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Vapor barrier inside double stud wall — is this OK?

Jim Sweazey | Posted in GBA Pro Help on

I’m building an energy efficient house this spring 25 miles Northwest of Minneapolis in the upper portion of zone 6. My wall design plan since the ’80s has been a double 2×4 stud with the vapor/air barrier in the gap between the two walls. This is similar to Joe Lstiburek’s Ideal Double-Stud Wall Design with 2 taped layers of 1.5” Foil Faced Polyisocyanurate foam board replacing the ½” OSB in Joe’s design. I believe this design would keep the inside polyiso vapor barrier well above dew point under the most demanding winter conditions. In the summer the outside surface of the polyiso would also be above dew point.
I have been getting pushback from insulation contractors that are scared of the vapor barrier being inside the wall. I have read much of the information written about moisture in walls by GBA and others. I have done wall temperature analysis that shows that this wall should work but it is such an important issue that I still need some reassurance. I would like your opinion.
This is the breakdown of my wall construction.
Double 2×4, 16” OC, 11 3â„4 in. deep, 2 layers of 1.5” Foil Faced Polyisocyanurate, staggered with seams taped in the gap between walls. Both stud walls insulated (exterior Mineral wool? Interior Fiberglass) R 45

Cold Weather
Description Thickness R Value Perm Rating
1 LP Smart side
2 Furring strip (rain screen)
3 Exterior Air Film 0.17
4 7/16″ Zip sheathing 7/16″ 0.62 12-16
5 2×4 16 OC Mineral wool 3 5/8″ 15 30
6 Polyiso Board 2 layers of 1.5″ 18.6 0.05
Total R of Interior Vapor layer to outside 34.39

7 2×4 16″ OC Fiberglass 3 5/8″ 9.2 High
8 1/2 ” Gypsum 1/2″ 0.45 50
9 Interior Air Film 0.68
Total R of Interior Vapor layer to inside 10.33

By the way the other wall options I’m considering are 6” or 8” SIPS or a 2×6 wall with vertical I joists filled with dense pack cellulose.

Thanks in advance for all you help

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  1. GBA Editor
    Martin Holladay | | #1

    Your suggested wall assembly has been used for over 30 years in cold climates, and it works just fine.

    Here is a link to an article about a Vermont house built 33 years ago with this type of wall -- it's held up just fine: A Superinsulated House from 1984.

  2. Jim Sweazey | | #2

    Thanks so much Martin. I'm sure you understand why this is so important to me.:)

  3. Charlie Sullivan | | #3

    I would go beyond calling that wall just fine. I'd say it's excellent.

    It's understandable that insulation contractors would be scared of it. If you don't really understand the physics, it's wise to be scared of doing something different from established known-good designs. But this in in fact a known good design, based both on experience and understanding the physics and practicality considerations.

  4. Brendan Albano | | #4

    You've gotten the thumbs up from Martin already, but if you wanted even more insurance, you could always swap the zip sheathing out for something more vapor permeable like fiberboard, gypsum board, or diagonal boards if you're feeling particularly old school, plus your vapor permeable house wrap or fluid of choice.

    Just out of curiosity, how come something like 2x6 with cellulose + 6 to 8" of EPS/polyiso/mineral wool didn't make the cut? Is it really more expensive than your double stud foam in the middle design?

    Similarly, why not Joe Lstiburek's exact design with the sheathing in the middle, and cellulose on both sides?

    It's always educational to see the hurdle people encounter that shape their designs in the real world!

  5. Jim Sweazey | | #5

    I'm amazed at what a quick knowledgeable response I have gotten. I also appreciate the questions and suggestions.

    Brendan- There are a lot of ways to skin a cat. Some of what drives my decision might be old school prejudice. My final decision for windows still hasn't been made. US windows are attached to the outside with nailing fins. High performance triple pane windows are heavy so I don't like hanging them outside the framing. European windows mount inside or half way between. This makes window flashing venerable. When I think of cellulose used where it could get wet, I get nervous. I have seen wet newspaper take a long time to dry. I know it has been used successfully but I still have concerns. "my wall" can dry to either side and has a solid vapor barrier. I think my wall is much cheaper than using enough foam board to keep the sheathing warm in this climate. It might be over kill but I look at the worst case conditions. Although the condensation issue should be addressed I believe most moisture problems come from flashing failure and walls that cannot dry quickly.

    The idea of using something other than Zip might make sense. I was looking for a sheathing that would give me another air barrier but would still dry if needed. Fiberboard with a house wrap/rain screen might be better. Most house wraps don't provide enough air gap to dry sufficiently.

    I'm sure that Joe knows better than I but I feel that an absolute vapor barrier that provides warm surfaces in both seasons if better than OSB.

    That said, I am so happy with the response that I have another somewhat related question.

    Should I be concerned about the Polyisocyanurate foam board shrinking? I read it can up to 1%

    Jim Sweazey

  6. Brendan Albano | | #6

    Thanks for all those details Jim!

    There are solutions to the window issues. Usually involving plywood boxes extending into the foam: It looks like one method is to support the windows in the plywood boxes with straps, and use the window flanges for water management and tape them to the house wrap that goes over the foam. (Things are different for mineral wool).

    But like you said, lots of ways to skin a cat!

    I think the reason Joe is so fond of using sheathing as the air barrier/vapor retarder is precisely due to issues like "will the polyiso shrink too much". I certainly couldn't tell you! Hopefully someone with more knowledge will chime in on the foam shrinkage issue. The impression I get is that Joe is fairly conservative in terms of durability concerns, which is why he loves taped sheathing so much over using membranes/foam/etc. as the air barrier.

  7. Reid Baldwin | | #7

    Is this a single story house? If it is a two story, how are you handling the floor framing for the second floor without interrupting the mid-wall air barrier?

  8. Tim R | | #8

    Do you have a good reason for 16" oc studs on both walls? At the least the interior wall is not load bearing so why the 16" oc. Go to 24" oc studs If done for drywall use 5/8".

  9. Jim Sweazey | | #9

    Reid, the house is a modified two story, two bedrooms and bath above half of the house. I think the question would pertain to both floors. We are using floor trusses. I would like to have the outside wall support the windows and roof. The inside wall could support the floor. The two walls would be connected at the floor truss with 1/2" plywood. Our thought is that this would complicate the build too much and is not doable. I haven't completely given up on this but at some point we just need to get it done. So the more realistic approach is to use closed cell spray foam in these areas, overlapping the Polyisocyanurate foam board. I have seen Poly wrapped around the exterior of the truss but I think that would not be practical with the spray foam. The difficulty of sealing this area is part of why I think we should use the zip sheathing as a secondary air barrier. I hope this makes sense. Do others have suggestions?

    Jim Sweazey

  10. Jim Sweazey | | #10

    Tim, thanks for the suggestion. The original thought was to make the double stud wall as easy for the framer as possible. We would use a little more wood in the walls and not push for advanced framing or 24" OC since we are avoiding thermal bridging. We also were concerned about wavy siding with 24" OC. However 24" OC interior might work. I'll ask my builder.

    Jim Sweazey

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