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Open Cell SF in double stud wall?

JMRtbay | Posted in Pretty Good House on

Howdy folks, 

My architect and I have designed a variation of the “double stud wall simplified” as detailed on 475 High Performance Building Supplies. https://ca.475.supply/blogs/design-construction-resources/the-double-stud-wall-simplified-low-cost-high-performance

We have a 5” gap between interior and exterior walls, for a total wall thickness of 12” on the main floor and 14” in the basement, and have planned for dense packed cellulose in combination with Pro Clima membranes.  

It is proving slightly challenging locating a dense pack installer in my area. I’m keeping my eye open for a second hand dense packing machine, but if I cant find either I’ll be needing to look at alternatives. Other than using batts, the only other option would be open cell spray foam. I’m aware of the environmental implications and not really looking for that debate at this time.

If one were to use OcSF in this wall assembly, what modifications would be needed? I don’t believe an interior vapour barrier would be needed. There would be a minimum of 8.5” of OcSF, but we could also have the entire depth of the cavity sprayed. The interior stud bay would be treated as a service cavity then filled with batts or OcSF once mechanical/electrical is complete. 

I’m not 100% sure of my climate zone (6 or 7?)but I’m near the north shore of Lake Superior in NW Ontario, about 350 miles N/E of Minneapolis. We have cold dry winters (-30c and beyond occasionally) and mildly warm but fairly dry summers (25-30c).

Thanks for any input!

 

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    You would have no need of that "inside the wall" air barrier if you fill the space with spray foam. Open cell spray foam that thick is a good air barrier itself. You'd have a wall that would perform very well, it would just be using expensive insulating materials if you go with spray foam, so that's the only real downside here. You'd also need to keep things open enough for the crew to be able to spray the entire interior of the assembly evenly.

    Remember that common installation practice with open cell spray foam is to overfill stud bays and then trim it flush with the surface of the studs. That means you need a back side barrier to spray against. Your Pro Clima membrane would work here, but you'll probably find it will bow outwards somewhat after the foam expands and solidifies. Making sure that membrane is pulled very taught and is well secured (stapled) to all the studs should help minimize that. The spray foam will seal everything, so you don't have to worry about getting some holes in the membrane while tensioning it.

    I see no need for an interior side vapor barrier here. You could probably eliminate any/all membranes except your outermost one that would be the barrier getting sprayed against.

    Bill

    1. JMRtbay | | #4

      Thanks for the input, Bill. You confirmed what I expected.

      We’d be using diagonal strapping on the exterior which would hopefully help cut down on the bulging. Not needing a membrane on the exterior face of the interior stud wall would greatly improve build ability.

      1. Expert Member
        BILL WICHERS | | #5

        Spray foam just needs something to spray against, which can be OSB, plywood, planks, even batts stuffed into holes. It just needs a surface. If those slats are placed sufficiently close together, you should have minimal issues with bulging. You'd want a membrane to keep the spray foam from blowing through gaps though, unless the planks are fairly tightly fitted together.

        Bill

        1. JMRtbay | | #6

          Yes, we would still use a vapour open WRB on the exterior in addition to diagonal strapping for a rain screen.

  2. Expert Member
    Michael Maines | | #2

    As often noted on GBA, there are downsides to using foam: higher global warming emissions than other types of insulation, and because it sticks to everything, it makes future renovations and building disposal more challenging and less environmentally friendly. Not everyone considers our buildings' impact on our shared environment to be part of "green" building, but it's still very important to me.

    1. Expert Member
      BILL WICHERS | | #3

      You should probably add that spray foam is usually also the most expensive way to insulate, often without any performance benefits over other, cheaper, insulating materials as long as you get the air sealing details right.

      I did notice that the "service cavity" here (which is something I don't think is worth the effort in nearly all cases) would be filled with spray foam, which would defeat the purpose of the service cavity.

      For the OP: I'm one of the most pro-spray foam people on GBA, and while I did respect your request not to get into the green/not-green argument here, I do think there are better ways to insulate this wall. I would fall back to batts before using spray foam here if you can't find someone to do the dense pack cellulose.

      Bill

    2. JMRtbay | | #7

      Unfortunately my Region is not blessed with a very diverse insulation contracting market. What’s the total environmental penalty if I need to hire a company to drive their 20’ box truck 16-18hrs to come blow in cellulose vs a local company spraying a slightly less environmentally friendly product?

      There are many OcSF’s on the market with very low GWP. They have 1/1000th the GWP of CcSF and less than mineral wool.

      1. Expert Member
        Michael Maines | | #16

        For some reason, spray foam companies only share the GWP of their blowing agents, not that of the solid portion (resin). Open cell foam is usually (or always?) blown with water. The GWP of the resin content is significant.

