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Unvented cathedral ceiling between vented attics

user-6651407 | Posted in Energy Efficiency and Durability on

Climate Zone 5. Unvented Cathedral Ceiling sits between Two Vented Attics.

Closed Cell in the Unvented Cathedral on the Roof Deck.
Blown in Fiberglass in the two vented attics on both sides.
Closed Cell was sprayed on the back wall inside the two vented attics where the unvented ceiling meets.

Are there any special considerations needed, having the joining wall sprayed with closed cell in the vented attics? This is very hard to explain in writing. I hope you follow my question. If you are standing looking up at the cathedral ceiling, there is an upside down “v” shape on both sides not sprayed, that is where the vented attics are on the opposite side. They instead sprayed those upside “v” areas on the other side of the wall.

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds like you have insulation in the right places. Here's the idea: Your home needs a continuous thermal barrier (a barrier of insulation) that wraps around the house. This barrier mustn't have any gaps or interruptions. There should be an air barrier (also continuous) adjacent to the insulation.

    In the areas that have closed-cell spray foam, the spray foam is the insulation as well as the air barrier. In the areas that have blown-in fiberglass, the air barrier is probably the drywall under the fiberglass.

    The "upside-down Vs" are triangular walls. They have to be built like exterior walls -- because one side of the wall faces an unconditioned attic.

    Your builder may have done a perfect job, but it's hard to know without knowing:

    (a) If a blower-door test was performed. (A blower door helps identify any leaks in the home's air barrier.)

    (b) If the closed-cell spray foam is thick enough to meet minimum code requirements. (Many installers of closed-cell spray foam skimp on insulation thickness.)

    -- Martin Holladay

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