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Q&A Spotlight

How to Build a ‘Perfect Wall’

Exterior foam will limit thermal bridging, but choosing between cellulose and spray foam can get contentious

Reduced thermal bridging

Rigid foam can be part of a 'perfect' wall system as long as it's thick enough for the climate. What to use for cavity insulation is another question.
Image Credit: FHB

Andrew Homoly is building a house in Kansas City, Missouri, and plans to use a “perfect wall” system consisting of 2×6 studs, Icynene open-cell foam insulation and an additional 1 1/2-in. layer of rigid foam on the building’s exterior.

His question in this Q&A post at GreenBuildingAdvisor concerns the best way to install window and door flashing, and where to install housewrap.

“Should the window flanges go over the foam at the window openings or should you line the window openings with a 2×4 so the window can be mounted into a solid piece of wood?” Homoly asks. “This would provide a more stable connection, but negates the benefit of the thermal bridging provided by the foam.”

Homoly is planning on using Huber’s Zip wall system under the rigid foam, and applying housewrap over the foam, an approach he recognizes as “a bit overkill” but reassuringly redundant.

The debate over how best to attach and flash windows in walls with exterior foam has been going on for years. What does the forum have to say? That’s the topic for this week’s Q&A Spotlight.

Sorry, this wall system isn’t ‘perfect’

For starters, Brett Moyer points out, there are three problems with the wall system Homoly is proposing, leaving it something short of perfect. He cites these shortcomings:

â— There is “zero moisture buffering capacity.”

â— The Zip System is expensive and unlike more traditional methods it relies on tape rather than overlapping layers of material to keep moisture out of the wall. Further, Huber hasn’t revealed what the perm rating of the material is.

â— There’s no need to use petrochemical foam where it…

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2 Comments

  1. Glen M | | #1

    Alternate Sheathing Option
    Excellent comments.

    I agree the Zipwall system doesn't seem to make sense in this application as well considering the Spider. Although we have been using the Spider with good success, Andrew may consider "equal" products such as AsureR Plus, Jet Stream Ultra, or Propink L77. Cellulose is fine too (I have it in my house) and the concern over it holding moisture can be overblown, notwithstanding hurricane zones, although that doesn't hold much creditability for me either since how often does that occur and how much can be saved/prevented in the aftermath is a big unknown.

    Andrew didn't comment on what siding product is being planned, but does imply it is a nailed-up siding such as fiber-cement or similar.

    One other option to consider is structurally insulated sheathing. I am only familiar with Dow's product (SIS) which comes in 1/2 and 1-inch thickness, so R-value is less, but eliminates the wood sheathing and simplifies window/siding installations.

    I would have a hard time promoting a double-wall system in this application. Yes it has all these great benefits, but it costs more and uses more resources. In this climate I just don't think it can be justified from a cost-benefit perspective. If you are looking to increase the R-value, use the rigid with closed-cell foam.

    If you use the 1 1/2" rigid, I would go with an "outie" design with the 2x2s (as noted). In addition to ease of installation it gives you a nice "deep" window well.

  2. Michael Roland | | #2

    Rock Wool Rigid Board and Batts
    An alternative to exterior rigid polyiso as a thermal break are rock wool boards. Two mfgs. I'm familiar with are Roxul and Thermafiber. They both make batts as well, which have a slightly higher R value than fiberglass and allow less air circulation. They're sometimes difficult to get hold of since they're used mostly in commercial construction. But they can be found. Lowe's is one source for Roxul.

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