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2×8 rafter cathedral ceiling insulation

user-7067054 | Posted in Green Products and Materials on

Looking for info on improving insulation for a cathedral ceiling in a zone 5b house built in the 70’s 2×8 rafters. Horrible ice dams in winter. Heat radiation in summer. Want to know how to insulate with out tearing down roof or drywall IF possible. I’m thinking spray foam is the best option for air sealing. i’m I correct? Thanks for any info.

Stephen, Utah

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that explains all of your options: "How to Build an Insulated Cathedral Ceiling."

    You can certainly install closed-cell spray foam against the underside of the roof sheathing if you want -- that's one perfectly acceptable option -- but you'll need to remove the ceiling to do that work.

  2. Expert Member
    Zephyr7 | | #2

    You’re going to have to tear off either the roof or the drywall. I’m assuming this is an unvented “hot” roof. You can either remove the interior drywall, apply closed cell spray foam directly to the underside of the roof sheathing, or you can remove your roof, put sufficient rigid foam on top of the roof sheathing (lots of info on here about how much is “sufficient”). Both ways have pros and cons.

    Closed cell spray foam probably involves the least overall rework, but it’s indoors where it’s most disruptive. You’ll have a brand new ceiling when you’re done though. Rigid foam on top of the roof has the advantage of eliminating thermal bridging from the rafters, but it’s more difficult to find roofing crews expierienced with such assemblies.

    Total cost for either method is probably going to be comparable, but it depends on how big of an area you’re working with. I’d probably go with closed cell spray foam myself.


  3. Expert Member
    Dana Dorsett | | #3

    How long until it's time to re-roof?

    Dense packing cellulose into the 2x8 cavties is relatively low risk if you're going to re-roof and add 3.5-4" of polyiso above the roof deck in short years, and that would have much better best ice-damming mitigation compared to installing 7" of HFO blown closed cell foam on the interior to achieve R49.

    The cellulose safely shares and distributes the wintertime moisture burden with the roof deck without losing function, but the shaded roof pitched and north facing roof pitches would still have some risk of excessive moisture accumulation in the roof deck until the exterior insulation goes up.

  4. GBA Editor
    Peter Yost | | #4

    Just a quick added note: ice dams are almost always a combination of convective and conductive heat loss, particularly at the eaves. Any effort to insulate/air seal has to get at the top of the eave walls and connect your air control layer from the roof to the one from the wall. If the wall does not have one, then be even more careful to project air sealing as far out and down the eave wall as you can.


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