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Zone 7 wall help – 2″ foam exterior, 3/4″ plywood to interior – need vapor retarder?

northman56751 | Posted in General Questions on

Need help on this wall. Live in far northern MN. Talked with the local building inspector and found out that he is insisting on me using an interior vapor retarder (class II) on a high-r wall that I would like to use. After 25+ years in the window/door industry and seeing the damage that a “tight” wall can do, I am trying to avoid this. Any help would be much appreciated/

Here’s the rundown of the wall construction…..exterior to interior

* House wrap
* 2″ insulated sheathing (XPS, etc.)
* 2×6 studs
* Cellulose fill insulation
* 3/4″ plywood on interior of studs – sheer strength for the wall and able to hang large objects where I want instead of on the studs
* 1/2″ Drywall over plywood

Question is, with the plywood on the interior like it is, do I still need a class I or II vapor retarder?
From what I can find out, the only way I can get by with a class III vapor retarder on the interior is if I the exterior insulated sheathing meets an R-15 (3″ insulated sheathing on the exterior).


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  1. Expert Member
    Dana Dorsett | | #1

    At 3/4" OSB runs less 0.5 perms, when dry (under 12% moisture content) and is thus a class-II vapor retarder when located on the inside of the stackup.

    While more vapor permeble than OSB, any 3/4" plywood would also be well under 1 perm when dry, and also a class-II vapor retarder, but becomes substantially more vapor open should it ever become wet- it is a "smart" vapor retarder. (So is OSB, but plywood is even more so.) Even 3/8" plywood is a class-II vapor retarder.

    See p.36:

    See also:

    and this:

    The plywood meets the Class-II vapor retardency requirement when installed on the interior side where it stays dry, and no additional vapor retarder is needed to keep wintertime moisture accumulation under control.

    Just be sure to build it air-tight (both exterior & interior sheathing, and be obsessive about lapping the window & door flashing properly.

  2. GBA Editor
    Martin Holladay | | #2

    The basic problem with your planned wall assembly is that the rigid foam is not thick enough for your cold climate. This type of wall in Climate Zone 7 will not work unless the rigid foam is thick enough to keep the interior face of the rigid sheathing warm enough to prevent moisture accumulation (condensation) during the winter. That means that you need at least R-15 of rigid foam.

    For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  3. Expert Member
    Dana Dorsett | | #3

    Having a 3/4" plywood "smart vapor retarder" on the interior will still work, as long as it's air tight, but it isn't as forgiving as putting the requisite R15 on the exterior instead of the R10-ish 2" of "...XPS, etc...".

    At 3/4" plywood has to have a fairly high moisture content to go above Class-II vapor retardency. There are many PassiveHouse designs in much higher R houses that use half-inch OSB or half inch CDX as the interior side vapor retarder & primary air barrier. My only concern here is that it it may eventually start leaking too much when employed as a means of being " to hang large objects where I want instead of on the studs...". As long as the fasteners don't fully penetrate the plywood into the insulation, or the holes are sealed when the objects are removed it's not a problem.

    Using 2" rigid rock wool or 2" unfaced EPS instead of XPS or polyiso would give the assembly an exterior drying path, which would help. At 2" EPS is still only a class-III vapor retarder, and rigid rock wool is as vapor open as the housewrap. There is no exterior structural sheathing to damage with moisture accumulation, so it's really a matter of not abusing the buffering capacity of the cellulose. With ~R8 of thermal break from th rock wool or EPS the stud edges would stay warm enough to limit it's moisture content, as long as the cellulose doesn't get saturated. At 2" XPS is already down into Class-II vapor retardency, and may not pass the moisture readily enough. (And in 50 years it's thermal performance won't be appreciably better than Type-II EPS or rock wool.) Polyiso is normally sold only with Class-II or Class-I vapor retarders as facers, and would take a serious performance hit in this climate & application. Polysio would be the worst possible choice of foam here, independent of it's higher labeled R-value. Rock wool would be best, but EPS would still be fine, and XPS would be borderline.

