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Double Stud Wall with Mineral Wool Batts

Doug_Epperly | Posted in Green Building Techniques on

All,

We have settled on a double stud wall design for our new house. We had planned to use dense pack cellulose to fill the cavity, with a goal of an R40 wall. Unfortunately, we’ve only been able to find one firm in our region (Billings, MT) that does dense pack cellulose and they’ve never done a twelve inch cavity like we are proposing. They are willing to try, but having read a few warnings on GBA about finding contractors experienced with dense pack, we’re looking at other options.

Wanting to stay with the double stud wall design, we have looked at mineral wool batt insulation as an option. It seems like we could run mineral wool vertically in the outside 2×4 stud wall, run 5.25 inch batts horizontally in the space between the stud walls, and then run batts vertically again in the inner 2×4 stud wall. I’m unsure how to calculate the true R value of this wall, but based on the mineral wool packaging it would be R15 + R23 + R15. We recognize the true R value would be less than R53, but would it be R40 or greater?

Any other thoughts on this approach? Pitfalls? Concerns? Tips?

Thanks, Doug

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Doug,
    Your plan will work. There isn't much of a thermal penalty for the framing elements of a double-stud wall. If your wall is 12.25 inches thick, the whole-wall R-value will be in the range of R-42 to R-45.

  2. Doug_Epperly | | #2

    Thanks Martin!

  3. westerman | | #3

    I have a similar idea, using a double 2x4 wall but attaching EPS rigid foam or possibly Roxul comfortboard along the length of the studs for the thermal break (1-1/2" wide x 3-3/4" deep) for a cavity of 10-3/4". Roxul batts would fill the exterior 2x4 wall to R-14 and the interior 2x4 wall would have an R-28 batt (designed for a 2x8 wall). Would this work or do you see a problem with attaching the rigid insulation? I think it could be framed conventionally as a tip-up wall, with exterior plywood sheathing attached. What do you think?

  4. user-1072251 | | #4

    I know a builder in Vermont who built a similar house - 12" d.s. wall with three layers of Roxul as described. Was very happy with the ease of installation & performance.

  5. Expert Member
    MALCOLM TAYLOR | | #5

    Amber,

    It's an interesting idea and you end up with a well performing wall. i can't see it ever becoming popular with production builders, even those doing high performance houses, because the foam is so labour intensive - but that may not be a deterrent to one-off builds.

    I've heard of it being done with smaller gaps where the foam was only 1 1/2" deep. How fiddly securing and maintaining 3 1/4" foam in place during construction is the real concern. It might be worth mocking up a small wall section to try before committing to it.

    Substituting batts for cellulose in thick walls was raised as a concern by John Straub in a recent blog post. How great a vulnerability it poses I don't know.

  6. ROBERT OPALUCH | | #6

    I've considered doing 3 layers of Roxul ComfortBatt as well as two layers, one of which fills a stud cavity and the between-stud cavity. However I'd suggest a smart vapor barrier be attached to the inner layer of one of the stud walls. That would help avoid condensation on the exterior sheathing, and address Straub's concern. Ply sheathing instead of OSB would be be more vapor open and less susceptible to rot from water vapor condensation.

    I wonder if layers of Roxul ComfortBatt were installed horizontally in the cavity between the stud walls, what support might be needed to avoid some (probably minor) compression of the layers. Over time, compression might leave an air gap at the top of the cavity? Using batts that span both the cavity and one stud bay avoid this problem but result in that gap behind the studs that needs to be filled with EPS or ComfortBoard. The studs or batts on either side should hold those pieces in place no problem.

  7. ROBERT OPALUCH | | #7

    Offset studs (the double walls have studs that don't align with each other) would improve the R-value of the double stud wall assembly, but can increase the number of studs when they align at door and window bucks.

    If high performance windows and doors are custom width, you could order window widths so that one or both king studs to be the regular 24" or 16" o.c. stud in the wall. That would reduce framing work but also result in fewer batts to be trimmed to fit between studs.

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