Cathedral Ceiling Insulation
I have a home built in 1978 that uses 2×12 rafters. There are fiberglass batts between the rafters, it looks like two 6″ paper faced bats with one paper face toward the roof deck and one toward the ceiling. 2/3 of the ceiling is smooth cedar planks. I need a new roof and need to know how to proceed. I was thinking of putting EPS panels on top of the existing sheathing to accomplish R-20. However based on the table about installing rigid foam above the sheathing, I will need 41% to be from the rigid foam. Using a figure of 3.7 R/inch of fiberglass bats, the thickness of the rigid foam would have to be R28. Does this make sense? Do I have to install this much rigid foam? One problem I will have is that I have clerestory windows which will not be able to tolerate even a 4″ added thickness of rigid foam. Any suggestions about this? Thank you very much for your advice.
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Replies
ilovebatz1,
The ratio is dependent on where you are. What climate zone are you in?
I am climate zone 5, in eastern WA.
ilovebatz1,
Sorry - you are right. The existing batt insulation (roughly r-40) dictates the necessary (r-28) depth of the foam above. To me the complications of working from either the inside or exterior of the roof to get more R-value outweigh any gains you might see. I would look for low hanging fruit elsewhere, like air-sealing or other areas where the insulation is obviously too low.
Malcolm, thank you for your replies. One of my concerns is about condensation on the underside of the deck if I leave everything as is. So far it doesn't seem that I have had a condensation problem but I have been using a whole house humidifer the last three years, set at 40% RH in the fall/winter/spring heating seasons. Is this likely to cause problems? My cathedral celing is unvented. Thank you.
ilovebatz1,
You are most likely to see moisture problems near the peak. Given the age of the house, if they haven't occurred yet, they probably won't.
There are a number of effective assemblies you can retrofit, but all are very disruptive to either the exterior or inside - and your roof has a number of end and side-walls which if you add any height to the roof surface, will need the cladding removed for re-flashing. So the question becomes: in the absence of moisture issues, are they worth doing?
Thanks again, Malcolm.
I tend to agree with the "don't lose sleep over what isn't broken" philosophy, but I think you should have a plan in mind in case the roof tear-off does reveal rotted sheathing. When you say there doesn't seem to be a condensation problem, do you mean you removed some of the planks to have a look in there?
Although most people just use the ratios implied by table R806.5, there is also the alternative in sentence 5.1.4 of calculating a ratio based on your average winter temperature. If you are in a relatively warm-in-winter part of CZ5, the required ratio might be closer to 35%.
You might be overestimating the R-value of the batts. If they were installed when the house was built, they would most likely be R-19. Compressed into a 2x6 space, Owens Corning says the R-value drops to R-17 (https://dcpd6wotaa0mb.cloudfront.net/mdms/dms/Residential%20Insulation/10017857/10017857-Building-Insul-Compressed-R-Value-Chart-Tech-Bulletin.pdf).
So hypothetically, if the batts are R-34 and the required ratio is 35%, you would only need R-18.5 on the exterior, doable with 3.5" of polyiso.
Thank you Frank. This gives me some more things to think about. I haven't torn off any planks, although I am going to pull out the one can light that is in my living room ceiling. I do have a very small attic space and have looked at the insulation there. I have attached a picture of the roof deck that I could see along one of the rafter bays after moving some insulation.