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I need help insulating my unvented cathedral ceiling

user-6499243 | Posted in GBA Pro Help on

Here’s what I have: I am building a house in my existing pole barn. When I built it, I put my roof nailers on 2′ center, then put down fan-fold insulation, then placed my metal roof. I am going to leave my trusses exposed on the inside for a cathedral ceiling, and was wanting to put metal up for my ceiling between the trusses.

I am trying to figure out how I can insulate this. Can I use rigid foam board and put it all the way against the fan-fold and make it airtight, then put up my interior metal?

All the help on this would be much appreciated. I am stumped. Thank you.

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  1. user-6499243 | | #2

    Iam from east central Indiana

  2. GBA Editor
    Martin Holladay | | #3

    You have two basic choices. You can make this a vented roof assembly, or you can make this an unvented roof assembly. (For a thorough discussion of these two options, see How to Build an Insulated Cathedral Ceiling.)

    If you decide to build an unvented roof assembly, you have to insulate with closed-cell spray foam. The spray foam can be installed directly against the underside of the fan-fold insulation.

    If you decide to build a vented roof assembly, there are many steps.

    1. Verify that your building has soffit vents and a ridge vent.

    2. Establish vent channels. You can do this by installing 1"x1" or 1.5"x1.5" sticks in the upper corners of each rafter bay, up against the underside of the nailers (purlins). For more information on these "sticks" (installed parallel to the rafters), see Site-Built Ventilation Baffles for Roofs.

    3. Then you need to install some type of air barrier / ventilation baffle up against the sticks. You can use thin plywood, rigid foam, fiberboard, and stiff cardboard, installed in an airtight manner.

    4. Then you need to install your insulation -- for example, fiberglass, mineral wool, blown-in fiberglass behind netting, or dense-packed cellulose behind netting. You are aiming for R-49 of insulation, so you may need to deepen your rafter space.

    Most of these steps are discussed in greater detail in the two articles I linked to.

  3. user-6499243 | | #4

    Is it a option to put 2" of spray foam in then fill the rest with batt insulation for a unvented roof !!

  4. GBA Editor
    Martin Holladay | | #5

    Q. "Is it an option to put 2 inches of spray foam in, and then fill the rest with batt insulation for a unvented roof?"

    A. You really should read the article I linked to (How to Build an Insulated Cathedral Ceiling). That article contains the answer to your question. The article probably also has answers to the next two or three questions you are itching to ask.

    I'll indulge your question, though. You are in Climate Zone 5. As the article notes, the minimum R-value of the spray foam layer in the type of roof assembly you are talking about is R-20. That means that you need at least 3.5 inches of closed-cell spray foam, not 2 inches, in your climate zone (as well as R-29 of batt insulation).

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