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Cathedral ceiling vs. standard ceiling: Insulation issues

aunsafe2015 | Posted in General Questions on

I’m building a 20×20 detached garage with a finished loft area. Loft area will have 2 foot knee walls and a 14/12 roof pitch to maximize space where I won’t hit my head on the ceiling. I’m trying to decide whether the loft should just have a cathedral ceiling, or if I should add in framing for a standard 8′ ceiling.

From an insulation perspective, if I do a cathedral ceiling, should I just do an unvented roof and closed-cell spray foam insulation? I read the article on this webpage about unvented cathedral ceilings, and that seems to be the way to go. I’m in the Washington, DC area by the way — hot, humid summers and cold-ish winters, but only about 20″ of snow per year (maybe even less on average).

Other option is to go ahead and just have standard 8 foot ceilings framed into the loft, which would leave a small “attic” area above the ceiling. If I were to go that route, what would preferred insulation method be? Should I still just go ahead and do an unvented roof and do spray from on the entire roof? Or would I be better off doing a traditional vented roof, not insulate the roof itself, and just insulating the 8′ ceiling?

Any advice appreciated. Cost not particularly a concern. I just want to do whatever is going to be most reliable and last the longest.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Aun,
    If cost is not a concern, then the best approach, by far, is to install rigid foam on the exterior side of your roof sheathing. The rigid foam will interrupt thermal bridging through the rafters, and provide the best possible thermal performance.

    For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.

  2. aunsafe2015 | | #2

    Thanks for the response. The garage is modular (i.e., mostly pre-fab) and I'm not sure if that will be an option. I'll look into it and see.

    If that's not an option, would it be best to do unvented roof + full spray foam, regardless of whether I do a cathedral ceiling or frame in a standard 8-foot ceiling?

    One last question. If I do end up going the unvented + spray foam route, should I have the builder go ahead and install soffit vents and a ridge vent, and just let the spray foam professionals seal those with foam? Or should I just ask the builder to omit the vents to begin with?

  3. GBA Editor
    Martin Holladay | | #3

    Aun,
    You may want to consult with an architect or a residential designer. If these concepts are new to you, I'm a little worried that I may be providing information that is just enough information to get you into trouble.

    In my opinion, if cost is not a concern, and if you are insulating on the interior side of your roof sheathing, then it always makes sense to include a ventilation channel between the top of your insulation and the underside of the roof sheathing -- even if you plan to install spray foam. The best ventilation baffles are stiff, site-built baffles. For more information on this issue, see Site-Built Ventilation Baffles for Roofs.

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