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Insulating 4-inch exterior walls

Steve Karkau | Posted in Green Building Techniques on

I am in the process of total gut remodel project, zone 5, 2×4 walls and installing vinyl siding.
I am trying to decide if I should use structural insulated sheathing such as Huber Zip-R and closed cell foam to fill stud cavity’s. or install foam on outside exterior sheathing and use open cell foam.
any feedback would be appreciated.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Closed cell foam in a stud cavity is a waste of good foam, and comes at a high environmental price, due to both using 4x the amount of polymer, but also from the high global warming of the HFC245fa blowing agent used (compared to water, the blowing agent for open cell foam.) The difference in "whole wall" performance between open cell and closed cell is only about R1, due to the high conductivity of the ~25% framing fraction that is only R4.2 dragging down the average.

    Putting continuous foam on the exterior of the framing delivers it's full performance-R and that's where you get the bang/buck, since it isn't undercut by the framing fraction. R5 would be sufficient dew point control at the sheathing boundary to be able to use fiber insulation or open cell in the cavity with only standard interior latex as the interior vapor retarder, but more is better. The 1.5" ZIP-R would be the minimum necessary in that product line to keep the fiber or o.c. foam dry, or you could use other sheathing and fully exterior foam. For exterior foam 1" of polyiso or 1.5" of EPS would be greener and better than 1" of XPS. At 1" /R5 XPS would initially have sufficient R for dew point control, but over a few decades it's performance decays asymptotically to R4.2-ish as it's blowing agents bleed out, which isn't enough. At 1.5" EPS would be R6+ now, and 100 years from now. At 1" polyiso would be R6-ish initially, but needs to be derated for temperature to about R5.5, falling slowly to R5.

    Both polyiso & EPS are blown with relatively low environmental impact pentane, whereas XPS is blown with a combination of HFCs, with something like 200x the lifecycle global warming impact of pentane.

    In many locations it's possible to buy reclaimed 2" roofing foam at a fraction of the cost of virgin stock foam, and since it's not using new materials, is far greener than any virgin stock foam. At 2" any foam would have sufficient dew point control, no matter what temperature or longevity derating is used.

  2. GBA Editor
    Martin Holladay | | #2

    Steve,
    Dana gave you good advice. For more information, see this article: How to Design a Wall.

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