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Help with my walls, please

Nfoote | Posted in General Questions on

I have read and reread the info about exterior foam insulation and moisture issues.  I really want to stop the thermal bridging of my 2×6 walls.  I live in northern Utah, Zone 5, with an average winter temp of 29 degrees.  My builder is calling for the following wall system: 1/2″ gypsum board, 2×6 studs with 5.5″ of open cell foam, 7/16″ osb sheathing, Tyvek wrap, 1.5″ XPS, 1″ airspace, and a brick veneer.  I have calculated the sheathing temp at 37.2 and the dew point at 40.  I’m hoping to get the most efficient 2×6 wall system possible without moisture problems, and without too much extra work for my mason (anchoring brick veneer through too many inches of foam.  Thoughts?  Thanks so much for the help!

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  1. GBA Editor
    Martin Holladay | | #1

    The wall assembly proposed by your builder will work. The exterior rigid foam has an R-value of R-7.5, which meets the minimum requirement for a 2x6 wall in your climate zone. (To read more about the rules for the ratio of rigid foam to between-the-studs insulation, see this article: "Calculating the Minimum Thickness of Rigid Foam Sheathing.")

    Even though this proposed wall won't have any moisture problems, it's not ideal from a green building perspective. Most green builders avoid the use of XPS, a type of rigid foam that is manufactured with a blowing agent that has a very high global warming potential. Either polyiso or EPS would be a better choice. For more information, see these two articles:

    "Choosing Rigid Foam"

    "How to Design a Wall"

  2. Nfoote | | #2

    Thank you for the reply, Martin. I will try and make the switch to polyiso- which certainly looks to be a better summer product. Based on your previous articles, it should do fine in the winter as well.

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