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Community and Q&A

Double stud wall with flash-and-fill insulation?

user-4405197 | Posted in Energy Efficiency and Durability on

I’ve been busy reading in the Q&A forum for the past year and a half, but just recently started posting some questions as our project nears – hope I’m not wearing out my welcome. Can’t believe how responsive its been – thanks Martin. I keep reevaluating our possibilities for our wall and insulation assemblies, though.

Martin – how does a double stud 2×4 wall flash and filled with 2 inches of closed cell spray foam and dense packed cellulose sound? I’m comparing this to my other consideration of a 2×6 wall air sealed with sprayable caulk, filled with Roxul Comfort Batts, sheathed with OSB and thermally broken with exterior Roxul rigid insulation.

It sounds like option 1 might be the cheapest and easiest to build. Any thoughts? (Trying to avoid paralysis by analysis but am not succeeding very well).

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  1. BillDietze | | #1

    Brian, if you do the flash and fill double stud wall, spray the studs too: two inches of foam everywhere and use it to seal the top and bottom plates as well. What zone are you in? If cold then two inches won't be enough. The sheathing will be cold and dry slowly, so consider plywood.

  2. GBA Editor
    Martin Holladay | | #2

    I don't recommend installing any closed-cell spray foam in a double-stud wall. Double-stud walls are designed to dry to the exterior, and any closed-cell spray foam might limit outward drying.

    If you want to combine foam insulation with air-permeable insulation in a double-stud wall, it's almost impossible to get the ratio between the foam and the air-permeable insulation right without going to a ridiculously thick layer of foam insulation -- and that's not economical.

    The entire point of a double-stud wall is to use an economical (and environmentally friendly) type of insulation -- usually cellulose -- and to avoid the expense and environmental drawbacks of foam insulation.

    For more information on these issues, see:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How Risky Is Cold OSB Wall Sheathing?

  3. user-4405197 | | #3

    Bill I'm in climate zone 5 I believe - Oakland county Michigan.

    Martin, how about using open cell spray foam instead?

    Comparatively speaking, how cost intensive is my Roxul mineral wool option? (Comfortbatts in the cavities sealed with spray caulk and rigid exterior mineral wool)

  4. HarrisDwight | | #4

    I'm not an expert, but this is my understanding / limited experience:
    Roxul is expensive. The only practical reason to use Roxul in general is if you need superior sound or fire protection. I'd use fiberglass batts on the interior and roxul on the outside, if that's what you want to do. From what I hear, the spray caulk air sealing doesn't get as tight as carefullly paying attention to details would. It's more for a lazy builder to spec so he can make a homeowner happy. Better off using Zip walls and carefully taping, IMO and using acoustical sealant at your plates.

    If you're sheathing with plain OSB I'd think you would need house wrap even with exterior insulation, especially roxul, since that's not water proof like rigid foam with taped seams.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "How about using open cell spray foam instead?"

    A. It's your house. You can specify open-cell spray foam if you want. It will work.

    I'm not a fan of open-cell spray foam between studs. You don't get any R-value improvement over cellulose, and the stuff is expensive.

  6. Expert Member
    Dana Dorsett | | #6

    Open cell foam is often cheaper than dense packed cellulose- it depends on just how hungry they are. Cellulose in fat walls has a subtle but real thermal mass effect too. When all else is equal cellulose is both greener, and slightly higher performance at any given R-value (due to the mass effect.)

    With open cell foam OR cellulose in a zone 5 climate you still can't ignore vapor diffusion loading issues, as discussed recently on this site, even though code would not require an interior vapor retarder for an insulated 2x6 wall with wood sheathing in zone 5 as long as the siding is back ventilates. This has been discussed at length multiple times recently on this site, eg:

    While neither the cellulose nor the ocSPF walls suffered mold or rot in the first 2 years of occupation, the peak moisture levels were pretty high unless the interior humidity was kept low/very-low (lower than the 30% RH recommended for human health). A 2 year survival without problems isn't exactly something to brag about, when the moisture content of the sheathing of the cellulose insulated wall was over 20% into May, at temperatures that would support mold growth.

