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Spray foam in a 2×6 wall : partially filled vs flash and batt vs full cellulose

mangler66 | Posted in General Questions on

Looking at my inside insulation options now, and I am waivering between flash and batt (2″ spray foam + 3.5″ kraft faced minerall wool batt), 3″ spray foam (leaving 2.5″ air cavity behind drywall) or seal the osb cavity with caulking and canned foam and just go full dense pack cellulose.

I already have 2″ of EPS foam on the outside of the  OSB sheathing, so with 2″ of spray foam only I would technically meet the 2012 OBC requirements (R-23). The advantage of flash and batt with 2″ as I see it is the spray foam would not interfere with wiring too much, so I could have it done now and get the  wiring done later. I could probably spend this winter without batt and drywall if needed.

3″ of spray foam would place me well over code, but should I be worried about the 2.5″ of air left behind the drywall?

Last option is probably the cheapest (full dense pack cellulose), but relies on me doing all the air sealing in each cavity before the insulation contractor comes. The only available contractor also uses cellulose with ammonium sulfate, whichIi want to avoid.

So, which option would you pick?

PS Zone 5, house is about 3000 squ ft net (windows and doors removed).

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  1. Expert Member

    Mai Tai,
    - If your air-barrier is the OSB, are you sure y0u need to air-seal each cavity? Can't you just tape the seams?
    - I don't know if it affects your decision, but you should check the OBC as to whether you can occupy a dwelling with exposed wiring. I know in un-finished basements the bottom of each wall has to be drywalled as mechanical protection. That may apply to the house above too. As the sequence of drywalling is typically, ceiling, upper-wall, lower wall, it doesn't make much sense to provide the protection, rather than do the whole job.

  2. mangler66 | | #2

    Thanks Malcolm. Sealing the OSB from the inside may be overkill, as we are taping the seams on the 2" foil faced Amvic EPS foam (perforated foil for high permeance), and then taping the tyvek house wrap over top. I just feel bad looking at the horizontal OSB seams on the inside of the wall.

    I don't plan on moving in with exposed wiring, at least not on the living level. But I will need to heat this place until spring when we are planning on moving in. My basement was never drywalled on the current house, but that was 2006. They may have gotten nuttier since then. Plus your point is well taken, as my first level is not technically a basement, so may be more strict.

    PS I miss the option on the old website to ask a question to a specific group (e.g. GBA Pro Help). My only option this time seemed to be "General".

  3. Expert Member

    Mai Tai,

    I've never understood why GBA offers a range of categories for questions, as they all appear on the same Q&A page. Maybe Martin can explain. Perhaps there is something I'm not taking advantage of.

  4. GBA Editor
    Martin Holladay | | #4

    Mai Tai and Malcolm,
    Other GBA readers may disagree, but I've always thought that the need to choose a category for questions was a silly feature. I always choose "General" when I post a question. I don't think it matters which category you chose.

    Mai Tai, I posted an answer to your question yesterday morning -- or at least I thought I did. It seems to have disappeared. I guess I'm still getting used to this new web site.

    Here's a modified version of what I wrote: Most builders who install exterior rigid foam establish an air barrier at the sheathing layer, by taping the OSB seams. It seems you didn't do that -- you probably should have -- but you've done the second-best thing, which was to use tape on the rigid foam seams and the housewrap seams. Since your rigid foam is foil-faced, that's a pretty good approach. I don't think you need caulk or spray foam between your studs.

    Q. "Which option would you pick?"

    A. Dense-packed cellulose. The main reason I say that is that I don't think it's a great idea to sandwich your OSB sheathing between two low-permeance layers of foam. For more information on this topic, see "How to Design a Wall."

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