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Community and Q&A

Cathedral unvented ceiling in California

viper630 | Posted in Energy Efficiency and Durability on

I live in Santa Barbara, CA (ideal climate) and I am remodeling a home. The roof was built in 1975 by the prior owner. The roof from top down and ceiling are comprised of shingles, paper, 1/2″ plywood sheathing, 3/4″ foam board, then 1 3/8″ tongue and groove. All materials are sandwiched together with no air gaps. The roof is supported by exposed rafters/beams.

I made a tough decision to cover the tongue and groove because I want recessed LED lights, plus the tongue and groove has some defects from the old walls, nails etc. that were removed during remodel. I found low profile LED lights that fit in a 1 1/2″ junction box, so we have rough wired them and attached wood furring strips on the tongue and groove to establish the right level for drywall. Thus, I will have a 1 1/2″ cavity potentially to fill with insulation between the tongue and groove and drywall.

I was going to install 1 1/2″ foil faced foam board tightly in that space, but after reading many articles, I question whether this is a bad idea. I realize the foil will serve little or no purpose without an air gap.i would think the foam can’t hurt, but I don’t expect much since heat will transfer through the furing strips, exposed beams, junction boxes, etc.

What do you advise? Should I skip the foam board entirely and leave the 1 1/2″ unventilated void? It sure would save a lot of work. I could go to 1″ foil faced foam board and leave a 1/2″ air gap between the tongue and groove and foam board, with the foil facing up for a radiant barrier? Or I could skip the foam board and staple some foil across the furing strips for a basic radiant barrier /with 1 1/2″ air gap? Then install the drywall against the foil?

I appreciate your advice.

Regards, Jeff

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  1. GBA Editor
    Martin Holladay | | #1

    It's too bad that you didn't post your question before you installed the electrical boxes and the furring strips.

    You have wasted a once-every-40-years opportunity to improve the R-value of your roof assembly. What your roof assembly needs is a thick layer of continuous rigid foam -- not rigid foam cut up into narrow rectangles and interrupted by furring strips. (If the rafters are 16 or 24 inches on center, of course, you can't install large pieces of rigid foam. But if you have a post-and-beam house with rafters that are 4 feet on center, there would be an opportunity for larger pieces of rigid foam.)

    I don't know much about the requirements of Title 24 (California's energy code) for Santa Barbara, but I'll bet that the code requires at least R-30 for ceiling insulation. Installing rigid foam that is at least 4 or 5 inches thick would have been a good idea.

    If you don't want to remove the furring strips and the electrical boxes, you should go ahead with your plan to install strips of 1.5-inch-thick rigid foam. Whether or not the rigid foam is faced with foil is irrelevant. If you've already bought the foam, the foil facing won't hurt anything. If you haven't bought the foam yet, you can buy 1.5-inch-thick polyiso, with or without foil facing.

    Don't install foam that is less than 1.5 inch thick.

    Your next opportunity to improve this roof will come when it's time to replace the asphalt shingles. At that point, you should install more rigid foam on the exterior side of the sandwich.

  2. viper630 | | #2

    Thank you for your response. Fortunately, since it is a remodel with a preexisting roof structure having no attic, I am not required to meet normal title 24 roof insulation requirements. The beams/exposed rafters are 4' on center. I suppose I could install almost full sheets of foam board if I removed the furing strips. However, wouldn't it be less than ideal to install drywall over the foam and screw through the foam into the tongue and groove? As far as going any thicker than 1 1/2" foam, I would lose my desired reveal of the exposed beams. One more inch would be OK, but not more.

    Any thoughts of attaching the drywall to the ceiling over the foam without furing strips?

    Regards, Jeff

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