  4. northman56751 | | #4

    Thanks for the input guys.

    Thinking now about bumping it up to 3" (XPS) on the exterior. I see from Owens Corning that on 1" XPS the perm rating is 1.1, 2" goes down to .70 and nothing listed for 3". Assume .4 -.5 range?
    I now know with this (3" = R-15) on the exterior wall that I can avoid an interior vapor barrier, which I HATE!

    On the interior plywood and perm rating. From Pg. 36 and 3/4" plywood
    Plywood used in typical “protected” applications, where the moisture content is less than 12% is relatively impermeable (i.e. 1and < 10 perms).

    What would the normal moisture content of the plywood be in a wall system like this? I highly doubt that it would stay around 12%. With the interior drywall and latex paint, is the plywood still going to allow the wall to dry out?

    Exterior house wrap.....based on the perm rating now going to 3" foam sheathing, does it really matter the perm rating on the house wrap/drainage plane?

    Now comes the big question on the windows............innies or outies? Moving the windows out that extra 3" can make a huge difference on frost accumulation or condensation.

  5. GBA Editor
    Martin Holladay | | #5

    Don't worry too much about inward drying if you have 3 inches of rigid foam. If you have 3 inches of foam, that's enough to keep your wall cavities warm and dry -- so there won't be any moisture that needs to dry out. In any case, the plywood is permeable enough to keep you out of trouble.

    Over exterior rigid foam, you can choose almost any housewrap you want -- the vapor permeance of the housewrap is basically irrelevant in this application.

  6. Expert Member
    Dana Dorsett | | #6

    XPS is blown with HFC134a, which is a powerful greenhouse gas (1400x CO2), and as it leaks out over the next half-century it's R-value drops asymptotically to that of EPS of the same density. So eventually that R15 becomes R12.6-ish.

    At 3" I wouldn't count on XPS being more than 0.3-0.4 perms. The vapor permeance specifications are usually maximums, not typical /average. It's not a drying path, which means you can't /shouldn't "cheat".

    Wood that is effectively indoors (as yours would be) typically runs 5-12% moisture content unless the house is kept unusually dry or unusually humid, so interior-side plywood IS a reasonable vapor retarder. The healthy relative humidity range for humans is 30-50%. For comfort it's usually OK even up to 65%, but above that is the threshold for mold to get going on some household materials.

    If you keep the place at 70F, 35% relative humidity in winter the equilibrium moisture content of the plywood would be about 7%. If you kept it at 50% RH it would settle in around 9% moisture content. If you let it get as high at 65% (the max recommended for conditioned space air comfort) the wood will settle in around 12% moisture content. Play around with this handy web-tool a bit to get a handle on it:

    In climate zone 7 outdoor winter air is super-dry, and it's easy to control interior RH in winter by adjusting the ventilation rate. If you keep it between 30-40% RH indoors the plywood won't pass moisture into the wall at a rate fast enough to cause a moisture accumulation problem, but air leaks potentially could.

    If the inspector throws a hissy-fit about whether plywood is or is not a vapor retarder, install a layer of Certanteed MemBrain between the plywood and wallboard. Like plywood it's a class-II vapor retarder when dry but would become vapor open if the wood ever become wet. Unlike plywood, it looks like a sheet of plastic (because it is- it's thin clear sheet of nylon, which has some interesting properties), and should assuage the inspector without creating a moisture trap. It's about $100 for an 8' x 100' sheet, and it comes in wider sheets as well. (In this application you'd be better off spending the $100 on a fancy dinner though- taping the seams of the plywood makes more sense. MemBrain would barely affect the performance of the wall if the plywood is air tight.)

    Latex paint runs 3-5 perms, and won't materially restrict the drying rate of the plywood or the cavity- the 3/4" plywood is a much bigger factor.

    The vapor permeance of ANY housewrap product is irrelevant in comparison to even 1" of XPS, let alone 3". It's an order of magnitude or more vapor-open than 1" XPS (which is the problem of using XPS.)

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