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Does it make sence to combine exterior ridgid ins with flash and batt?

Mark Brousseau | Posted in Green Building Techniques on

I’m in zone 7 (southern Ontario, Ottawa) building new. I had been planning to install 1 1/2″ rigid foam to benefit from bridging the studs. For the wall cavity I’m considering DIY flash and batt. Assy in to out- 1/2″ sheetrock, no poly, fiberglass batt, 2X6 stud, 1″ closed cell spray foam, 7/16 OSB, 1 1/2″ ridged foam, 3/8 vertical strip firing, cement board siding. Does this arrangement make sense or would I be better off to forget about the exterior foam and spend that money on filling the stud cavities with closed cell foam? The house is roof tite now. Any feedback would be much appreciated. thanks, Mark

Replies

  1. Brett Moyer | | #1

    Mark,

    Forget the spray foam. Install as much rigid insulation on the exterior as you can afford, fill the cavities with cellulose, and use the Airtight Drywall Approach when installing gyp board.

    Spray foam is expensive and you are sandwiching the OSB with impermeable/no-so-permeable layers.

    I recommend the following:

    -Exterior Cladding
    - Furring Strips
    - Foam Sheathing (3+ XPS, 2+ inches Polyiso)
    - Draining housewrap, properly flashed and integrated with windows and other wall penetrations
    - OSB
    - 2x6 wall, dense pack cellulose
    - ADA Drywall, sealed to plates and penetrations (use airtight electrical rough-in boxes)
    - Vapor Retarder (Latex) Paint

  2. Mark Brousseau | | #2

    Thanks for the quick reply.
    Oh but this raises a few more questions...why would you recommend using 2 kinds of ridged foam, do you mean 3" XPS board and another 2" polyiso board? Is that thick of a wall worth it? For the housewrap, I assume you mean something like Home slicker? I guess I would need to flash inside the walls with the housewrap to the rough openings and again on the very outside with the siding?
    For the cellulose, I thought that dense pack meant blown in, how is that done for new construction, wait till drywall is done then drill holes?

  3. Brett Moyer | | #3

    Mark,

    Sorry. Allow me clarify...

    For the rigid foam, choose one: either 3+ inches of XPS or 2+ inches of Polyiso.

    Draining housewrap- It is important to provide a small gap between the rigid foam and the OSB for hygric redistribution and to reduce hydrostatic pressures. Draining housewraps provide this gap. Examples include: Drainwrap (Tyvek), Raindrop (GreenGaurd), and Weather Trek (Barry Plastics).

    Dense pack cellulose is best installed behind netting/insulweb stapled to the studs. Make sure the contractor installs the cellulose at 3.5 pounds/sq ft.

  4. John Klingel | | #4

    Mark: I would also consider using plywood instead of OSB. If you look on builditsolar.com (if I recall) Robert Riversong explains how he dense packs walls after the sheet rock is up, which saves the netting. I also read on one site that some folks use a roller to flatten any bulging in the cellulose if you use netting; I have no idea how important or necessary that is, but it seems that some bulging is going to happen.

  5. Mark Brousseau | | #5

    Thanks for the great advise guys. Too late to use plywood, I already have the OSB installed. I just read about the Mooney wall system and that sounds excellent! I think I'll go with...in to out...drywall, insul mesh, 2X2 horiz, 2X6 wall, dense pack cellulose, 7/16 OSB, draining housewrap, 2" polyiso, vertical firing, ext cladding.
    I have 2 other areas that I'm not sure the best way to insulate. One is a cantilevered floor section with an interior room above that protrudes out about 4 feet from the house. It is 14" TJI joists with a LVL rim. The other is a shed roof section that will be finished cathedral on the interior. It is made of 2X8 rafters 16" OC and has 2 big operating sky lights. It is too shallow to get the code requirement of R40 in conventional blown. I think I need a hybrid type solution here as well. Any ideas would be most appreciated.

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