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Old 01-13-2008, 01:25 PM   #1
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Adding insulation to a low-slope roof


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, 02:44 PM   #2
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Adding insulation to a low-slope roof


How thick are the joists?

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Old 01-13-2008, 02:46 PM   #3
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Adding insulation to a low-slope roof


I think they're either 6" or 8". My neighbour's house had 8 inch joists, but I suspect mine are only 6". The home inspector hypothesized they'd have about 4" of insulation in them, regardless of size. The house was built in 1961.
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Old 01-13-2008, 02:56 PM   #4
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Adding insulation to a low-slope roof


Does your neighbor also have a low sloped roof?
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Old 01-13-2008, 03:06 PM   #5
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Adding insulation to a low-slope roof


Yeah, his is similarly sloped, though it's a split level, and ours is a bungalow. When he had his roof done, he had the roofers life the plywood and cram insulation in to fill the space. But, that doesn't seem overly efficient to me -- compacted insulation isn't as effective, and it would still only probably bring it up to R-20 at most, which, in a province where -40 is common, doesn't help the old gas bill that much
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Old 01-13-2008, 04:36 PM   #6
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Adding insulation to a low-slope roof


I was just reading about packed insulation which is fit too tightly in the rafter space.

True, it does not offer the rated r-value, but it will offer more r-value than a thinner version that properly fits the space.

A 2/12 is too shallow of a pitch for a shingle application anyways though, unless special provisions are made.

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Old 01-13-2008, 08:35 PM   #7
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Adding insulation to a low-slope roof


it`s recommended to cister 2 x 12`s w/cats between them if you want to be able to insulate properly
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Old 01-13-2008, 08:45 PM   #8
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Adding insulation to a low-slope roof


What do you mean by cister? Sorry, I'm a roofing rookie. Will it be hard to get fascia that is tall enough once the roof height is extended? Also, should I worry about the additional weight on the structure of the building? It was built in 1961. Thanks so much!
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Old 01-14-2008, 07:26 AM   #9
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Adding insulation to a low-slope roof


Just for clarification, he probably meant to say, Sister the rafters, where you naul an additional rafter side by side to the existing one and the other word cats, was probably supposed to be batts, as in batts of insulation.

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Old 01-14-2008, 11:56 PM   #10
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Adding insulation to a low-slope roof


cister is the right word for securing an additional joist to the side of the one existing,cats are sommetimes called cripplers,basically they go horizontally between the added joists and keep them secure so they don`t twist,you would leave the cats about one inch shy of the sheathing for airflow puirposes,then you can add whatever batts or rolls of insulation to meet code-if there is existing insulation ,it is important to use unfaced insulation above it so you don`t trap condensation between the insulation layers
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Old 07-31-2010, 09:23 PM   #11
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Adding insulation to a low-slope roof


Quote:
Originally Posted by cbeingessner View Post
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 07-31-2010, 09:25 PM   #12
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Adding insulation to a low-slope roof


I give up where is the reply to the low slope insulation?:thumbsup :
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Old 07-31-2010, 09:41 PM   #13
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Adding insulation to a low-slope roof


Please don't use the report button, unless you are reporting a post or thread that violates the rules. Thank you.
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Old 07-31-2010, 09:50 PM   #14
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Adding insulation to a low-slope roof


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I give up where is the reply to the low slope insulation?:thumbsup :
the question was regarding insulating an attic area,on a home with a low slope roof,not adding insulation to a low slope roof system,for that search out topics regarding "tapered insulation"
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Old 08-11-2010, 01:50 PM   #15
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Adding insulation to a low-slope roof


Normally what is done is to remove the plywood, cover with 4" to 6" of rigid foam board (polyurethane is R6.5 per inch) over the rafters, and blow in cellulose underneith the foam to fill any remaining space. Tape the seams in the foam, then cover with 2x sleepers fastened through the foam down to the rafters. The gaps between the sleepers are you new air vent channels, put screen at the eave to keep bugs out, and a ridge vent at the top. Since this is so low, you have to be concerned with snow loads... you may need to sheath with 3/4" plywood.

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