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Community and Q&A

Cathedral ceiling insulation

Coopsdaddy | Posted in Energy Efficiency and Durability on

Zone 3 needs R-38.
2×8 truss, 12/12 pitch.
Metal roof, 16 square roof.

Would you prefer:
7 inches open-cell foam, then 2.5 inches on polyiso nailbase on roof, or
BIBS (R-30) touching roof deck and 2 inches polyiso nailbase on top of roof deck, or
something else?

Is one option way better than another or way cheaper but as effective?
Or is there a better option?
…than If you had the option of using a vented nailbase instead of nailbase and a fascia vent for some venting under the metal. Would you, at a cost of about $2,500 extra?

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  1. GBA Editor
    Martin Holladay | | #1

    You are the only one who can perform the research to determine a method of insulating your sloped roof assembly that is "way cheaper" (as you put it) than your proposed methods. All prices are local, so you have to talk to your contractor. If you are doing the work yourself, start creating some estimates.

    In most locations, BIBS will be cheaper than open-cell spray foam, but you never know what's cheaper until you do the research.

    The main reason to prefer a vented roof to an unvented roof is to address ice dams. I'm guessing that ice dams aren't a big concern in Zone 3.

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