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Foil Faced Polyiso & DrainWarp for OSB drying

airfix | Posted in Green Products and Materials on

A week or two ago there was discussion about vapor permeability of foil faced polyiso exterior foam and the effect it has on drying of the sheathing.

I came across this Tyvek Drainwrap which sounds like it is similar to regular building wrap except it provides a drainage route between the sheathing and exterior foam. It seems like it would also be a way for the sheathing to dry to both the inside and outside.

Does anybody know of its effectiveness or is it just marketing hype to sell more expensive building wrap?

Does anybody know of other products like this that I should look into?



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  1. CMObuilds | | #1

    I looked at drainwrap, which is just crinkled housewrap, and thought it was bs yet I got to do a drain test with it and other drainage wraps with a plexiglass setup (this was by tyvek so maybe it was rigged, Im not sure how) and it actually blew my mind how well it worked.
    Hydrogap, Homeslicker are others. I like Hydrogap, Drainwrap is less expensive but even though the lumberyard test was slick I dont trust the gap to be large enough to clear water if thats what you need it to do.
    I have torn apart lots of houses with regular housewrap or felt and given what Im seeing behind the wrap after 20 years or more the rainscreen gap in my climate isnt a necessity.

  2. Expert Member


    We don't have any choice here as rain-screens are mandated by our code, but after building that way for a few years now, I really like them of a number of other reasons. They seem to make the detailing of flashing and trim easier and more logical. It sort of breaks the wall down into discrete layers you deal with one at a time. It also makes renovations or repairs a lot easier, especially window replacement and re-siding. Those probably aren't sufficient reasons for someone in your climate to adopt rain-screens, but I bet if you moved somewhere that they made sense, you would enjoy building with them.

  3. Expert Member
    Michael Maines | | #3

    Steve, there are two types of drainable housewraps. Some, like Tyvek Drainwrap, allow liquid water to drain. Hydrogap goes a bit further and provides a capillary break as well. But if your goal is to clear water vapor you need a thicker gap. Obdyke's Homeslicker is 1/4", and available in 5/16" to meet Canadian code. A competitor to Homeslicker and Slicker Classic is Keene Driscreen.

    I've used all of the above and they all work for what they're intended for. None are intended for the situation you describe.

    You might find some good information here:

  4. Jon_R | | #4

    Yes, Drainwrap is effective for drainage and diffusion redistribution. You don't want large (like 1/8" or more) gaps between your sheathing and insulation.

    When is comes to gaps for ventilation (outside the foam), "3/8-inch gap is very, very conservative. It can be smaller".

  5. Expert Member
    Michael Maines | | #5

    I almost posted the same BSC paper as Jon. Read it carefully, including the footnotes, and draw your own conclusions.

  6. airfix | | #6

    T Carlson, what is your climate? I'm 6a.

    So it sounds like these are designed for rain screen gaps which is actually how a came across them. I was chatting with my synthetic stucco guy and asking him how be drains the back side of his stucco. I was assuming it would be channels in his EPS foam. I was surprised to learn be uses Tyvek Stucco wrap and swears by it. However how does he know if it's effective unless he's disassembled one of his walls or had stucco failures. I'd personally like to see a more well defined gap behind my stucco.

    The application for drainwrap I was thinking of was as an answer to mounting exterior foil faced foam directly to your standard Tyvek WRB thus preventing vapor drive through the WRB from the sheathing. I tight a drainwrap might help but it sounds like I'd need a larger gap to allow vapor to pass. Just out of interest, why are these products good for water to drain but not good for allowing wet sheathing to dry. Is it because of the relative area of the crinkle being so low compared to the area of the sheathing?

    I have my rain screen gap on the outside of my foam created by furring strips that my siding is mounted to. With the foam being 2" with taped seams I won't need or want a rain gap between the inside of my foam and the WRB.

    I am concerned with rain gaps on the part of my house that has the synthetic stucco and rock veneer hence my discussion with my stucco guy.


  7. airfix | | #7

    Stupid cell phone autocorrect. I'm sure the smart reader can decipher my bad autocorrect.

  8. CMObuilds | | #8

    Steve Im on the 6/7 border. The tyvek products are some of the highest perm wraps out there whether good or bad. Stuccowrap is tested at 50 perms, regular tyvek products are in that neighborhood as well. The stuccowrap is behind the "sacrificial" wrap and thus will drain between those 2 layers with everything flashed to the stuccowrap layer.

    I dont use EIFS ever, after seeing so many failures, but I have heard the systems have gotten better. I dont know if its better details or in the product itself or both but Ive seen some dandy failures with grade wicking, improper flashing or blown joints getting bulk water behind everything, and some that seem like they were just not drying out coupled with our interior poly requirements and made some mold farms.

    I dont care for what Im seeing but there are a ton of installs and plenty of good ones Im sure, I only get called out to the bad ones.

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