We are planning a new construction with ZIP System in Houston, TX. (see attached pictures) Except on the east side, the second floor is set back from the first floor so that we have separate 2' overhangs for each floor.

First floor is brick veneer with some full stone cladding with the standard air gap behind it. Second floor is mainly hardie plank and standard installation seems to be nailing it directly onto the ZIP.

Reading the two articles below at GBA, I wonder whether we should add some sort of rainscreen behind the hardie on the second floor. The exposed wall is in many areas only 6' down from the overhang, only on the east side it is a full 10' down.

Marc Rosenbaum in that article said he wouldn't even use ZIP as a WRB period and always put housewrap on top anyway (makes sense to trust shingling more than tape chemistry). But that was also 7 years ago and nobody seems to do this?

Long story short: given the 2 ft overhangs and the rather short hardie wall exposure, should we
a) just nail the hardie onto the ZIP as even shown in installation instructions
b) add furing strips and nail the hardie to that or
c) put a drainable housewrap like TAMLYN or Benjamin Obdyke behind all hardie areas
or d) just believe Marc Rosenbaum and install drainable housewrap around the whole house, even behind the brick

http://www.greenbuildingadvisor.com/blogs/dept/musings/airtight-wall-and-roof-sheathing

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/zip-sheathing-tips

A rainscreen is always a good idea in a place as rainy/windy as Houston.

It'll meet code without it, but it'll improve the drying rate of both the sheathing and the siding, and (usually) extend the lifecycle to the exterior finish.

I'm not convinced that drainable housewrap is anywhere near equivalent to a real gap.

The brick veneer should not be tight to the sheathing or housewrap, it should ALWAYS have a gap.

Ok thanks! So sounds like you would go with option b (furring strips) if it were your house.

Regarding ZIP system in general, you would use it and rely on the tape and not worry about adding extra house wrap over it?

If you've taped properly, with Zip system, there should be no need for an additional house wrap over the top.

As stated, just maintain some spacing to allow drying.

A very biased source, but I'd at least consider this:

http://www.dupont.com/content/dam/assets/products-and-services/construction-materials/assets/Tyvek-vs-ZIP-System-Tech-Talk-K17924-1.pdf

It would be interesting to see more data regarding drainable wraps vs larger gaps.

Thanks! I had already come across that in my early research as well as the article below which also makes a lot of sense. The main point I agree with is the reverse shingling the tape creates which goes against any traditional thoughts about water management.
http://www.connormillbuilt.com/zippetty-do-dont/

Nevertheless, Joe Lstiburek has nothing negative to say about ZIP, i see a lot of high end builders use it, also ones that really care a lot about building science like Matt Risinger. Martin Holladay on GBA only wrote in a few comments that "some conservative builders add housewrap on top of ZIP " but he didn't say that he thinks himself that it is necessary.

That's where I think to myself: well maybe you are overthinking this and if none of the "gurus" have problems with it, don't worry about it.

I expect that a rainscreen significantly improves Zip bulk water performance, increasing the odds for "don't worry about it". Choice of siding and overhangs should also change the odds.

I'm curious - why did you choose Zip over taped plywood + wrap? The latter is more breathable, more durable, shingled and AFAIK, about the same price. Perhaps still superior to Zip when used with Drainwrap instead of a larger gap with Zip (reducing the cost of the former).

I'm the homeowner, trying to build a home that we plan on staying in for 40 years, who has been reading online for over 6 months about building science. Finding a builder that is willing to pay extra attention to water management and air sealing was much harder than anticipated. Many of the concepts I read on on buildingscience.com, GBA, JCL, Matt Risinger etc. where completely unknown to all of the 7 we interviewed. We found one that was at least intrigued by what I had to share and was very interested in applying these techniques, whereas the others had more of a "I've done it like this for years, never had a problem, not changing anything" attitude.

This "open-minded" builder uses ZIP as default. Plus here in Houston I have seen many bad Tyvek home wrap installations with cheaper window flashing products (e.g. Tamlyn tape). Many times, the house wrap is blowing around for weeks before all windows are installed and then there is quite a lot of taping as well that pieces it back together, so we would rely on the tape chemistry there as well. I also liked the lower perm rating of the ZIP system which in Houston's climate is a plus since we mainly dry cavity walls to the inside and the lower perm rating should help with solar driven moisture from the brick. And last but not least the superior air tightness of ZIP is something I liked since we are using fiberglass or cellulose insulation and not spray foam and I want to exceed the 5 ACH50 goal that Houston requires.

I think if I could be guaranteed a great house wrap install my preference would have been Plywood taped with Siga Wigluv + Tyvek Commercial D + Dupont FlexWrap and Straightflash. But when I asked our ZIP-builder to price it, it came out a couple thousand more. Plus I figured that if his framing crew per default does ZIP, their house wrap install may not be their strong suit.

So overall I think it was less of a product selection and more of a selection of what I think would provide the best outcome with this particular builder.

That being said, I'm still a bit nervous about the tape really lasting 30-40 years but I guess nobody will know for sure until 2030 ...

> the superior air tightness of ZIP

Is there any proof for this? Ie, taped ZIP is more airtight than taped plywood?

Another shingled option is adhered wrap (like Blueskin VP100), but it's expensive. Or, for horizontal joints, possibly short pieces of building wrap attached to the back side of the upper ZIP panel and then brought to the front of the lower panel.

>Is there any proof for this? Ie, taped ZIP is more airtight than taped plywood?
No, not at all. I meant compared to the option that would have been equal in cost: Plywood (not taped at seams) + Tyvek Homewrap taped where it overlaps + Dupont FlexWrap and Straighflash. The couple thousand upcharge came from taping the seams prior to installing the housewrap and the Commercial D instead of homewrap.

>Or, for horizontal joints, possibly short pieces of building wrap attached to the back side of the upper ZIP panel and then brought to the front of the lower panel.
sounds interesting, but how would the taping of the seams work in that scenario?

> sounds interesting, but how would the taping of the seams work in that scenario?

Possibly with tape so wide that it could cover the seam and the small lapped piece of wrap. Easier would be some Z flashing that sits in horizontal seams. But this is speculation - taped ZIP might be perfectly fine.

We build double stud walls which we sheath with CDX fir plywood, since it is far more resilient to moisture than OSB (ZIP is painted OSB). We tape the joints with 3M 8067 which is also an acrylic tape. Why not use a building wrap instead? For one, if we have a problem and do a fog test, with tape we can see the leak, but is impossible to find if the wall is covered in Tyvek. (We had that problem once with a modular which had been covered in Tyvek.) For another, using building wrap as your main air barrier means being absolutely certain that the Tyvek is air tight at all seams and all edges and all corners - which means tape in all of those locations, plus taping over the 1000's of staples which poke holes in the fabric. We typically have very tight homes with this method; under 1.0 ACH50.

Bob, I understand (and agree with) taped sheathing joints as an air barrier. But then you put non-taped building wrap over that, right?

Sometimes we do, but we don't use it as an air barrier. When we use exterior insulation with horizontal strapping - for board and batten or shingles - we use a drain wrap under it.