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Flash-and-fill double-stud walls?

cjm5199 | Posted in General Questions on

After reading the Building science article regarding moisture monitoring in high R value walls:

I became interested in the idea of a flash and fill double stud wall. (new construction climate zone 6 – northern New England) as a “safe” wall for my location.

It seems that all of the literature on this approach includes using 2-3 inches of closed cell spray foam, depending on hdd, and then using dense pack cellulose for the remainder of the cavity. Would there be no benefit for the remaining portion of the cavity to be filled with open cell spray foam instead of the cellulose? Installation costs are similar in my area.

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  1. user-626934 | | #1

    Installed costs in your area are about the same for dense pack cellulose vs. open cell? Wow.

    Anyway, there likely would no benefit to the open cell foam (vs. cellulose). All of the air sealing between the studs is already going to be accomplished by the closed cell foam...mind you, that's not all of the air sealing that NEEDS to be done. There are a lot more leakage pathways outside of the studs than inside the studs. The R-value is about the same between open cell and cellulose. Cellulose would be quieter, with lower embodied energy, less risk for installation errors, and slightly more wall mass for better swing season and summer performance.

    Having said all that, if you want to build a "safe" wall, then why not build one using exterior insulation? A 2x6 wall w/ cellulose and 4" of exterior rigid insulation (mineral wool, EPS, polyiso) is going to give you about the same whole-wall R-value as a 10" double stud wall (using a 30/70 split between closed cell and cellulose)...for no more cost, and with less moisture durability risk.

  2. cjm5199 | | #2

    John - I have no labor costs for framing walls, and also like the idea that the the framing / flashing of windows and doors would be the same with double walls vs traditional walls are reasons I was leaning towards the double wall configuration.

    I may need to look further into the installed costs of dense pack cellulose vs open cell.

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