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Old 01-13-2008, 01:27 PM   #1
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Insulation of a Vaulted (Cathedral) Ceiling

I live in Canada, and have a low slope (2-12) roof that is shingled. The r-value of the existing insulation would be somewhere between 4 and 7, I think, though there's no way to tell for sure without lifting the roof off, because we have vaulted ceilings inside. The shingles are poor, and I'm looking to replace them, and add insulation at the same time. If I can get the R-value up to at least 28 (and I'd prefer 40 or 60) I can get up to $1800 in grants from the government. If I lift the plywood off the roof, I can add in some insulation, but with only 6" of room, I'd be short of the target. Any thoughts on how to best do this? I've thought of strapping the existing rafters with 2X6s on end, and adding insulation that way. Any better ideas?


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Old 01-13-2008, 04:51 PM   #2
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That is an awful lot of expense to get $1800. To make the roof structurally sound for snow loads, I believe you would have to stack the new rafters directly on top of the old and then tie them together with plywood gussets. Also they would need to be blocked to prevent them from rolling. this also means that you have to run an entire new band of facia around the house in some sort of stacked configuration, as you will now have facia boards somewhere between 14 to 16 inches wide. The first money you spend should be a couple hundred dollars for the opinion of a structural engineer, to asses if the existing roof structure can even handle the additional weight, then get him to design the proper configuration of gussets and connections to the original wall plates.


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Old 01-13-2008, 11:19 PM   #3
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Im not a roofer just a diy'er but if you are going to be taking off the roof and only have 6 inches of space I would suggest to use layers of pink XPS foamboard along with some sprayfoam like from handi foam. You can get more R value than with fiberglass this way and also do some serious airsealing while you are up there with the foam. I think the sprayfoam is R7 per inch. THe foamboards come in different R values depending on the thickness. 2 layers of R10 layed down like a sandwich and sealed with foam on the joists gives you R20 at 4 inches. One more layer of 1.5 inch board on top of that which is R7.5 and you have R27.5 right there. If you seal over everything with more sprayfoam to secure it to the ceiling joists you will have R28 with a total of 5.5 inches of foamboard total. If you do it right there will be no penetration of conditioned air from below. Here is a link that lays out a possible solution to your problem. Its long and from a while ago but still good info.
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