        Cellulose still has significantly lower embodied carbon, though a quirk of cellulose reporting is that they don't count the raw material as having a carbon footprint.

        Mineral wool GWP varies by brand but low carbon emissions are not its strong point, and I avoid it when possible as a result, though it poses less health risks and is much friendlier to future renovations so I use it occasionally.

        Cellulose is not the only alternative, but it is the only type of insulation, other than wood fiber, that I would use in a double-stud wall, because in a cold climate the sheathing WILL get damp in late winter and cellulose protects everything it touches from microbial action thanks to its mineral borate content.

        This is an open, free forum and many others will likely read this thread, now or in the future. Even if the environmental aspect isn't important to you, by sharing my thoughts, someone else may be convinced to avoid foam.

        1. JMRtbay | | #17

          The wet sheathing issue is exactly why we are using this wall assembly as detailed on 475. Unfortunately my architect wasn’t comfortable going completely sheathingless, there is significantly less than a standard double stud design.

          For the record I never said the environmental aspect wasn’t important to me. I stated that I wasn’t interested in debating it in this forum. I’m simply trying my best to research all possible alternatives in case “plan a” doesn’t work out. I didn’t want want the environmental argument (regardless of its validity) to overshadow information about the mechanics of how OsSF would perform in this wall assembly and what modification might be necessary, yet alas, here we are.

  3. Expert Member
    Akos | | #8

    You can also try to search for a BIBS installer, in some areas they are more common.

  4. begreener | | #9

    If you are going to use foam - have you considered redesigning your wall assembly?

    Major reason for double stud wall is for thermal break & then to achieve the desired rvalue with a blow in insulation ...

    What about a zipr sheathing (continuous insulation) for thermal break?

  5. Malcolm_Taylor | | #10

    JMRtbay,

    The wall looks ideal for batts. What's your hesitation in going that route?

    1. JMRtbay | | #11

      I have a strong distain for fiberglass batts. Mineral Wool batts are bloody costly, but then again so is OcSF. Although mineral wool could be a diy install. If I went the batt route it would make sense to bump the wall gap to 5.5”.

      1. Expert Member
        BILL WICHERS | | #12

        Why are you so against fiberglass batts? If it's from past experience with them, I understand completely! I used to hate the stuf too -- itchy, dustry, nasty stuff. The new material is surprisingly better though, much softer, less itchy, and less dusty. I still wear a dust mask and gloves when working with the stuff, but I don't itch like crazy for a day or two afterwards anymore. It's really impressive how much better the stuff is now.

        You could easily use unfaced batts in this wall assembly. You can even get high density batts that get about the same R value per unit thickness as mineral wool. You might reconsider using fiberglass here if your options are limited.

        Bill

        1. JMRtbay | | #13

          Thanks for the insight Bill. Along with being horrible to work with, their lack of density and general inability to restrict air movement make them undesirable.

          But I’ll definitely look into these higher density batts you refer to.

          1. Malcolm_Taylor | | #14

            JMRtbay,

            High density fg batts are a good option, but apart from the short term annoyance of workings with regular ones (maybe two days?) they perform well. Their lack of density doesn't appreciably reduce the R-value, nor do they lose much insulating value from wind-washing - especially in air-sealed cavities such as your walls. See tables #8 and #9 in this link: https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf

          2. Expert Member
            BILL WICHERS | | #15

            The new stuff is way less horrible to work with. I was really suprised myself, as when I was younger, I spent a lot of time laying in blown fiberglass in old attics running wires. I learned to hate that stuff with a passion. I don't know how they did it, but the "prickly" feel of the stuff is gone now, it's much softer and way better to work with. The difference is very noticeable between the decades old stuff and the current product. I usually use Knauf brand fiberglass if it makes any difference. Their stuff is a brownish color with a pattern like you'd see in hard candy, and it's made from recycled bottles.

            The higher density batts are much stiffer. R15 fiberglass batts are light lighter mineral wool, and while they are a bit fluffier, they do hold their shape much better. I wouldn't use the old R11 stuff, but R13 is pretty workable, and R15 is actually fairly nice.

            The air movement issue isn't really a problem if you have an air barrier on both sides, which you do in your assembly. The air movement issue was mostly with low density vertical batts with no air barrier on one side, such as is commonly seen with attic knee walls. If you put air barriers on both sides, that issue is pretty much gone.

            Bill

  6. begreener | | #18

    You might consider - to save money - using mineral wool only in the space between walls for extra air movement protection & then knauf batts in the stud walls...

    https://www.knaufnorthamerica.com/en-us/products/batts-insulation

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