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Rigid board insulation for an unvented cathedral ceiling: fill the void?

Hamlet Jones | Posted in General Questions on

Hi there, first post.

I’m building a tiny cabin, Pacific NW, right on the boarder between zone 4/5, 40miles
east of Portland.

I have vaulted ceiling/gable roof. No interior walls in yet.

I have enough rigid board insulation to fill the void (iso). The sheathing was sealed to the rafters with adhesive sealant when laid down. Blocking at the soffets was sealed with silicone. There is rolled mineral roofing over that, all edges sealed with lap cement. All this was done with the anticipation of filling the void with pink fiberglass rolled insulation, with drying to the interior.

Plans change,

I was given enough 2″ and 3″ iso board to fill the void between the rafters. Great.

My question: Do I fill the void completely with this foam,

or,

can I stuff the 3″ board flush to the rafters, (leaving an unvented airgap of 2 1/2″ above the iso), then overlay everything with the 2″ iso for a higher r-value? (rafters insulated). I don’t have enough iso to do both.

There will be no penetrations in the ceiling sheetrock, no can-lites, no skylites, nada. There will be no air path from the walls. I would prefer not to vent this space, as this will just increase the risk for imperfections that the mice will eventually exploit (I hate mice.)

Thanks!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Too bad you didn't have the goods in hand BEFORE the roofing went down, since rigid foam above the roof deck buys you pretty much the full-R of the foam, whereas high-R foam between rafters has it's performance cut off at the knees by the thermal bridging of the rafters.

    But, if that's where you are, sure, go ahead and fill the rafter bays with cut'n'cobbled rigid, taking care to air-seal the edges to prevent convective moisture transfer to the roof deck. If you have enough to put 2" of thermal break on the rafters on the interior side, it's fine to leave 2" of gap between the iso and roof deck, but again, air-sealing the foam to the rafters is critical to prevent that from becoming a convection path.

    IRC code spells out that the foam needs to be in contact with the roof deck, but if it's truly air-tight, it doesn't much matter. With a 2" gap you COULD put in a screened 1.5" or 2" round plug-vent on each bay to give it at least some venting (not exactly a mouse-path unless they're able to crowbar it out). eg: Speedi-Products SM-RSV 2 (sold in some box stores eg. http://www.homedepot.com/buy/speedi-products-2-in-white-round-soffit-vent-sm-rsv-2.html#.USZ46WeBCwc )

  2. Hamlet Jones | | #2

    I'm uncertain of your terms, could you clarify?

    "If you have enough to put 2" of thermal break on the rafters on the interior side, it's fine to leave 2" of gap between the iso and roof deck, but again, air-sealing the foam to the rafters is critical to prevent that from becoming a convection path."

    My plan:

    The 3" iso will be jammed tight between the rafters, flush with the inner rafter face, and can-foamed where appropriate (eave and ridge pole). The 2" iso overlay will be can-foamed at these places as well, with tape on the seams. Over this will be finished gypsum board with tape & mud seams and transitions. I am very picky with these kinds of details, so my confidence of pulling it off is high.

  3. GBA Editor
    Martin Holladay | | #3

    Hamlet,
    First, install 1x2 sticks in the upper corners of your rafter bays, so you have something to install the rigid foam against.

    Then install the first layer of rigid foam. Be sure that each piece of this rigid foam is installed in an airtight manner -- with caulk or canned spray foam around the entire perimeter -- to create a top-side air barrier.

    Then install a continuous layer of rigid foam on the underside of the rafters, with carefully taped seams and with air sealing at the perimeter of your ceiling.

    The installation won't be ideal. As Dana pointed out, the rigid foam really belongs on top of your roof sheathing, not between the rafters. But at this point, that's probably the way to go.

  4. Hamlet Jones | | #4

    Thank you Dana & Martin.

    I did a site search for "cut-n-cobble" that was helpful. (I thought I was the only one who did it like this!)

    As I originally planned to use fiber batts in the wall and cathedral ceiling, I was very OCD about gluing and sealing the sheathing and sub-floor on when building the shell, to prevent air leakage, which as I understand it, is holy grail and first mandate for energy efficiency. I also made sure everything was square. Squareness and the use of a table saw to cut the iso board has made insulating the walls with the cut-n-cobble technique much less of a PITA. Luckily, the cabin is a very simple design, and I built the design with the idea that I wanted to make it easy to apply foam to the exterior if I ever felt it necessary...

    Ahem,

    another thought occurs to me, & as has already been pointed out, as using full sheets with taped and overlapped seams is the best practice. There is nothing to preclude me from lowering the ceiling by 5", that is, laying both my 2" iso and my 3" iso on the underside of the rafters, staggering the seams.

    I have done this on ceilings of my first home, but the sheet thickness was only one layer of 2" iso.

    If I decide to do all of the iso board to the underside of the rafters, where do I find 6" plastic-cap-nails, and where can I find 6" or 6.5" drywall screws???

  5. GBA Editor
    Martin Holladay | | #5

    Hamlet,
    Check out this article: How to Install Rigid Foam Sheathing.

    The article lists sources for long cap nails and long screws.

    Cap nails can be purchased in a variety of lengths up to 8 inches. Here is a link: http://www.simplexnails.com/InsulationCapDataSheet.html

    Some suppliers of long screws include:
    Best Materials — Dekfast 6-inch roofing screws and Dekfast 9-inch roofing screws
    Wind-Lock (a source of long screws and plastic hold-down buttons)
    FastenMaster (a source of screws up to 10 inches long)

  6. Expert Member
    Dana Dorsett | | #6

    Putting it all below the rafters is ideal, since it take the performance-robbing thermal bridging of the rafters out of the equation!

    Maximizing the fastener spacing minimizes the thermal bridging of the screws, which is why holding it in place with furring with 24" spacing between the screws is recommended, then screwing the gypsum to the furring.

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