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chrisser
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# Posted: 25 Feb 2014 10:43pm
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Been reading a lot here and all over the web.
I'm planning a 12x12 cabin with a standard rafter roof and loft.
Would like to use some salvage corrugated metal roofing as the roof surface.
Having trouble figuring out the insulation.
The property is very remote, and the only power is our small generator. Looked at dense packed cellulose, but getting a contractor's truck to the site would be pretty much impossible. Looked into spray on foam insulation kits - big bucks.
So it's looking like I'm going to have to use fiberglass or possibly foam board.
Reading on the forum, I was going to use OSB on top of the rafters, covered by foam board, then OSB, then 15# felt paper, and then screw the metal roofing into that. So far so good.
Then inside, I was going to vent the eaves and ridge, use the foam spacers against the underside of the sheathing and fill the rafters with fiberglass.
Seems like that gives me good insulation on the roof, venting to take care of moisture, and the additional insulation outside the sheathing should minimize condensation should any water vapor get through. It's (relatively) inexpensive.
Then I came across this article: http://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditioned-attic
It's about attics so maybe isn't 100% relevant, but it states:
"•Air-permeable insulation (like fiberglass) is risky, since it allows moisture-laden air to reach the cold roof sheathing. If you want to use air-permeable insulation in this location, you have two choices: include a ventilation channel between the top of the insulation and the underside of the roof sheathing, or install rigid foam insulation on top of the roof sheathing to keep the sheathing warm. (Don't do both, since these two solutions are incompatible.) "
Any idea why "these two solutions are incompatible"?
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MtnDon
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# Posted: 25 Feb 2014 11:01pm
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If you mean why not have foam on top of the roof and have batt or blown insulation underneath and a ventilation channel between the two, the answer is the ventilation channel. There's no point in passing cold or hot air (Hot or cold depending on season. ) between the two insulated layers. That would negate most of one of the layers of insulation.
If you mean something else, please expand on the thought.
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bldginsp
Member
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# Posted: 25 Feb 2014 11:07pm
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If you provide an air space above the fiberglass as you describe, the foam on top of the roof will do nothing at all, or very little, since cold air will come in to the vented space. I think that's all they meant. But perhaps if you do all you said, but with no air space above the fiberglass, hmmm.. Interesting. If there is no foam on the roof, and you don't vent above the fiberglass, the cold plywood above the fiberglass condenses moisture that gets through the fiberglass. But, if you put foam on top, the plywood over the fiberglass won't get so cold, and perhaps would not condense moisture. But, I'm just musing, speculating, theorizing, and my theories have occasionally not worked in the past, much to my consternation. So now I stick to the tried and true, which is to always ventilate above the insulation.
So, if you are willing to spend all that money on insulation, why not do this- fiberglass in rafters with spacers and venting under plywood. Then put the foam board under the rafters. All the insulation, all the ventilation, half the plywood.
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chrisser
Member
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# Posted: 26 Feb 2014 12:55am
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I guess I read "incompatible" to mean that it would cause some sort of problem. But I can see how the venting would make the insulation outside the roof sheathing kind of pointless.
That's OK with me. The foam board costs start to add up pretty quickly compared to fiberglass, as does the second sheet of OSB.
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bldginsp
Member
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# Posted: 26 Feb 2014 08:44am
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Yes, so you have to have a good reason to do it. In my vaulted ceiling design, from bottom up, I have log rafters, finish material, vapor barrier, plywood, foam insulation, 1" air space, one more layer plywood. All that because I wanted the log rafters exposed on the inside.
In your case, where it sounds like you are leaning toward a simpler vaulted ceiling, the amount of fiberglass you can get in there is a function of the width of the rafters. But, be aware that there are some kinds of fiberglass insulation that are denser and provide more insulation for the thickness. Also, Roxul mineral wool insulation gives more. But these cost more. So, you can spend more on your rafters and less on insulation, or less on rafters and more on insulation.
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Shadyacres
Member
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# Posted: 26 Feb 2014 10:41pm
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I am going to use the high density R-38 fiberglass which is only 10 1/4 inches with my 2 X 12 rafters.
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