like Sherman Williams Moisture Vapor Barrier, then sand/scuff my finish coat so I can start over with the vapor retarder primer. As Martin H. has pointed out in numerous places, vapor diffusion is nowhere near as important as air leakage, but still I'll feel better if I get the Class II stuff on my exterior wall. And that way I don't have to worry about my inspector having a different standard than the building official.

  6. Expert Member
    Malcolm Taylor | | #6

    A general question: If you are relying on paints or primer to meet a code requirement, how does the inspector know what you have used?

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