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Another attic & cathedral ceiling insulation question

tdm64 | Posted in General Questions on

New here, and I have read a number of the related questions and articles, but as usual, every situation seems a little bit different. I have a small 3-season cabin in eastern Ontario, used from April through October.I am renovating the interior and looking to add some insulation – mainly in hopes of keeping some of the heat out in the summer, and enough to keep the drafts out on cooler spring/fall days (heat is provided by central wood stove only). 2×4 walls have 2-3 inches of spray foam already, windows are new and quite airtight with a good reflective coat. Cabin sits on peers and floor is uninsulated (I am not planning to address the floor now, but ideas would be good for the future).
My ceiling is uninsulated – 2×4 rafters with collar ties. I have soffit ventilation all around, and a ridge vent running the entire length of the cottage. I will be covering the ceiling with T&G cedar. My rafters which act as a cathedral run about 8′ before they hit the 8′ wide collar ties. True attic would be above the collar ties. Unfortunately, a 2×4 space in the rafters does not provide sufficient room for an air baffle and batt insulation. I could put at most a 2×3″ extension on the 2×4’s, and hence get about 3.5″ (R12) in the rafter portion (and as much as I want in the collar tie portion). I would vapor barrier the interior before putting up my T&G cedar.
Alternatively, I was thinking of putting up 1.5″ to 2″ thick 4’x8′ Silverboard on the interior of the 2×4 rafters followed by 1×3″ strapping, then vapor barrier, and then the T&G cedar. This would leave a full 2×4 air channel under the roof deck in the rafter portion, plus the reflective Silverboard could help with radient heat. In the collar tie (attic) section, I was planning on R40 batts, then vapor barrier, then T&G cedar.

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  1. GBA Editor
    Martin Holladay | | #1

    All of your options are explained in this article: How to Build an Insulated Cathedral Ceiling.

    If you want to use fluffy insulation like fiberglass or mineral wool, you need a ventilation baffle above the insulation, and an air space between the baffle and the roof sheathing. You also need an air barrier on the interior side of the fluffy insulation, and tongue-and-groove boards don't meet that need. I would advise that you install taped drywall, with your T-&-G boards on the interior side of the drywall.

    Obviously, your R-value will be quite low unless you are willing to deepen your rafters.

    The unvented option would require the installation of spray foam on the interior side of the roof sheathing. Since spray foam is an air barrier, you wouldn't need the taped drywall if you go this route (as long as your local code inspector approves the use of T-&-G boards as a thermal barrier to protect the spray foam).

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