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Community and Q&A

Spray-Foam Insulation for Exterior Wall and Ceiling

christian_330 | Posted in General Questions on

Hi again everyone. I’m to the point in my 1970 house complete remodel where I need to start planning insulation. I’m in climate zone 6 in northeast Ohio, my home has 2×4 exterior walls that are sheathed in something called flintkote which is about 1/2in thick. It’s like a Fiberous type material that I’m sure some of you are familiar with. There’s about about a 3/4in cavity outside of the sheathing in between the nailers for the cedar siding on the outside. My plan was to use the froth pak 200 kit I bought and picture frame spray and batt the interior of my wall cavities. I got kind of spooked with the chemical concerns with spray foam and also the moisture accumulation that could happen on the face of the foam in the winter (it didn’t seem like I’d be able to get a very narrow bead with the sprayer). I’ve since decided to use the can foam to cover all gaps and protrusions for an air seal using as thin of a bead of foam as possible to avoid condensation building up on it in the winter. I was planning on using r-15 Rockwool and covering everything with the certainteed MemBrain barrier/retarder. Is this a solid plan and what would you do differently if not. Thanks.

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  1. Expert Member
    BILL WICHERS | | #1

    Moisture accumulation on the surface of spray foam means you didn't apply a thick enough layer and the interior side of the foam is falling below the dew point of the interior air. This isn't a problem with the spray foam itself, it's a problem with either or both of an improper application (too-thin layer) or improper application of the product (using it somewhere where it can't be applied correctly).

    I'd use urethane caulk on the gaps between those sistered studs. I've done in the past exactly what you're doing now, and I'm guessing you'll find that if you run a putty knife over that bead of canned foam it will pop right off with none of it staying in the skinny gap between sistered studs. Canned foam works best on larger gaps, caulk works best on smaller gaps.

    I think you have a solid plan with mineral wool in the stud cavities and MemBrain over the interior side of the studs. My home office where I am right now writing this post is built exactly the same way (I recently renovated the space), and it was done based on input from GBA just like you're getting :-)


    1. christian_330 | | #2

      Great! Thanks. Yeah since I wasn’t planning on spraying a thick coat of foam and only wanted to use it to cover the gaps, that’s why I elected to use the can and use the smaller amount needed. You’re exactly right with the sisters studs and small gaps haha.

  2. christian_330 | | #3

    I’m planning on using the Accuvent system for venting the attic. These are thin pieces of PVC that staple to the top plate and then up the roof to ensure air passage through the soffit into the attic. I’ve had to trim them to fit and I’m using can foam around the edges to seal them up from drafts. Does anyone see a problem with this and possible moisture accumulation on the face of the PVC right at the corner where the wall meets the rafters in the winter months?

    1. Expert Member
      Akos | | #4

      You don't need to air seal the baffle to the rafters. It is just there to keep the insulation out of the vent space. The air sealing there doesn't really do anything.

      The one you want to air seal is your top plate. This should get sealed to your ceiling and to your sheathing if that is your main air barrier. The simplest is to spray the whole thing with an 1" or so layer of two part foam.

  3. christian_330 | | #5

    Ok, thanks! I was using the canned foam mostly to prevent wind wash from blowing right into the side of the insulation but I guess if I use the bigger froth pak kit to seal all around that are the point would be moot. Spraying it thick enough would also prevent any moisture accumulation on the PVC correct? One problem I may have is that I was planning on using craft faced r19 fiberglass batts that would be stapled to the ceiling joists as my first layer of insulation (got a good deal on a few bags of it) . I think this would make spraying the foam after the batts and drywall installed difficult in those area. Could I just leave the areas near the exterior and interior wall top plates not stapled and kind of loose, then go in the attic, pull it back, spray, let it cure, and then push it down? I’m also undecided on what to layer over the r19 batts whether it be blown in loose or another batt going the opposite direction (I kind of like the batt idea because it seems cleaner to me. I’d assume to get the nose efficiency I’d need to at least blow in some in those hard to reach area like we’re talking about. Also, do I even need a vapor retarder on the ceiling as long as there’s enough insulation? Thanks!

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