Zone 6 wall construction – Is Zip R sheathing (R12.6) adequate
A contractor I am considering for a new house build in Fond du Lac Wi (zone 6) is recommending using the foam zip sheathing (2″foam – R12.6) and HD batt. The wall is 2×6 So the total R value is (20+12.6). After reading the articles here on gba for calculating the minimum ridged foam thickness, I’m concerned this isn’t enough because the foam is de-rated at extremes temps. But since the zip foam faces the wall cavity, I wasn’t sure if the dew point being inside is still an issue as long as the wall can dry to the inside by providing the class III vapor barrier. I was thinking of asking the contractor to go back and give me a quote for zip sheathing (non insulated – just osb) and 3″EPS foam over the top of it. What do you recommend?
Another contractor I am considering just uses 1″ridged foam and batt insulation also with 2×6. And no wood sheathing. I think the total r value here is 25. I realize this isn’t nearly as energy effiecent as the first contractors recommendation but would this wall have condensation/moisture issues?
Thanks I appreciate everyone’s perspective.
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I think this thread will provide some pros and cons on Zip-R: https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/100998/zip-system-r-sheathing
Some folks on GBA don't like ZIP-R because they feel the foam is on the wrong side. That may be a valid argument. If you decided to use exterior foam, you would need R-11.25. Three inches of EPS wouldn't get you there, but 2 inches of XPS would be pretty close. See this article for more information: https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing