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Hot roof cathedral ceiling insulation

kikicrow | Posted in General Questions on

Hope I’ve put this in the right category. I’ve read through the information on here about insulation options for my non-vented Cathedral ceiling and am none the wiser…

I live outside Los Angeles in zone 3.  New
master bedroom cathedral ceiling we are currently building has been designed to be a hot roof – no ventilation, and uses 2×8 beams. Roof sheathing is OSB with reflective coating facing into the roof cavity. 

Best I can tell from my reading, I need to use either spray foam or rigid board. I can’t tell if I then need to fill the extra with batts or not to fill up the space. 

We need R-30 in this cavity  per our Title 24 report. 

The structural engineer designed the ceiling with a parallam so the framer is putting a small cross base a few inches below the parallam  so that we can hang a light. This will give space to run electrical wires. I assume that we’ll need to fill this space with insulation before drywalling?

We we’re planning to put 4 can lights in the angled portion of the ceiling in order to get the lighting we need in the space, but I’ve only come across something today (after several weeks of research) that can lights in a “hot roof” is not the thing to do. 

Can lights are in, wires run, but they aren’t live yet.

So confused… so much of what I read is for areas farther north that get ice in the winter. 

Can anyone point me in the right direction?



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  1. user-2310254 | | #1


    I will give your post a bump. I'm just a homeowner with an interest in energy-efficient construction.

    So... If it were my house, I would install reclaimed (much cheaper) rigid foam over the sheathing and re-roof. If that option was not practical, I would install spray foam (perhaps closed cell that uses water as the blowing agent) on the underside of the sheathing and reinstall the drywall following airtight best practices. I would not install traditional canned lights in this area but would opt for low-profile, surface-mounted LEDs.

    Let see if one of GBA's construction experts will chime in.

  2. kikicrow | | #2

    Thanks for the bump-up!

    It’s all new construction, so we have a variety of options. Just that I’m getting conflicting info/bids from each insulation company who is bidding, and one roofer is now suggesting a roof ridge to allow condensation to escape. Which doesn’t make sense .

    So confused.

  3. user-2310254 | | #3


    Confusing input from contractors is not that unusual. You might be able to create a vented cathedral ceiling if your roofline is simple and the rafters are deep enough. You do want to keep indoor air out of the ceiling, even in a dry climate like LA. I doubt the roofer was suggesting a vapor diffusion port, which you can learn more about here:

    You might want to search for a local "green" roofer with a track record of installing the types of projects

    1. kikicrow | | #6

      Steve, he definitely meant a roof ridge. He said he just went to cut one in for a friend of a friend who’s roof/cathedral ceiling is about 3 years old.

  4. Jon_R | | #4

    As I see it, Table R806.5 footnote b) allows you (with your mild weather) to use no foam (ie, all fiber fill to R30). Local codes may say otherwise.

    1. kikicrow | | #7

      Thanks Jon, but California has its own stringent codes. I’ve tried to figure out what the latest codes are and infos all over the place as well.

  5. user-2310254 | | #5

    Jon, how would you get to R-30 in the cathedral sections with 2X8 rafters? Prescriptive approach? Deepen the rafters? Just curious to know what would be most cost effective.

  6. Expert Member
    BILL WICHERS | | #8

    If you really want can lights, at least get the shallow cans -- even if you have room for the regular ones -- so that you can fit more insulation between the back of the can and the roof sheathing. can lights are notoriously leaky, and that's especially problematic when installed in an unvented roof assembly. Make sure you get the "IC-AT" type which are rated both for insulation contact and air tightness.

    You shouldn't try to install rigid foam board between the rafters in an unvented roof assembly. If you want to use rigid foam, it should ideally be installed ON TOP of the roof sheathing. Is that an option for you here?

    The easiest way to go is probably going to be a layer of spray foam against the underside of the sheathing thick enough to be sufficient for dew point control in your climate zone, and then fill the remaining space with batts. It shouldn't be a problem to reach R30 in a 2x8 cavity if you're using even something like high density batts. Mineral wool batts alone would get you to about R32 in that space.


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