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Community and Q&A

Kraft Paper as Vapor Retarder

lekawa | Posted in General Questions on

I’m in the process of insulating a new addition with unfaced mineral wool insulation in zone 4.  I am not  required by code to install a vapor barrier between insulation and drywall.  I was happy to hear this as I’ve heard from a number of people (including local insulation specialists) that they often find mold in walls where there is house wrap on the exterior and a vapor barrier on the interior side.

I was just wondering if covering interior walls with Kraft paper after mineral wool insulation is in place might be of any use.   I don’t see such a product when I try to Google it.  Just regular Kraft paper, which I believe is lacking the”vapor retarding coating” that Kraft faced fiberglass batts supposedly have.

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  1. Expert Member
    BILL WICHERS | | #1

    Yes, it’s a good idea to add an interior side vapor RETARDER (not a vapor BARRIER) on the inside I never you don’t have sufficient exterior rigid foam to prevent condensation issues inside the wall.

    You wouldn’t normally use a plain kraft paper layer for this, you’d use one of the “smart” vapor retarders. Two examples of these are certainteed’s MemBrain and Intello. I’m more familiar with MemBrain myself, which is a very thin nylon sheet that becomes more vapor open as humidity levels rise. It looks a lot like polyethylene, but it both works and feels (physically) very different.


  2. creativedestruction | | #2

    Search for "Grade B asphalt kraft underlayment". It comes in 36" rolls, serves as a class 2 vapor retarder and has similar if not equal characteristics to common kraft facings on batts. "Aquabar B" is one product name.

    It's not a very tough material, and it's not an air barrier. I would use only in combination with the airtight drywall approach or another air barrier strategy.

  3. lekawa | | #3

    Thanks to both! I looked up Intello, MemBrain, and Aquabar B. The least the site at which I viewed it... didn't say anything about it being a vapor retarder...just said it was to hold blown in insulation in place and it's expensive, so if it's not necessary, would rather avoid the expense and the trouble. MemBrain is expensive too. The Aquabar B is inexpensive, but it said it's a "floor underlayment"...found another source of it that did mention it's potential use as a membrane for walls and concrete backer. Found one comment though that said it smells like asphalt. I assume the smell would disipate? Any knowledge or experience using?

    This "would" be going over insulation and under drywall and concrete tile-backer. Not sure what you mean by "air tight" unless most drywall is considered to be so.

    Also read somewhere that there are vapor retarding paints. I'm thinking maybe a layer of Aquabar B and a good coat or two of vapor retarding paint might at least be better than what the city is requiring.

  4. Expert Member
    BILL WICHERS | | #4

    Drywall itself is an air barrier. To make a drywall wall air tight, you need to detail it as such (proper tape and mud in joints, a bead of sealant around the perimeter to seal it against top and bottom plates). It’s not particularly difficult to detail drywall to be an air barrier and I’d recommend you do so. There are articles here about the “airtight drywall approach”.

    MemBrain is an air barrier too, although it’s not always easier to detail the MemBrain to be an air barrier than it is to detail the drywall. I personally would spend the extra money to use MemBrain over the asphalted kraft paper as I believe MemBrain to be a superior product (and I’ve used it, although my experience with the kraft paper is just as a backing on Fiberglas batts).

    If you do choose to go with the aquabar product, it would probably be easier to detail if you ran it vertically and not horizontally. Vertically installed strips would have seams on studs where they’d be easy to keep tight together. Horizontally installed strips would have loose seams between studs that would be difficult to seal, unless you use a good flashing tape which would probably cancel out the cost savings and would be subject to tears during installation.

    Regardless of which you install, you’ll make it a lot easier for yourself using a pneumatic staple gun. These things are awesome for this kind of thing, HUGE labor savors. You want one that shoots regular handheld staple gun staples, not the larger narrow crown staples.


  5. lekawa | | #5

    Thanks Bill,

    I've heard of the MemBrain, and really would have no issue with using it if it weren't for the cost and the trouble getting it installed correctly...and, above all, I'm imagining, after spending the money and time to get it in perfectly, workers inadvertantly tearing holes in it....which may be harder to identify than with the Kraft paper. (After months on this project I'm starting to really "get" the law of "diminishing returns")

    I agree with your point about installing aquabar vertically. I have this staple gun...Is this what you're talking about? Can also be sure to have drywall installed as proper air barrier.

    1. Expert Member
      BILL WICHERS | | #7

      I don’t know why you’d be worried about getting membrain installed correctly: it goes up just like a sheet of poly, easy. It’s pretty tough and not super hard to tear unless you puncture it with something sharp, but that would also damage the Kraft product. My only problem was when my three year old daughter “helped” by poking holes in the membrain with a drywall screw she found. Nothing some tape can’t fix! :-) I’ve heard some of the stories about it being “hard to install” too, and I think that’s because it comes in a sheet the full height of the wall, so it’s pretty big. If you’re careful it goes in pretty quick and easy though. The pneumatic staple gun helps, a lot.

