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Membrain: smart vapor retarder

Mill_house | Posted in General Questions on

Climate zone 7/8
Insulating kneewall.  From the attic portion of the kneewall working inwards, the assembly is 2″ rigid board attached backside of studs, roxul batts in the stud bays, then drywall.  Could I put membrain between the roxul batts and the drywall?

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  1. Expert Member
    Dana Dorsett | | #1

    In zone 7 it takes R10 exterior to a wood sheathed exterior 2x4 framed wall for dew point control to protect the sheathing, but it sounds like you have no sheathing to protect here(?), and only need to control moisture at the foam/fiber boundary. It's also not exactly an exterior wall- there is no rain-wetting of the foam and there is a large drying cavity (the mini-attic).

    The foam type matters a bit. If it's unfaced Type-II EPS it's sufficiently vapor permeable that you wouldn't even need MemBrain. At temperatures that matter the EPS will be performing at about R9, and while some amount of moisture might be pulled through latex paint & wall board from the interior, at 1.5 perms or so the foam will pass a good portion of that out to the (presumably vented) attic space. If it's foil faced polyiso there is no exterior drying, but even if derated to R4.5/inch for temperature, the exterior side foil facer looking at a big air gap adds another ~R1 . The worst case would be 2" XPS, which will eventually hit the same thermal performance as Type-II EPS, but it's a lot more vapor tight, less than 1 perm, passing less moisture through through. Even so you're probably safe without it.

    But sure, since you're somewhat on the margin MemBrain would be cheap insurance, even if not really necessary and better than installing 4-6 mil polyethylene if there are facers on the foam which could create a moisture trap.

  2. Mill_house | | #2

    That's correct. It's the kneewall that I'm insulating. So no exterior sheathing. Just concerns about the moisture at foam/fiber boundary. (I have an unconditioned unvented attic. Zone 7 Canadian prairies.)

    The foam was recommended by GBA as a way to protect insulation in the kneewall studs from air movement. Upon additional review, I see that GBA says foam is needed to protect FIBREGLASS batts. Just to reiterate, I am using mineral wool batts in the 2x4 kneewall stud, not fibreglass. Is foam still necessary vs say two layers of mineral wool batts (vertical batts in kneewall stud bay and horizontal batts held in place attic side of kneewall)?

    The foam recommendation of 2" was based on exterior wall. But it still applies to the kneewall too right?

    Just checking to be certain/confident before I place my supply order.


  3. GBA Editor
    Martin Holladay | | #3

    You'll be fine without a vapor retarder. Just pay attention to airtightness.

    In general, walls with exterior rigid foam don't need an interior vapor retarder. For more information on this type of wall assembly, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

  4. Expert Member
    Dana Dorsett | | #4

    This isn't the general case, since there is no wood sheathing to protect here, only the dampness of the fiber insulation.

    While at 2" that's not very risky (even without MemBrain) no matter what type of foam was installed, we're still in the dark about what type of foam it is (?).

    1. Mill_house | | #5

      Climate zone 7 Canadian prairies with an unconditioned unvented attic. Rigid insulation will be used on the attic side of kneewall. Roxul batts will go in 2x4 stud bay of kneewall.
      The chart you referred me to says says R10 for for 2x4 walls. 2 inches of EPS type II is just below R10 so I was thinking of doing 3 inches unfaced, EPS type II. What are your thoughts on this decision?
      And membrain or no membrain?

      1. Expert Member
        Dana Dorsett | | #9

        At 3" of EPS it would be no need for MemBrain.

        At 2" EPS the performance would be just shy of R10, and would probably be OK, but MemBrain would be cheap insurance.

  5. Jon_R | | #6

    My guess is that the highest moisture performance in this case would come from putting the low perm foam on the interior side. Well vented, external fiber, no external sheathing has been shown to work even at 50% interior relative humidity.

    Is MemBrain beneficial (even if it were not required)? Sounds like a WUFI question.

  6. GBA Editor
    Martin Holladay | | #7

    Q. "I was thinking of doing 3 inches of unfaced EPS, Type II. What are your thoughts on this decision?"

    A. It's OK with me.

    Q. "And membrain or no membrain?"

    A. As I wrote on another Q&A thread where you posted the same question, you don't need a vapor retarder on a wall with exterior rigid foam. In all cases, it makes sense to pay attention to airtightness. The only reason to install interior MemBrain would be because a building inspector required it.

    1. Jon_R | | #8

      > you don't need a vapor retarder on a wall with exterior rigid foam.

      I believe that IR code requires a Class III vapor retarder - so don't leave the wall unpainted.

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