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Michael Peters-Rodbell
Posted: Mar 16, 2009 07:29 PM
Insulating roof with Cathedral Ceilings
Hi all,

We have a home with cathedral ceilings for which we've had a long standing battle with ice dams. We believe that there is a significant issue with heat transfer onto the roof and a contractor has suggested removing all ceilings and the existing fiberglass insulation with spray foam.

We're now trying to decide on the material that will best keep the heat off of the roof. The original quote came in for applying 6 inches of half pound open cell foam. I have the following questions:

- will the corresponding R24 (6" x ~3.9 R/inch) insulation be sufficient to keep the heat and corresponding ice dams off of the roof?
- I'm hearing that after 4 inches of the open cell foam, there's not a great deal of additional insulating value provided by the additional foam
- Would closed cell foam do a better job of insulating? It appears to have a denser quality.
- there are recessed lights in the ceiling, for which there will be a bit less than 4" between the light can and the roof. How well does the foam prevent heat transfer in those areas?

Roger Morrison
Posted: Mar 17, 2009 07:30 AM
The basic trick to eliminating or minimizing ice damming is to have a uniform temperature profile of the roof from peak to eave. This can be done with insulation or ventilation. For residential construction, insulation is generally the more practical way to go because of the complex details within the roof structure.
Generally in cold climates where ice damming is an issue, condensation can also be an issue, especially if you have recessed lights. Under these circumstances, I'd recommend a closed-cell SPF. With closed-cell, you will be able to achieve a higher R-value and you will avoid the condensation issues. (It's very unlikely that you will get an R-24 with 6" of open-cell SPF.)
The recessed lights are a real problem and may be a major factor with your current ice damming problem. It is very important that you get the same thickness of insulation between the recessed lights and the roof deck as you are installing in the remainder of the roof. But few SPF manufacturers allow the application of SPF directly to a recessed light. If you cannot do this, I suggest removing the recessed lights or redesigning this feature.
Michael Peters-Rodbell
Posted: Mar 17, 2009 08:25 AM
Hi Roger,

Thanks for the feedback. What you're saying seems to make great sense. I spoke with the installer yesterday & he was leaning that way as well. He's proposing a 4" coating of closed cell, which (using my admittedly simple math) would also result in around R-24? He should have adequate clearance under the recessed lighting with the closed cell.

Does that sound like something that would work well?



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