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Community and Q&A

A couple of envelope questions

user-4704465 | Posted in General Questions on

Good morning All, I’ve been framing my pretty good house, and have arrived at another couple of envelope questions: I have a 2×6 frame with taped Zip sheathing for an air barrier, followed by 3″ of recycled poly-iso, skinned with 1/2″ CDX for a nail b…

I think I covered it in the question- I’m sure I’ll hear about it if not!
Oh, coastal New England, marine zone 4.

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  1. Expert Member
    Michael Maines | | #1

    Hi John! Your question did not post completely. From what I see so far, I would recommend adding 1x or 2x strapping over your polyiso, both to allow the back of the cdx to dry, and to allow for some shimming--recycled foam varies in thickness, so depending on how particular you are, you might want to shim to a flat plane.

  2. GBA Editor
    Martin Holladay | | #2

    If you try to cram too many sentences in the title block of your post, the title cuts off the end of the paragraph.

    You should try again. Post your question as a comment on this page.

  3. user-4704465 | | #3

    Yes, I see that my question got cut off. My question was about comparing exterior weather barriers over the CDX. My original intention was to use 15# felt paper, a tried and true material with good characteristics. But, with winter coming, I'm tempted to use a self-adhering WRB- Henry Blueskin- because it won't blow off in the nor-easters that my house will be exposed to in the time that I'm working on it. As a builder, I know the pros and cons- cost, application, practicality, etc. What I don't know is the relative performance; will the Blueskin allow adequate drying? I did not shim or space the CDX- my foam was very flat and consistent, and my experience has been that cedar shingles are very forgiving- they self-vent and dry, and they don't leak.
    Thanks for any and all insight.

  4. GBA Editor
    Martin Holladay | | #4

    You never quite completed the original description, but it sounds like you've got a site-built SIP (Zip sheathing, 3 inches of polyiso, and plywood), with cedar shingle siding.

    Henry Blueskin is good stuff. It's a self-adhering housewrap that is waterproof and vapor-permeable. It will work just fine.

    Your next question will probably be: "Do I need an air gap -- something like Cedar Breather or Benjamin Obdyke Slicker -- between the WRB and the cedar shingles?" But since you haven't asked that yet, I won't go there.

  5. user-4704465 | | #5

    Thanks Martin,

    I've used the Blueskin and it's remarkably durable once installed, standing up to repeated storms with high winds- nice for winter time construction on a coastal island. Although tough to install on a windy day. My question was whether it had comparable characteristics to felt paper, which reputedly has variable permeability depending on moisture conditions.
    No, I wasn't going to ask about cedar breather or air gaps, so no need to go there! I've spent a lot of the last 30 years tearing cedar shingles off and replacing them, so I've got a pretty good understanding of how they work, what works behind them, and what the failure modes are. I use cedar breather on roofs, but not on walls.
    Again, thanks for your response, and for all of the other good information I've gleaned from this site.

  6. GBA Editor
    Martin Holladay | | #6

    According to Henry Company, there are three kinds of Blueskin:
    Blueskin TG is a vapor barrier.
    Blueskin VP100 has a vapor permeance of 33 perms.
    Blueskin VP160 has a vapor permeance of 29 perms.

    None of these products, to the best of my knowledge, is a "smart" retarder with variable permeance.

    Asphalt felt behaves somewhat differently from a plastic product like Blueskin, of course. Asphalt felt can absorb and redistribute moisture, to a limited extent, in a way that Blueskin cannot. I doubt whether that fact matters much under cedar shingles, but it might -- especially if you decide not to install Slicker behind the shingles.

    Certainly, asphalt felt is the go-to choice of WRB behind cedar shingles for traditionalists. It's time-tested.

  7. Expert Member

    Wasn't there an article on GBA a while ago about how housewraps lose their water resistance quite quickly when not protected by cladding?

  8. GBA Editor
    Martin Holladay | | #8

    You're probably thinking of this article:
    Can Your Water-Resistive Barrier Take UV Exposure?

    Allison Bailes discusses a presentation by Marcus Jablonka of Cosella-Dörken. Jablonka's employer manufactures WRBs, so the company has a vested interest in undermining its rivals' products -- a fact that taints the research conclusions, I feel.

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