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Community and Q&A

Plan for exterior wall construction and insulation

k8RQsvj3yc | Posted in GBA Pro Help on

I am planning a new home in PA – zone 6. The options for insulation/exterior wall construction are mind numbing. My wall will be 2×6 single wall construction. I am focusing in on the design plan #5 outlined out in the Dec 2009/Jan2010 issue of FHB. In summary this technique calls for a wall constructed as follows (from the inside out)
2×6 studs with cellulose blown in
two layers of Polyiso Foam (2in per layer/staggered and taped joints)
3/4″ strapping screwed to joists

Here are my questions
1. Is a Vapor Barrier on the interior wall needed?
2. How will this wall handle moisture created inside the home?
3. Are there any repercussions from using two layers of 1.5″ foam vs the 2″?
4. Any repercussions from using sprayed foam vs the blown cellulose?
5. If I use something like Zipwall for sheathing and tap the joints, can I eliminate the housewrap?

and last but not least, considering the fact that I want to stay with the single 2×6 exterior wall and there any other variations on this that I should consider?

I’m sure this question has been raised in manner variations, but I need to ask anyway.


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  1. iEffyBYLtm | | #1

    Sounds like you are tied to 2x6; if not, my suggestion would be to simplify the process, reduce labor, and decrease your duration by using a 6 inch Insulating Concrete Form for your exterior walls.

  2. Gregory La Vardera | | #2

    Please consider the USA New Wall:

    You'll get advised to use an "interior drying" configuration. I would not do it. In zone 6 in PA? In north central PA you are outside the humid summers of the coastal regions, and you have much more vapor push in winter from the inside than you do in summer from the outside. If you will have AC then use an interior vapor retarder like Membrain from Certainteed. If no AC, then in zone 6 you can use a vapor barrier, although I think the Membrain is always a better choice.

    The toughest part is not poking your retarder full of holes, so you should consider an interior wire chase as shown in the USA New Wall. And you should consider Swedish Platform Framing to bring up the overall performance of the wall and eliminate thermal bridges - it also makes it much easier to make the vapor retarder continuous. See link from same page, link list top right.

  3. jbmoyer | | #3

    6" ICF walks?

    Wow. What an excellent suggestion..

  4. Foamer | | #4


    Your proposed wall will have a combined r-value of about 45, an excellent value. More than half of that will be contributed by the polyiso, which means that the potential condensing surface (the innermost skin of the poly) should be above the dew point. this fact provides the answers to your first 3 questions.

    1) since there is no risk of condensation, a vapor barrier isn't needed. On the contrary, it should be avoided. You should never have two vapor barriers in a system.
    2) condensation is the enemy - no condensation, no problem. I assume that you will control indoor humidity within the normal range.
    3) reducing the thickness of the polyiso lowers your safety margin. There is an article somewhere on GBA on how to calculate the amount of exterior foam needed given climate and cavity insulation. You should run the numbers before making this decision.
    4) Foam is fine. It will cost you more than lose but it will add to air tightness.
    5) you should be able to skip the house wrap regardless. Use OSB, foam the wall cavities and tape the isoboard. Just be careful with the window flashing!

  5. GBA Editor
    Martin Holladay | | #5

    Torsten has accurately answered your questions. (The others seem to be promoting their own favorite wall instead of responding you your questions.)

    Here's a link to the article he was talking about: Calculating the Minimum Thickness of Rigid Foam Sheathing. In your climate, the foam on the outside of a 2x6 wall needs a minimum R-value of R-11.25, so even a mere 2 inches of polyiso would work. However, thicker foam obviously gives you a better-performing wall.

    I'm not sure what you mean in question #2: "How will this wall handle moisture created inside the home?" Sometimes (usually in winter) the relative humidity indoors is higher than the relative humidity outdoors. Sometimes (usually in summer) the opposite condition holds. If you want to change your indoor RH, you need either mechanical ventilation or air conditioning. As long as your wall is robust, and as long as there is no reason to think that condensation or moisture accumulation is happening, your walls will be fine.

    According to the manufacturers of Zip System sheathing, you don't need housewrap if you use their OSB and tape. If you like their system, go ahead and use it. Some builders prefer to use housewrap with Zip sheathing, because the housewrap is cheap insurance against tape failure. Your choice.

    There is one reason to prefer cellulose to spray foam between your studs: if your OSB ever gets wet due to a flashing error, the OSB well be better able to dry if you choose cellulose.

  6. Gregory La Vardera | | #6

    Jim, what Martin and Torsten have described is an "interior drying wall'. And if we want to dismiss anybody's opinion as "promoting their own favorite wall", then this is it.

    Martin, I was sharing an alternative wall with Jim, not promoting as you put it. The information on the USA New Wall is offered for free. I'm not selling anything. The configuration of the USA New Wall is very similar to Jim's wall so its a relevant comparison. I see plenty of instances where people asking questions here are steered towards an interior drying wall configuration, when it apparently has nothing to do with their questions.

    I've no doubt gotten on your bad side because I disagree with the almost universal advice here to configure heating climate walls for interior drying. I'll stay away if you wish. If you want a diversity of info discussed here I'm happy to continue posting when I think its relevant.

  7. GBA Editor
    Martin Holladay | | #7

    I think you misunderstood me. I certainly did not intend for you (or anyone else providing information here) to think that I want you to "stay away." Thanks for your links.

    This was what I meant to say: Jim designed a wall. He asked 5 questions about the wall. Torsten took a stab at answering the questions, whereas the others (including you) didn't really answer the questions.

    I didn't mean that your post wasn't interesting. Just that you didn't answer Jim's questions about his planned wall.

    Please keep posting here.

    And by the way: I'm not sure what an "interior drying wall" is. The wall that Jim described will only get wet from the exterior. It will also dry to the exterior. In other words, rain may soak the siding. Some moisture may get past the siding and get the furring strips wet. Any liquid water will drain down the rainscreen gap, and the wall will dry to the exterior.

    Since the sheathing will be on the interior side of 4 inches of polyiso, it will be warm and dry. There isn't any reason why moisture will accumulate in the stud bays, so there is no reason for this wall to dry to the interior. If, however, there is some type of slow plumbing leak -- there really shouldn't be any pipes in exterior walls, but there might be in theory -- I suppose the stud bays could dry to the interior. But these stud bays really won't ever get damp.

  8. homedesign | | #8

    Gregory, please don't go away.
    Martin has no idea that he is being not-so-nice...or BIASED
    He has driven away and or banned some very fine folks...
    Martin may not believe me...but I respect him and GBA
    he just needs to realize how cruel his words can be...
    and that there is a BIAS to promote outside Polyiso (Martin's favorite)

  9. GBA Editor
    Martin Holladay | | #9

    I'd like to apologize to you and Gregory for any cruelty. None intended! I was just trying to answer Jim's 5 questions.

    Again, if this isn't clear: I certainly hope that you and Gregory will continue to post comments.

  10. Gregory La Vardera | | #10

    No - I'm good. I just don't want to waste his time or mine. Back to the regularly scheduled program.

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