      What you show in your link is ye olde way to do this. Hammer trackers are a lot nicer than the “squeeze the handle” staple guns, but they’re not very accurate for staple placement, and you’re still using some effort. Use something like this:
      That’s what I use, and it works great. It’s very easy to place a staple wherever you want it, and it takes very little effort to squeeze the trigger, since the air pressure is doing all the work.


  6. tjanson | | #6

    Do kraft faced batts actually function well as a vapor retarder? Is it in the code just to keep production builders happy? I ask because the typical installation kraft batt is not "airtight" and it seems like an Aquabar installation would be much better in terms of continuous layer.

    I'm climate zone 6. I've been using mineral wool, airtight-ish drywall, and Ben Moore 573 vapor retarding primer as I remodel my house.

  7. lekawa | | #8

    Bill, I'm guessing you have to tape all of the seams...and over all staples when installing MemBrain properly (to work the way it's "supposed to", and for the cost of it, I wouldn't want to be careless about it) Also, just a girl here, who will have no further use for any fancy- schmancy tools (nor the space) Does sound awesome. Could probably rent one (?) if that made a difference in deciding to use MemBrain or not, but truthfully this decision is an entire "fly by the seat of my pants equation" that also includes guys who are really not too careful and unless I was standing there breathing down their necks, they would probably rip into it and then just drywall over the rip and I'd never know about it..

    Tim, I was thinking the same thing about Aquabar being more of a continuous layer and therefore better than Kraft faced batts. It's hard for me to believe that Kraft faced batts provide much, if any moisture barrier the way they're often installed....But I'm thinking the fact that they ARE paper probably means they wouldn't "sweat" the way I imagine poly maybe they could absorb a bit of vapor now and then...and could hold it until it has a chance to dry out. This is another "fly by the seat of my pants theory" but I think at this point in my project, it's either "drywall and paint", or "Aquabar, drywall and paint". It certainly couldn't hurt to try it.

  8. Expert Member
    BILL WICHERS | | #9

    You don’t need to need to tape over all the staples. Taping the seams is usually a good idea (a big overlap works too, but not as well and is much less elegant), but since it comes as a bit sheet, you don’t have many seams to deal with. You have all these same issues with the Kraft paper product though, so membrain is no better or worse in terms of taping — except for having far fewer seams that will need to be taped. Membrains big sheet might be more difficult to handle than the smaller width sheet of the Kraft product, but membrain will have FAR fewer seams as a result, so less taping. Less stapling too if you just span over large sections of wall. You don’t need to fasten. Membrain to every stud. I tack up the perimeter and a few places along the wall just to keep it laying flat to make drywall installation easier.

    Membrain and kraft paper are both subject to damage during construction. There is no different between them in that regard either. Neither is any better or worse than the other in terms of the potential for damage. Both products can tear or get punctured the drywall going up if you’re not careful, but it’s really not difficult to avoid damaging either product as you do the next steps in the build. It just takes a little care.

    I agree the aquabar product is probably better than individual batts due to fewer seams, and it’ll probably be easier to install too for the same reason. Vapor barriers don’t have to be completely sealed the way an air barrier has to be. Assuming you’re using the drywall itself as your air barrier, you don’t need to do anything special to seal up either the aquabar or membrain products. Just staple them up and call it done, then seal your drywall. Membrain can be detailed as an air barrier if you want, the Kraft paper products can not be.

    Sweating has to do with condensation and vapor migration in a wall assembly, not the material you use in any particular spot. Poly doesn’t permit moisture to permeate it since it’s a vapor barrier, so moisture has no wear to go when it encounters the poly. If the poly is below the dew point, you get condensation on the poly. Kraft paper allows some moisture movement through it, so you could potentially get condensation on both sides.

    Don’t think about a vapor retarder as absorbing moisture. Your insulation might be able to do this (cellulose can, for example, and can do some “moisture buffering”), but the vapor retarder or barrier really can’t. You’ll still get condensation inside the wall if conditions are right, what the vapor retarder does is limit how much moisture can get into the wall to condense.


  9. lekawa | | #10

    "just takes a little care"....I hate to say it, but I haven't been seeing much "care" from workers. I have not yet hired a drywaller, but at this point, my expectations in regard to "care" are pretty low...

    I AM planning to use moisture resistant drywall on all bathroom walls (roughly 1/3 of the entire addition) Do you think moisture resistant drywall acts as somewhat of a vapor retarder?

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