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Community and Q&A

Cathedral ceiling with service cavity

ayyoitsp | Posted in Energy Efficiency and Durability on

I am building a vented cathedral ceiling, but am now looking at adding a service cavity to allow space for recessed lights without penetrating my air barrier.

I’m looking at some ultra-thin LED lights, which run about 1.5″, so I was thinking I could simply use 2x2s or 2x4s laid flat against my existing rafters. The transformers are short as well and can fit into a shorter 1.5″ junction box. They are designed to be placed directly against insulation, so they don’t need any space behind them as well, though they’ll still have 1″ or so, and space all around.

Also, I was planning on installing t&g plank below my drywall. With a service cavity, it seems like I could just install the drywall directly against the rafters, mud and seal to achieve my airtightness, then install the service cavity below it, with only t&g attached to the service cavity. I still want the drywall for fire resistance.

It seems like I’d save a layer of OSB or other material that would normally be placed between the service cavity and rafters. Are there any energy issues or code violations with this approach?

Also, is fire blocking required for service cavities as well?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Are there any energy issues or code violations with this approach?"

    A. Not that I know of.

    Q. "Is fire blocking required for service cavities as well?"

    A. I don't think so. But for a definitive answer, ask your local code official.

    Q. "I could just install the drywall directly against the rafters."

    A. Even better: You could install a layer of continuous polyiso (with taped seams) directly against the rafters, followed by (optional) drywall on the interior side of the polyiso. (Whether or not the drywall is required depends on local code requirements.) Then you could proceed with your plan to install 2x4s to create a service cavity.

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