Insulating a cathederal ceiling roof?
mercierfamily
| Posted in General Questions on
We have a home with a cathedral ceiling and fiberglass batt insulation. We are considering while we are replacing the roofing in installing spray foam insulation. The proposals are to apply 5 -6 inches of spray foam to the ceiling deck not the back of the roof deck as we would have to remove the drywall ceiling inside to apply the spray foam to the underside of the roof deck. Is this a viable/ effective application? What might be the problems or issues?
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Advice will be climate-specific, so you should tell us your location. Also be sure to read this article: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
We live in northern Alberta, Canada which has very cold winters and significant snow fall!
Steve is correct: If you are replacing your roofing, the best approach is to install one or more layers of continuous rigid foam above the roof sheathing. Here is a link to an article that describes this method: How to Install Rigid Foam On Top of Roof Sheathing.
Depending on your location in Alberta, you are in Climate Zone 6 or Zone 7. Here is a link to a map of climate zones in Canada.
In your climate zone, you want to aim for a minimum roof insulation R-value of R-49. Your plan to install 5 to 6 inches of spray foam is insufficient. This amount of spray foam has an R-value that ranges from R-18 (for 5 inches of open-cell spray foam) to a maximum of R-38 (for 6 inches of closed-cell spray foam). That's not enough, in my opinion.
Instlaling rigid foam against the ceiling drywall from above (once the roof has been opened up) is an accepted technique. It works. The only disadvantage is that this method doesn't address thermal bridging through the rafters.
-- Martin Holladay
If you are replacing your roof, your best option is to install exterior rigid insulation above the roof decking. Another option would be install a combination of exterior rigid foam with interior air permeable insulation, but you would need to remove the cathedral ceiling (which I'm assuming is some type of shiplap or similar board nailed to the roof joists). The article I referenced earlier describes this approach along with guidelines for installing all the foam on the interior side of the roof sheathing.
I am not sure how Alberta equates to U.S. climate zones, but no doubt some one will clarify that point (and likely offer other advice).