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PERSIST – Vapor-open or vapor-closed insulation and WRB?

Irishjake | Posted in General Questions on

So I just got back from Building Energy 15′ – what a great conference! I had a few conversations about various vapor barriers, insulation and how each works with the PERSIST technique. I wanted your feedback though……

Most of the time PERSIST walls get built using Grace Ice & Water on the outside of sheathing, and no interior insulation. REMOTE walls also use Grace Ice & Water. Both proven techniques. Grace Ice & Water is vapor impermeable (0.05 Perms (2.9 ng/m2s Pa). So in essence the exterior sheathing (OSB in this model) and interior walls breath to the inside because there is only a class III barrier (2 coats of latex paint on the inside) and the exterior side of the Grace I&W, insulation, and siding all breath to the outside.

Well that vapor barrier on the inside, only works with 2 coats of latex and you shouldn’t use vinyl wallpaper or other impermeable “stuff” on the walls, because that would mean there is two vapor impermeable barriers in the wall and we all know what that leads to……..So what happens as the years go by, and my kids want to change the color of the walls in their rooms, or new folks move in and change the colors of the walls? That is more paint…..lots more (I’ve had seen 10-12 coats of paint on walls during remodels). So what it the vapor permeability of that now???? It’s not really, so now I have two vapor impermeable barriers, and other bad stuff happening in my wall assembly.

So because I am trying to build a home that is resistant to the human element and mother nature – both of which are variable and always ruin or change any design plan (kinda like kids do to a clean room) I started thinking I’d go with a different WRB on the exterior side of the sheathing (a vapor permeable membrane that is a water and air barrier), and different sheathing too (plywood, which is vapor permeable). Sweet everything breaths to the outside, right?

Well I’m planning to use 8″ of Neopor (graphite impregnated EPS on the exterior), and even though Neopor/EPS is vapor permeable, that much NEOPOR/EPS insulation would render that assembly essentially impermeable to the outside…….wrong or right?

If the answer is “right”, I should switch the insulation to a more vapor permeable insulation like: Roxul Comfortboard IS (rock mineral wool) (R 4.0), or Gutex MultiTherm board (wood fiberboard) (R 5.3), or Corkboard, or Shroomboard – all of these would allow drying to the exterior. The Gutex MultiTherm doesn’t even need sheathing (the wall would need structural inlays though), and is it’s own WRB too, and the Shroomboard can be eaten in an emergency (not). Regardless, now my wall is breathing nicey, nicey….right?

Well what happens with solar push when these vapor permeable insulations are used? What about wInd-washing? Vermin eating through the Gutex or Cork or Shrooms(I know they can eat through the Foam too), but with Gutex there isn’t a need for sheathing?

So if it is “right” I should just use the cheaper OSB and use the Grace Ice & Water, and not splurge on the vapor permeable WRB because the wall is impermeable anyways, which brings me back too… at some point down the road I’ll have more than two layers of paint on the inside walls. (This post is kinda like the kids book – “If you give the mouse a cookie”!)

If the answer is “wrong” – stick with the Neopor/EPS, use a vapor permeable WRB and now my wall will breath nicey, nicey right??????

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  1. GBA Editor
    Martin Holladay | | #1

    You are much too worried about vapor diffusion. Take a deep breath and relax.

    If you build a PERSIST house, the wall sheathing and studs are on the inside of the house. If you keep your house warm during the winter -- most people do -- then your studs and wall sheathing will also be warm and dry, because they are indoors. They will be just as warm and dry as the coffee table in your living room.

    Even if your drywall has 12 coats of paint, it won't matter. There isn't any vapor drive through the interior paint layer, because the conditions are the same on both sides of the drywall. In your living room, it's 70 degrees with 35% RH. Guess what? Between your studs, it's also 70 degrees with 35% RH. No vapor drive.

  2. Expert Member
    Dana Dorsett | | #2

    Vapor diffusion is rarely the wetting path that leads to damage, but it's often a drying path that mitigates damage when moisture gets in. A vapor impermeable framing cavity is only a problem if there is bulk water leakage since it can take years (or decades) to find it's way back out, and if it doesn't dry between subsequent intermittent wetting events it will eventually rot.

    The vast majority of moisture problems in wall assemblies are from bulk-water intrusion- it's not crazy to design for at least a modest drying path to either or both sides.

    The interstitial spaces of EPS make it more vapor permeable than most other types of rigid foam. (Without pulling up the spec for Neopor...) , most 1.5lb density EPS would still be at about 0.35-0.4 perms @ 8" thickness, which is middle-of-the-road class II vapor retardency, and an adequate drying path for the sheathing toward that side of the assembly, as long as there isn't some low-perm membrane like Grace Ice & Water preventing it. (Asphalted kraft facers on batts are also about 0.4 perms- it' snot exactly a fast drying rate at modest vapor pressure differences, but is can still move some moisture.)

    The notion that fairly vapor open WRB such as housewrap or #15 felt would be characterized as a "splurge" relative to peel & stick membranes Iike Grace Ice & Water Shield seems a bit odd.

  3. Irishjake | | #3

    Gentlemen - thanks again for your quick responses to my questions......

    Martin - Yes, Thanks for setting my concerns aside in regards to the paint layers. I do understand the conditions inside the cavity are the same as the room inside, and that there is no vapor drive from the cavity into the interior or vise versa if that is the case.

    My concern was/is more focused on design changes down the road like tile, vinyl wallpaper, tin or look alike tin panels, sequencia, etc., etc., etc., and how a wall that is intended to dry to both sides, would act if those changes were to take place. I mean, I know that such changes "could" have a detrimental effect, and also "might" not even happen, so why worry right? I just figured I should plan for it, to decrease the likelihood of the human element changing the behavior of the wall, and therefore will plan to use a vapor permeable WRB on the oustside of the sheathing instead of Grace Ice & Water.

    Dana - I wasn't referring to 15# felt or housewrap (Tyvek/Typar) as being a "splurge" in regards to the cost of a WRB such as Gace Ice & Water - I was referring to products like: ProClima or Prosoco, or Tremco, or Henry or EnV, etc., etc., etc..

  4. user-1072251 | | #4

    Brad - there are, as you've found, various answers to that question, but the most resilient wall is one which is vapor permeable both ways. And Roxul is increasingly being used for exterior insulation for that reason.

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