[Greenbuilding] taping plywood seams vs. vapor barrier

[John Straube]

For those who dont know, ZIP is an OSB with a water resistant overlay bonded to it in the factory.  The joints are sealed in the field and the resulting assembly is an air and water barrier.
We have tested it a fair bit and used it in construction.
The air barrier is fantastic.  As good as almost any other approach.
The water barrier is good except for the concern of installation of the tape at the horizontal joints. Workers can screw up and make fish mouths and other errors.  It is pretty good, but not as good as a gravity lap.
Because of this concern, they have just launched a "liquid tape", a product with the consistency of peanut butter that you can smear into the joints and corners and penetrations.  Using this liquid flashing tape, I would rate ZIP better than almost any residential system I can think of out there. 

Re permeability.  The OSB in ZIP has the same permeance as normal quality OSB, that is, lowish.  About 0.3 to 1 under dry conditions (interior winter) and maybe 3 to 5 perms when damp (winter exterior conditions).  The exterior overlay that is added to render the system water resistant is about 16 perms, which is about normal for most house wraps.  We tested two walls side by side with high interior winter RH, one ZIP and one normal, and found the moisture contents of both systems to be about the same.

RE tapes.  We have tested a bunch, as has Martin Holliday. We mostly were looking for tapes that stick to foam, which is easier in some ways than plywood, but move ore.  OSB, when clean and dry is easy to stick to. People want to believe that tapes are like cars, and that the only good ones are from Europe.  In reality, there are some top quality tapes made here, like 3M 3015, and Dow Weathermate, Dupont Straightflash. Most tapes sold here are not that good because they are cheap because people did not want to pay more than a few cents per foot.   If you spend 10 or 20 cents per foot you get good stuff.  Of course, products like ProClima and Siga are in the 50 cents to $1/foot and are definitely good, but we have not seen a lot of benefits over the good ones at 20 or 25 cents cents/ft.  Wider tapes (2.5 to 3+") are definitely useful and we see more and more manufacturers with wider tapes (e.g. 2" has been found to be practically too little)


On 2013-07-01, at 9:53 AM, Alan Abrams <alan at abramsdesignbuild.com> wrote:

> John S--
> 
> what's your assessment of Zip Sheathing--as not only air barrier, but possible WRB.  Also, what about Zip's permeability VS plywood or OSB
> 
> thanks-
> 
> AA
> 
> On Mon, Jul 1, 2013 at 9:31 AM, gennaro brooks-church <gennarobc at gmail.com> wrote:
> I don't like any of the US tapes. Pro Clima from 475 here in the US or Siga are the only ones I would use. 
> 
> 
> Gennaro Brooks-Church
> EcoBrooklyn.com
> 347-244-3016
> 22 2nd St., Brooklyn.
> This email was sent from my phone.
> 
> On Jul 1, 2013, at 9:24 AM, Reuben Deumling <9watts at gmail.com> wrote:
> 
>> 
>> 
>> 
>> On Mon, Jul 1, 2013 at 5:53 AM, Sam Ewbank <g.l.ewbank at gmail.com> wrote:
>> 
>> Rueben- One west coast supplier of SIGA brand tapes is Small Planet Workshop.
>> http://www.smallplanetworkshop.com/siga-the-company-products/
>> That stuff looks interesting. Thank you very much for the link.  
>> 
>> I've had good luck using 3-m 8067 all weather flashing tape around windows but don't know if it will last as long as the SIGA
>> I'm happy to try that stuff too. Thanks. 
>> 
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> -- 
> Alan Abrams
> certified professional building designer, AIBD
> certified passive house consultant, PHIUS
> certified passive house builder, PHIUS
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John Straube
www.JohnStraube.com