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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 2x6 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 2x6. 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.

Asked by Jim Cuhel
Posted Nov 14, 2017 5:48 AM ET


1 Answer

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I think this thread will provide some pros and cons on Zip-R: http://www.greenbuildingadvisor.com/community/forum/green-building-techn...

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: http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minim...

Answered by Steve Knapp CZ 3A Georgia
Posted Nov 14, 2017 10:31 AM ET

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