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Insulating a 2×6 cathedral roof

Steve Karkau | Posted in General Questions on

Just read “How to Build an Insulated Cathedral Ceiling” trying to apply to my gut rehab project.
Zone 5a, house has a hip roof framed with 2×6. When I tore drywall off, found vented eaves with no venting on roof. Insulation was 6″ fiber glass bats tGN#?with no air channel space. Roof has brand new shingles so I want to insulate from below. I am thinking non-vented roof, fill 2×6 void with closed cell foam and then install 2″ foam on studs for thermal break. Does this sound like correct strategy? Should I install vapor barrier below foam board before drywall?

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    You want the roof assembly to dry to the interior since it cannot dry to the exterior with asphalt singles as the roofing material. You would need to install ventilation baffles and a ridge vent. I'm assuming you have a simple roof line and no dormers. See this article for more information on constructing baffles: https://www.greenbuildingadvisor.com/articles/dept/musings/site-built-ventilation-baffles-roofs

  2. GBA Editor
    Martin Holladay | | #2

    Steve Karkau,
    I disagree with Steve Knapp. Since you have a hip roof, you can't create a vented roof assembly. You have to design an unvented roof assembly.

    Unvented roof assemblies insulated with closed-cell spray foam are common. These roof assemblies aren't designed to dry to the interior. They depend on the closed-cell spray foam to keep the roof sheathing dry. (The closed-cell foam is an air barrier as well as a vapor barrier, so interior moisture never reaches the cold roof sheathing.)

    I think that your plan to use closed-cell spray foam, supplemented by a continuous layer of interior rigid foam, will work.

    If for some reason the idea of encapsulated roof sheathing bothers you, your only other option is to install rigid foam above the roof sheathing. But that approach would require new roofing, so I doubt if you are eager to use that approach.

    1. Eli Connors | | #7

      Is there a concern that the closed cell foam filling the cavity and foam board on the studs will cause a potential issue of drying - in case any moisture makes its way past the foam board?

  3. Steve Knapp CZ 3A Georgia | | #3

    Steve Karkau. Sorry, I missed your description of the roof (always a danger when I am viewing posts on the iPad).

    Martin. Do you have any concerns about sandwiching the sheathing between two impermeable layers (closed cell and asphalt shingles)?

  4. GBA Editor
    Martin Holladay | | #4

    Steve Knapp,
    I'm a big fan of rigid foam above the roof sheathing. That's where the foam belongs. However, in the real world, that approach isn't always possible. I can't count the number of times GBA readers have asked for insulation advice just one or two years after installing brand-new, expensive roofing. These people end up installing insulation from the interior side of the assembly.

    So, if you have a hip roof, you can't use a vented approach. If you don't want to pay for the cost of new roofing, your only choice is spray foam.

    Closed-cell or open-cell? That's an easy one: closed-cell, because of all of the reports of moisture problems associated with open-cell foam under roof sheathing.

    So you end up with vapor-impermeable roofing above the roof sheathing, and vapor-impermeable spray foam under the sheathing. That's not ideal, but if the sheathing is dry on the day the spray foam is installed, it won't get wet from the interior. The sheathing will stay dry until the roofing starts to leak -- and, as I've often said, all roofs leak eventually. Every time a roof leaks, there is always the risk of sheathing damage, no matter what type of roofing you install, and no matter what type of insulation you install.

  5. Jon R | | #5

    Would be interesting to see discussion of how easy it is to repair the sheathing (with CC foam underneath) when it does get damaged.

    Also, for new construction, these hip vents:

    http://www.gaf.com/roofing/residential/products/roof_vents/cobra_hip_vent

    and this vented hip roof:

    http://www.dciproducts.com/html/offpeak.htm

    perhaps combined with this for horizontal air movement:

    http://www.dciproducts.com/html/raftervent.htm

  6. Steve Karkau | | #6

    Thanks for the feedback, the house was a foreclosure and 1 of the only things worth saving was the new roof the bank installed. I only need it to last 20-25 years and it will be someone else's problem:)

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