Kraft Paper

What is the purpose of Kraft Paper on batt insulation?
The reason I'm asking is that today I finished the insulation in my office and there were a few odd shaped cavities that I stuffed scraps of insulation into. That insulation did not have the brown kraft paper on it.
I don't want condensation or thermal breaks or any other surprises after the drywall is up, so should I do anything to that exposed insulation?
BTW: I'm using Johns Manville batts, R13 3.5" - walls, and R30 10.5" - ceiling.
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Don wrote:

The kraft paper is part of the vapor barrier. There's some schmutz sprayed onto the back of the paper that reduces the permeability and bonds the fiberglass to the paper.
R
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RicodJour wrote:

Yeah...and you've seen a typical installation. Look at the seams...not a very effective vapor barrier. Maybe, just maybe, in a good drywall assembly with careful attention paid to the holes (electrical boxes, chases, etc)....
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"3D Peruna"> wrote

Don't know how typical mine is but in my honest opinion it *looks* like hell. I've never done batt insulation before and though its not physically hard it is a little demanding. Knife blades don't last long when cutting batts and I've went through prolly at least a dozen of them. When the blade dulls it tends to rip the kraft, so there are lots of areas where the batts are showing through. I did staple all of it though, 5 (1250 count) boxes of Arrow T-50's, 1/4". 2 days of stapling has my right hand in anguish.
Today will be comprised of cleaning up the insulation and getting set up to do the drywall. I have to create a stable cut table and (2) 8' T-squares to support the stuff while I get the first screw started.
Its cold as a mf right now (18 degrees) and snowing like hell. (no Ken, I don't have shorts on today :-) ) About 30 cardinals in the front yard and they really stand out against the white background. Looks like a christmas card or sumfink............
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In a previous post Don wrote...

Don:
I suggest that you cover the wall with 4 or 6 mil visqueen before you install the GWB. This will restore the vapor barrier. You will only need enough staples to hold the plastic in place until you put up the GWB.
Apply the plastic across the outlet boxes then make an "X" cut at the boxes and tape the edges of the plastic to the box with duct tape or aluminum foil insulation tape (my preferred tape for this application).
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Don wrote:

Don, you have to start asking these questions _before_ you do the work. From your description it sounds like you used a regular stapler - a hammer stapler would have worked far faster and not beat up your right hand so much. Odd, really. I would have thought all those years choking the chicken would have beefed up your forearm. :)~

Why would you want to use a table for cutting drywall? You'd have to handle the sheets twice - once putting them up on the table and another lifting them off the table to install them. Seems like a lot of extra work. I find leaning the boards up against the wall and cutting them standing up works well, or just cutting them from the stack on the floor. Of course you should be using one of those perforated 24" T-squares for drywall, they make ripping a, ahem, snap. (Amazon.com product link shortened)
R
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What he said.
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In a previous post RicodJour wrote...

I personally prefer the use of a 48-inch drywall square. Lean the sheets against the wall, put the square on top and cut down to the floor. Fold the sheet, cut the back paper and you're done.
For rip cuts mark the sheet, put the square on one end, cut to the middle, flip the square over to the other end, finish the cut.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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"Bob Morrison"> wrote

Thats exactly what I'll be doing, starting in about 30 mins. I have a Johnson adjustable 48" T-square. Previously I thought I'd lay the sheets on a table and cut the electrical box holes with a knife. But now I think I'll screw the ceiling sheets up and then use my Ryobi speedsaw to cut the holes around the boxes. Its like one of those Roto-zip things but its 18volt cordless.
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"RicodJour"> wrote

I used a Stanley electric stapler. Worked pretty good but I wish it could hold more than 1 stick of staples and occaisionally it had a tendency to shoot on full automatic, blowing 5-10 staples out before I knew what was goin on.
Just because I've been married forever doesn't mean I'm THAT addicted to choken that chicken! heh
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I've never used that stuff. The temp difference/vapor pressure is too great and the consequences too significant.
--


MichaelB
www.michaelbulatovich.ca
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I've never considered batt insulation with Kraft paper a vapor barrier insulation. As far as I'm concerned the sole purpose of kraft papered batts is for ease of fastening to vertical installations. In fact, kraft paper is quite breathable. That said, it doesn't have zero vapor barrier quality but it isn't the same as the same batts lined with plastic. Actually even JM's website calls kraft faced batts "vapor retarder" not vapor barrier http://tinyurl.com/2dwce7 . For a proper vapor barrier I would line the entire wall surface with a 6 mil polyethylene wrap layered horizontally top to bottom and againt the vertical seams between the batts
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Even though the outside of the building is wrapped with Tyvek?
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In a previous post Don wrote...

Yes. The vapor interior barrier is intended to keep room moisture from infiltrating the insulation and reducing its effectiveness. The exterior barrier (Tyvek) is intended to keep leaks outside the structural envelope and to allow any moisture vapor that might make make it into the insulation to migrate out of the building envelope.
Nothing like taking Alaska's Cold Regions Engineering Short Course to learn a ton of stuff about cold weather construction! BTW, it is a requirement for obtaining a license to practice in Alaska. Fortunately, the course is taught in Seattle.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Took the 'almost' exact words right out of my mouth Bob... thx...
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"Bob Morrison"> wrote

Ahhhh. Gotcha. Thanks Bob!
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