    But putting a layer of smart vapor retarder (Certainteed MemBrain, Intello Plus) would allow you to run the interior RH at just about any reasonable level. MemBrain is sufficiently vapor open in the presence of higher humidity air that if the assembly took on moisture the drying rate toward the interior would only be limited by the interior paint, which has a vapor permeance of about 3-5 perms). With Intello it has to be nearly saturated air to get much above 1-perm, but it's more vapor tight. An alternative is to use a "vapor barrier latex" as the interior side vapor retarder, which is rated at about 0.5 perms. That would limit the rate of moisture moving from the interior, but it would also limit the drying rate by quite a bit too, more so than Intello, and FAR more than MemBrain. Cost wise vapor barrier latex is by far the cheapest, but MemBrain can be had retail-online for about $100 + shipping for an 8' x 100' piece (cheaper, if you have a distributor near you who carries it). Intello Plus is about 3x that per square foot- it's both a heavier material, and it's imported, with limited distribution in N. America.

  7. user-4405197 | | #7

    Thanks for the info Dana. You bringing up Membrain got me investigating and it led me to some Q&A with Gregory LaVardera and his advocacy of Swedish wall design and the "USA New Wall." Here's the link:

    Is this something worth serious consideration? Problem is, everything is sounding like a viable option - I'm wondering what you or Martin might prefer if you were building a wall of your choice? The only thing that I saw in the Swedish design that was confusing was the smart air/vapor barrier that according to some recent GBA articles isn't necessary in my climate zone 5 - if I planned on keeping my sheathing warm with Roxul comfortboard on the exterior.

  8. GBA Editor
    Martin Holladay | | #8

    "Everything is sounding like a viable option" because it is. There are 50 ways to build a wall.

    Choose a way that makes sense to you (or your builder), is buildable, and is affordable. Then stop worrying.

  9. user-1137156 | | #9

    You will find that mineral wool boards are more than twice as expensive, per r sq ft. as mineral wool bats. If you are considering a double 2x4 stud wall. then putting OSB on the outer face of the inner wall, detailing the OSB as the air barrier and insulating with 3 layers of mineral wool bats covering the outside with NO sheathing, just a house wrap then siding will result in an economical, well air sealed high r wall.

  10. user-4405197 | | #10

    Good info. Thanks Dana, Martin, and Jerry.

  11. Expert Member
    Dana Dorsett | | #11

    If you're planning to rely on insulating sheathing to keep the sheathing warm enough for dew point control with only standard latex paint as the interior vapor retarder, it has to be at least 25% of the total-R in climate zone 5. For a double stud-wall design that's quite a bit of exterior R.

    IRC chapter 7 give some guidance:

    The "vented cladding over wood structural panels" doesn't really scale to high-R assemblies, as shown by Ueno's data. But the exterior-R/cavity-R ratios would still work, since that's what determines the average temperature of the sheathing. In a 2x4 assembly you'd get at most R15 cavity fill using standard fiber insulation, and an exterior R5 would keep the sheathing warm enough for zone 5. In a 2x6 assembly the most you'd get is about R23 cavity fill, and R7.5 on the exterior is enough. R5 is 25% of (R5+ R15=) R20, just as R7.5 is ~25% of (R7.5 + R23=) R30.5. For an R100 wall, you'd do fine with R25 outside the sheathing, R75 inside, etc.

    So, say you installed 11" of rock wool in a double studwall you'd be at about R45 for cavity fill, and you'd need at least R15 on the exterior. That's 4" of ComfortBoard. Using ComfortBoard instead of rigid foam gives dramatically better drying rates toward the exterior- you'd be able to cheat that a bit and go with 3" but don't push your luck, with only 2" (or less) without a smart vapor retarder on the interior side.

    Half-inch plywood is a 1-perm vapor retarder when dry- if you don't keep it warm enough it'll be taking on quite a bit of moisture with only 5-perm paint on the interior side. But with a smart vapor retarder membrane that is sub-1 perm when dry and more than 5 perms when at high humidity you gain a lot of resilience- the plywood could pass through moisture at a sufficient rate to mostly keep up, even if the exterior R was on the thin side.

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