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Community and Q&A

Retrofit unvented catherdral ceiling

Arlene_H | Posted in GBA Pro Help on

Hello and thanks in advance for help …

I am in zone 5 (by your map) Ohio and have a typical cathedral ceiling room addition from the late 70s. T&G knotty pine, followed by 6″ fiberglass in 2 x 6 rafters and then the roof plywood …of course now rotten and was previously replaced!. I am a senior lady trying to get the contractor to do it as correct as possible this time. None of the carpenters seem to know what to do with my situation so I have had to do the research.

The old ceiling is all down and there is a new roof just installed with new plywood, Titanium (?)underlayment, ice guard around the roof edges and valleys and regular shingles. The house is one story with a low slope and added back dormer.

The east side of the cathedral room has 2 hips with no ridge at the peak. This extends west to a dormer loft which does have a ridge vent.

Here’s the plan, please advise me if it will work or, if not, what would be suggested.
1. Fit 2″ rigid foam board between the rafters and against the roof deck. (Type? Need to seal?) Sprayed ISO Foam is not an option because of my respiratory health issues, fire fumes and other environmental factors.
2. 3.5″ faced fiberglass
3. Sealed drywall
4. Replace T & G

About the dormer / loft. It can be vented because of the ridge vent and rafters going into a vented attic. Would it be ok to vent it as it runs next to the proposed unvented cathedral ceiling?

Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    In your climate zone, the building code requires a minimum of R-49 in a roof or ceiling. The code may not apply in a retrofit situation, but it's a good goal to aim towards.

    Your suggested method provides about R-22 of insulation -- not much. It would be good to aim higher.

    Your suggested method is a variation of the cut-and-cobble method. When used for an unvented cathedral ceiling, this approach has resulted in a few failures (damp roof sheathing), so I don't recommend this approach. For more information, see Cut-and-Cobble Insulation.

    If you don't want to use spray foam insulation, you have (unfortunately) timed your question here rather poorly. You should have posted a question on the GBA site a short while ago, before you installed a new roof, so that we could explain that the correct way to insulate this type of roof is to install one or more layers of thick rigid foam insulation above the roof sheathing. Unfortunately, it's too late to do it the right way in your case.

    For more information on insulating this type of ceiling, see How to Build an Insulated Cathedral Ceiling.

    At this point, you'll have to come up with an imperfect approach -- either spray foam or a cut-and-cobble approach, meticulously performed, with enough rigid foam to get closer to R-49. If you do this, be sure to include a continuous layer of rigid foam under your rafters, installed in an airtight manner.

  2. charlie_sullivan | | #2

    The only other option I can see would be to figure out a way to vent it. With vent space added, you've have very little space left for insulation, so to get decent insulation you'd also need to add space between your rafters and the ceiling, which could be foam board insulation or strips of wood. Depending on how much thickness you are willing to add, you could end up significantly more insulation that way.

  3. Arlene_H | | #3

    Thank you for your quick response. I had read your article about How to Build an Insulated Cathedral Ceiling .....about 3 months ago and many times since. I had also discussed adding rigid foam above the roof sheathing with the roofer but he was not keen on the idea and frankly it would have been a large added expense that I couldn't handle since it would have to go across the entire front of my very long ranch style home, plus side and most of back (plus all the plywood and inside repair cost). I guess I didn't realize there was no other good solution.

    What type of rigid foam would you recommend? I did just read the cut-and-cobble method article but am still confused on which type foam board would be best to use in my situation. The installing carpenter is very meticulous and careful.

    We did also previously consider adding below the rafters but assuming that adding the drywall is indispensable, adding more to lower the ceiling (which runs continuously over the loft) will make the loft ceiling so low as to not be able to stand erect at the highest point. I know it's all a very bad design. Perhaps I need to rethink the foam. Is the polyurethane foam a viable option? I just can't take the health chance with the isocyanate given my asthma and other inhalant allergy problems.

    Thank you.

  4. GBA Editor
    Martin Holladay | | #4

    Any solution you choose now will involve a compromise. If your ceiling height is low, that's another reason why the needed insulation should have been installed above the roof sheathing, not between the rafters.

    You can use any type of rigid foam you want for a cut-and-cobble job. Green builders usually avoid using XPS, since XPS is manufactured with blowing agents that have a high global warming potential.

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