[jillmd]

Hi,

I'm building a cabin in the woods up here in Alaska. This is my first time building something out of wood besides a tree fort. I'm a welder.
Anyway my cabin dimension is 16' wide by 24' long with 10' walls. The roof pitch is 12x12 (heavy snow load). I built the roof with the idea of having a cathedral ceiling. Also so I could walk upstairs in the loft. The rafters that I built are 2x8. The roof sheeting is CDX plywood, grace's ice and water shield membrane plus 29 gauge metal roofing.
My questions is how do I insulate it? I was told just to seal off the eaves and peak and stuff the rafters with bat insulation, so it will just be a hot roof. Then I was told it will rot. Next was leave a gap at the eave and peak (at least a one inch) and insulate with R-25. By doing this the roof will now be a cold roof and this will prevent condensation.
So what is the right way?

Thanks,
Mike

[rtw_travel]

Cathedral ceilings are a pain in the neck to insulate. You need to have vents at the bottom, vents at the peak and air gap up each joist space behind the insulation (unless you use commercially sprayed foam) to connect the vents at the bottom to the top.

If your inspector allows it, you can use plastic waffle sheet material that you staple into the joist cavity. (its 22" (i.e. as wide as the joist cavity) and about 4' long and provides a 1" gap behind it. If I recall correctly, the local manufacturer is called koro-vent - its probably something different where you are.

R25 would probably not meet code where you live. You may end up having to spray foam ($$$) or make your joists thicker.

[jillmd]

Well this cabin is 30 miles from any major road and then 200 miles from any major city. So code is not a problem. I do want to built it strong. Also spray foam would break my bank account. Getting all the supplies out there , etc...
The plastic waffle sheet amaterial option sounds good. What insulation (r-value) that I'll need for the waffle barrier and 2x8 joist?
Mike

[ballengerb1]

Rtw has it on this one. I have a log cabin that's 40 years old. Know how they insulated cathedral ceilings back then, they didn't. I'm about the 5th owner and now I'm trying to insulated because we bake in the summer and freeze in the winter.

[rtw_travel]

You've got a number of options.

a) if you find the plastic waffle stuff... buy R28 insulation (i.e. ~8" thick fibreglass). You'll have to compress it slightly to fit because the plastic will take up an inch. The ceiling rated drywall (i.e. slightly stiffer drywall) will hold it in place without bulging but you'll need at least two people (3 would be better) & extra screws to put the drywall in place. If you're doing a wood t&g ceiling then it will be much easier... but watch for bulging as you install it.
b) if you can't find the plastic waffle stuff then either buy thinner insulation or add 2x2 strips to your 2x8 so that R28 will leave an air gap behind it.
c) of course, you can use the plastic stuff and add 2x2 strips. Then the whole thing will be a bit easier.

In our area, R28 is code for cathedral ceilings (R40 for any other ceiling). Alaska may be different (we get all our cold weather in Vancouver from you guys!), but you shouldn't be too far off the mark.

Edit: just realized I might have given you the wrong insulation values: R28 is for 8" rock wool insulation. I think fibreglass may have slightly less R value. Just buy whatever fits in the space