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Community and Q&A

Roof assembly

Ccloud | Posted in Energy Efficiency and Durability on

I live in zone 6. My main concern in my house assembly is air leakage. My original roof assembly plan was 2 inches of closed cell against the roof deck followed by 9 inches of open cell to reach the needed r value. I was considering what the down side would be of 2 inches of closed cell in the roof deck for air sealing and blown in cellulose in the ceiling to reach the needed r value. Both of these would be unveiled roof assemblies. Any input would be greatly appreciated.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Charles,
    In Climate Zone 6, if you want to insulate an unvented cathedral ceiling with a combination of closed-cell spray foam and cellulose, then the minimum R-value of the closed-cell spray foam layer (according to building code requirements) is R-25. To achieve that R-value, you will need 4 inches of spray foam, not 2 inches.

    Here is a link to an article that explains these code requirements: How to Build an Insulated Cathedral Ceiling.

  2. Ccloud | | #2

    Martin, I am having a normal truss roof. I am thinking of the description above for that type of roof. The closed cell as a vapor/air barrier and the cellulose as the primary insulation. Possible a vapor barrier in the ceiling also?

  3. GBA Editor
    Martin Holladay | | #3

    Charles,
    Your description is confusing.

    Is this a vented, unconditioned attic with cellulose on the attic floor? If so, installing closed-cell spray foam against the underside of the roof sheathing is a waste of money.

    If you have a vented attic, you are inviting cold, outdoor air into your attic. Installing insulation on the attic floor under those circumstances makes sense. But it makes no sense to install any spray-foam insulation when you have cold outdoor air flowing between your house and the spray foam insulation.

  4. Ccloud | | #4

    Sorry to be confusing. This would be an unvented roof assembly. The sealing would come from the cc foam. I would either cover the closed cell with open cell or use the cellulose in the ceiling deck. Is that clearer?

  5. GBA Editor
    Martin Holladay | | #5

    Charles,
    As I wrote in my first response, in Climate Zone 6, if you want to insulate an unvented cathedral ceiling with a combination of closed-cell spray foam and cellulose, then the minimum R-value of the closed-cell spray foam layer (according to building code requirements) is R-25. To achieve that R-value, you will need 4 inches of spray foam, not 2 inches.

    If you want to insulate with a combination of closed-cell spray foam and open-cell spray foam, then a mere 2 inches of closed-cell spray foam is permissible.

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