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How to insulate an unvented cathedral ceiling?

Roger Donage | Posted in Building Code Questions on

Hi Everyone –

I know variations of this question have been asked many times, but I couldn’t find an answer that applied in my situation.

I have a porch that was enclosed by a previous owner. It has a vaulted ceiling with slopes on all three sides. There is no ridge, so I can’t add a ridge vent, and there’s no gable to put a gable vent.

The porch is finished on the outside, and the roof is complete, it has not been finished on the inside (no insulation, drywall, plaster, etc). The total size of the porch is less than 100sf.

I’m in zone 5.

I am trying to determine how to insulate the patio ceiling. I know that the preferred method would be spray foam, but nobody around here is willing to do a job this small for any reasonable amount. Also, the roof is already shingled, so I can’t add rigid foam above the deck.

From the many other posts on this topic, I know that cut and cobble is not a good idea.

So I was thinking I would add several layers of rigid foam below the rafters (in full pieces) and tape them to make them airtight. I would stagger them so that the air can’t get through. I was thinking of using Super TUFF-R from Home Depot (which has aluminum on both sides, see http://www.homedepot.com/p/100322374?N=baxx ), with an R value of 6.5 per inch. The space between the rafters would be empty.

I believe that since I am not cutting the foam and there will be no thermal bridging through the rafters, the air should not be able to get past the foam. That should (if I understand correctly) keep the hot air from causing condensation on or below the roof deck.

1) Can anyone tell me if this would be sufficient and avoid ice dams and moisture issues?

2) If this wouldn’t work, would it work to fill the space between the rafters with rigid foam and then add a few layers of rigid foam below the rafters?

3) Can I layer the Super TUFF-R in the way described?

4) Are there any other alternatives?

Thank you all so much!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Roger,
    Q. "Can anyone tell me if this would be sufficient and avoid ice dams and moisture issues?"

    A. If you do a good job of air sealing, and install enough R-value to meet minimum code requirements, your approach will work.

    Q. "If this wouldn't work, would it work to fill the space between the rafters with rigid foam and then add a few layers of rigid foam below the rafters?"

    A. As I said, the approach will work.

    Q. "Can I layer the Super TUFF-R in the way described?"

    A. Yes.

    Q. "Are there any other alternatives?"

    A. Yes: (a) Spray foam; (b) Installing rigid foam above the roof sheathing -- an approach that would require new roofing.

  2. Roger Donage | | #2

    Thanks so much Martin!

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