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Least expensive wall system in Vermont?

willgoodwin | Posted in Energy Efficiency and Durability on

I was reviewing the Vt. energy code and it allows 6″ walls with high density spray foam between the studs; I don’t think that was an option last time I checked. It seems as if this method will be so much less expensive than external rigid foam and a rainscreen, that it will become the standard. What do the experts think?

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Replies

  1. STEPHEN SHEEHY | | #1

    I'm no expert, but you still need a rain screen with most siding. And closed cell foam is very expensive and does nothing about thermal bridging through the studs. If I were building another house, I'd look at Zip R sheathing and dense pack cellulose.

  2. Expert Member
    Dana Dorsett | | #2

    Closed cell foam between studs is a waste of (environmentally & financially) expensive foam. In a 2x6 assembly it adds less than R2 to the "whole-wall R" performance, after thermal bridging is factored in. It doesn't keep the sheathing any warmer in winter, and at ~0.2 perms it all but prevents drying toward the interior. A rainscreen (or vinyl siding) would be prudent to give the sheathing some drying ability, but with 5" + of 2lb foam the sheathing could rot away and the house still wouldn't fall down, due to the structural rigidity of the foam, not that it's a desirable outcome.

    IMHO i's highly unlikely that 5" of closed cell foam between 2 x 6 will become the standard. From a lifecycle verditude point of view it's more than 2x the polymer per R of open cell foam, and even open cell foam has a higher environmental footprint than cellulose, rock wool or fiberglass.

    See:

    https://www.greenbuildingadvisor.com/articles/dept/musings/installing-closed-cell-spray-foam-between-studs-waste

    https://media.licdn.com/mpr/mpr/AAEAAQAAAAAAAAv3AAAAJGVlYWRjNTk5LWEyMTQtNDk3YS04MjFjLTZlYjdjMWEzNjRkZQ.jpg

  3. GBA Editor
    Martin Holladay | | #3

    William,
    These two articles address your question:

    Installing Closed-Cell Spray Foam Between Studs is a Waste

    How to Design a Wall

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