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Old 12-10-2010, 02:22 AM   #16
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Originally Posted by Hohn View Post
Yes, sheeting=decking (does anything have more vernacular variation than construction? oy..).

Yes, problem is vapor reaching underside of decking. Finished drywall isn't a vapor barrier. It does slow things down a bit

Keep in mind that vapor permeance is really a sliding scale of time-- how long does it take X amount of vapor to move through a given sample of material under a given test condition. So when you're talking about permeance, a low rating is good (a "barrier" is <0.1, meaning it takes a LOONG time for vapor to move through it (though it still moves!!). A higher perm means vapor moves like the brown matter through a goose. MOre here:

Permeance is additive (or subtractive, if you want to think of it that way.) So if you have something that's rated 1 perm/inch, then 4" of that material is .25perm rating.

So the closed cell has less permeance for a given thickness. If you are planning 5" sf, you might not need closed cell.

I don't think the roof leak situation is reason to choose open cell over closed. Closed cell does everything you want SF to do better-- better R value and better barrier to vapor and air movement. I wouldn't hang my hat on closed vs open being the difference between being able to save a roof or not if it's leaking. Differences in vapor and air movement do NOT translate into passes liquid water or doesn't pass liquid water.

If the roof leak scenario is important to you, then maybe SF isn't the best option. You might want to consider other options.
good points. i will have to think it over. only issue is open cell is already expensive...closed would be just way beyond the budget. as you may notice or not i just spent over 7K on a new roof, so i really dont have any more room for more cost,haha.

i think i am going to go with Gary's idea and do paint on top of drywall to stop the vapor from rising through it.

that should suffice right
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Old 12-21-2010, 02:14 PM   #17
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Vapor barriers are a nightmare, specially when with traditional insulation methods. I would recommend closed-cell spray foam insulation, just two inches of it will provide with superior insulating performance and qualify you for the vapor barrier that you are looking for as well. Read more if you need more.

Last edited by Gary in WA; 02-17-2011 at 11:12 PM. Reason: ad in post body
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Old 12-21-2010, 03:42 PM   #18
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Wow, a ton of misinformation in this thread. Just because a Net link says so doesn't make it true for a particular area or , ahem...... jurisdiction.

Lstiburek says a few things that won't pass code in many areas and some things that are simply not real world and cannot be done in any work environment.

Not sure about anyone else, but here we MUST pass inspection and some of the things said here would never pass in cold climate areas.
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Old 12-21-2010, 04:12 PM   #19
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plastic covered

Lowes sells J manv. with a plastic covering that may also work as a vapor barrier. They say it keeps dust and itch to a minimum, but I think the plastic covering would vapor block to some degree, correct me if I am wrong.
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