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Community and Q&A

Double-stud wall insulation methods

CMB_arch_eng | Posted in Energy Efficiency and Durability on

We are building a new house in Wisconsin, zone 6, and are planning on using double wall construction, originally with dense-pack cellulose. None of the local insulators that we have talked to will install the cellulose.

I am still searching for another insulator, but in the meanwhile I have been considering a flash and batt method with 3 to 4 inches of spray polyurethane closed cell foam on the exterior side of the wall and either batt insulation or blown-in fiberglass making up the remaining cavity.

The space between the walls is 5 inches. Looking at a product like Roxul, can it be fitted where the SPCCF has been applied? It seems like that would be very fussy work. Also, the 5 inch space presents a challenge. I like the density of the Roxul, but I think this might not be the right application.

Part of the appeal is that we could do the batt insulation ourselves and are motivated to fit the insulation to the cavity. There will be no openings in the exterior walls except for windows and doors. (Electrical outlets will be in the floor.) Interior walls are 5/8″ drywall. Stud spacing is 24″ OC.

If we skipped the flash part and just used the Roxul, what type of vapor barrier should we use and where should it be placed? Exterior sheathing is OSB with Tyvek and cladding is cement board horizontal siding above and full stone veneer below. Rain screen is planned.

Insulation bids have been over $10K and that is for a 900 SF house and garage. That includes 2 inch flash foam of ceiling and 4 inch flash foam of exterior walls.

Does the double wall construction commit us to using a spray/blown-in method of insulation?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, the condensation risk on the interior surface of the cured spray foam increases when you add more fluffy insulation on the interior side of the foam. The guidelines for flash-and-batt insulation ratios are based on 2x6 walls, not 12-inch thick walls. So you'll need to do a dew-point analysis if you proceed with your flash-and-batt plan. (For more on this topic, see Are Dew-Point Calculations Really Necessary?

    Second: concerning your question, "If we skipped the flash part and just used the Roxul, what type of vapor barrier should we use and where should it be placed?" Neither building science experts nor the building code require the use of a vapor barrier. All you need is a vapor retarder, not a vapor barrier. I recommend the use of vapor retarder paint or a "smart" retarder with variable permeance (for example, MemBrain).

  2. jklingel | | #2

    Christine: Why not rent a "pump" and pack in your own cellulose? I have not done it yet, but am going to be, after doing some reading and watching youtube vids. I don't think it is all that hard, though I am sure one better do his/her homework and practice first. One key, according to Robert Riversong, is to have a blower w/ enough power to pack well. I believe a Force 2 or 3 is sufficient, but do some reading on that. BTW: One of the guys who packs in 18" has one of the installers walking around between the walls packing it in. Literally walking on what they've blown in, then blowing in some more. I guess that really dense packs!

  3. user-1137156 | | #3

    What stage of construction are you at? Are the walls up? Are both stud walls 24"OC. Are both 2x4? do you really have 5" between? The 5" inter wall space is a bit problematic with Roxul, it would be easier if it were 3",3 1/2" or 5 1/2" as these are the thickness of Roxul bats. If the walls aren't up I'd adjust the space to 5 1/2". I'll be building in northern Michigan, also zone 6 and plan on 3 1/2" space between walls. I have both 2x4 and 2x6 outer walls as I need 2x6 to support snow loads. My plan is to use r 15 bats in the 2x4 stud cavities AND in the inter wall cavity as well, but there they will be crammed next to each other. solidly from bottom to top. Out of curiosity what kind of foundation will you have. With Roxul you don't want any foam as it'll hamper drying. Well air sealed drywall with a vapor retarding paint should work well. Also be careful of the weight of ceiling insulation. My understanding is that drywall sag will occur with r60 cellulose over 24" oc framed drywall.

  4. user-901114 | | #4

    Or go with two layers of 23r roxul in an 11in double staggerd wall. It cuts and fits nice and tight... Home depot has the best volume discounts on roxul that I could find. You could go 24oc if code allows. Then put on a strapped 24oc raised heel truss roof with r60 cellulose on an airtight ceiling. Accessed from an outside gable end.

  5. jklingel | | #5

    FWIW, if you don't want to pump your own cellulose, fit in the Roxul w/ no foam. I did 'glass between the studs, then stacked batts like hay bails between the walls. That was in '80, and it's working fine. However, I'd never use fg batts again; Roxul, if I had to use batts, but better to dense pack.


    I also choose not to use cellulose in the double wall homes I build but install Johns Manville Spider microfilament fiberglass insulation instead.

    It doesn't hold as much water as cellulose and therefore doesn't get heavy and pull away from the top plate. (Please don't flame me for saying this - it is my opinion and experience that cellulose gets very heavy, settles, and contributes to mold growth on adjacent wood and drywall surfaces inside walls when it gets wet) I do still use cellulose on flat ceilings where I can get R-48 at reasonable cost with air circulation above to promote rapid drying when the insulation gets wet.

    Spider is advertized as a damp-spray system but we use a "Blown-in-batt" system instead where we staple a spun-bond polyolefin mesh "scrim" (like very thin Tyvek) to the interior studs with pneumatic staplers with the staples approx one inch on center and then we pack the walls full of the Spider with no binders at all. Water runs right through it and it dries very well. I've used it in attics but it's not recommended to be loose filled and is much more susceptible to air washing if not packed and enclosed on all six sides so we've gone back to cellulose in these situations.

    We off-set our studs about 1 1/2" so the right side of the outer studs is in alignment with the left side of the interior studs. This way we don't have to worry about getting a good pack in the space between the back of the front stud and the front of the back stud but we can still get one stud bay properly packed before moving on to the next stud bay. - if the studs are offset more than six inches it is hard to get a decent pack.

    See if your installers will quote a blown-in-batt with scrim. they may be nervous about the cellulose in the wide open walls and would feel more comfortable with the fiberglass. Even if you used conventional attic fiberglass rather than the Spider you would have a good R-value and drying potential.

  7. CMB_arch_eng | | #7

    Thanks for all your comments. We have just poured the footings, so we can still make some changes. The other folks on my team are concerned about the ease of installation of Roxul, so I probably won't push that. I agree that it would make the most sense to adjust the thermal break cavity to 3-1/2" to make it more compatible with the insulation system. Roxul also has sheet material in various thicknesses that could be used for that space. Any testimonials on the Roxul? As far as self-performing the insulation, I'm reluctant to do anything beyond batts.
    I will do the calculation to verify that the dew point would occur in the foam per Martin's advice.
    Michael, how do you deal with the air currents issue if fiberglass is blown in the attic? We are planning to ventilate (natural convection) and it will get mighty cold up there. If we have a contractor on site installing blown-in-batt fiberglass in the walls, is it worthwhile to use cellulose in the attic? This is beginning to sound like a Wisconsin insulation smorgasbord.

  8. homedesign | | #8

    Hi Christine,
    What is your Wall Air-Tightness strategy?

    If you decide not to use sprayfoam....
    Where is The Pressure Boundary (the Red Line)?
    Is THE Pressure Boundary on the Climate Controlled side? ....
    Or on the Not-So Climate Controlled Side of the wall?
    Or both?

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