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…