For thinking about NOT using rigid foam on the outside of my new double wall home? 

Zip
furing strips
Siding

?

Rigid foam on the outside is primarily used for a thermal break on the framing. This can double the clear-wall R-value of 2x4 construction, but with double-wall construction you have a GIA-NORMOUS thermal break since the bulk of the insulation between the pair of stud-walls, and the additional R of the foam is relatively small by comparison. (Larsen Truss clear-wall & whole wall R-values are very close to the center-cavity R-values.)

And, since the most-common foam boards used range from semi-permeable XPS to IM-permeable foil-face iso , it inhibits drying toward the exterior which can detrimental in a heating-dominated climate.(Inward-drying-only is just bad idea anywhere you have 7000+ heating degree day seasons.) Any sheathing you use should be fairly permeable. (something like Georgia-Pacific Stedi-R permeable structural fiberboard seems like the right stuff: http://www.gp.com/BUILD/DocumentViewer.aspx?repository=BP&elementid=2998)

If you're planning to use blown-in-netting cellulose (or similar) and furring strips with no sheathing, you'll still need some sort of drainage plane behind the siding to inhibit wind-driven wetting of the insulation & cavities from the exterior, be it drain-plane-capable Tyvek or 10mm thick Canadian-style rain-screen. You'll need to engineer bracing in the exterior framing to sustain wind-loads as well- some sort of structural sheathing may be easier sell the inspectors on. If you have 2' roof overhangs on all walls (especially for 1- 1.5 story structures) and are in a relatively low-wind environment the quality of drain-planes can be somewhat less of an issue though. Standard siding felts have been used for a century and work "well enough", but they're somewhat less permeable than modern houswraps. Canadians are apparently bigger on high-performance drain-planes than in the US: http://irc.nrc-cnrc.gc.ca/pubs/ctus/9_e.html

was considering the furing stripes over the Zip sheathing then Hardi Plank siding over that for the drainage plane down..

DOH!

"Zip" as in "Huber ZipSystem" not as in "nada, nix, zero". (my BAD!)

I'm not-so familiar with the product but I've seen it being used around southern New England quite a bit over the past few years (didn't know it by name however, but it's a distinctive color, eh?)

The installation instructions show it being used as the drain-plane, so I'm assuming it's good for it:

http://huberwood.com/media/2009/pdf/ZIP%20SYSTEM%20WALL%20MANUAL-ENGLISH-1-08.pdf

I haven't seen a permeability spec for it, only "designed to be breathable". From a strictly academic point of view it would be nice to have a permeability number to compare to Stedi-R (which is rated <15 perms at all thicknesses), but I'm sure it has to be better by far than CDX or OSB, and surely lower permeability than low-perm interior paint.

There are warnings in the literature about safe RH ranges when using it when applying wet-spray cellulose though- bear those in mind if that's the route you take.

And as for the foam sheathing, my comments stand- you absolutely DON'T need foam cladding in a double-wall superinsulation structure since you already have a substantial thermal break. If you need/want a higher R-value/more break you can space the walls an inch or two wider at minimal cost.

Thanks Dana, no foam for me....

The one thing thats troublesome for my mind is the fact that this Zip Sheating needs to be shedding water that sneaks by the siding for YEARS and YEARS and YEARS. We would be trusting this board to defend itself for a long time.

I'd worry less about the board itself than the long-term reliability of the taped seams 3-6-10 decades out.  If century-long performance is a concern, you might price out what a high-permeability housewrap layer (that Huber claims isn't necessary for ZipSystem) would run to install over the ZipSystem. Not having handled the ZipSystem material itself it's hard for me to say just how much "overkill" an housewrap layer would be.

I know, im sooo easily spooked by new technologies. This house is my baby and my last one. Im trying to use new tech for energy efficiency purposes at the same time im scared.


I just took this off Hubers site..

The following wall coverings can be applied directly to the ZIP System wall.
• Vinyl and aluminum siding
• Cementious plank or lapped siding
• Wood siding and shakes
• Brick or Stone (1” minimum air space for stacked installation)
The following wall coverings should have one additional layer of protection

Also just read that the TAPE has a 15 year warranty, thats not MUCH at all!


15-Year System Limited Warranty. We warrant to the Builder that, for a period of 15 years after the date of installation, the system, consisting
of ZIP System wall or roof panels and ZIP System adhesive tape, will satisfy the Air and Water Resistance Properties Test set forth in Huber
Engineered Woods (HEW) Publication Z-1. Our liability to the Builder under this 15-year System Limited Warranty is limited to the reasonable cost
of material and labor required to correct the failure to comply with this Warranty.
30-Year Panel Limited Warranty. We warrant to the Builder that, for a period of 30 years after the date of installation, the ZIP System® panels
will comply with the performance requirements set forth in U.S. Dept. of Commerce Voluntary Product Standard PS2-04 and will not delaminate.
Delamination is a visible separation of panel layers which reduces the panel’s structural strength. Our liability to the Builder under this 30-year
Panel Limited Warranty is limited to the repair or replacement, at HEW’s option, of the defective panel and of any exterior wall or roof covering
which must be removed for access to the panels.

I have used that system before, and let me tell you that tape is some tough stuff. I don't think it will deteriorate in 15 years, but as for 100+ years there isn't much you can put on exterior sheathing that will still pass tests after that long that is an air barrier and not a vapor barrier. My builder switched to sip system from tyvec house wrap because when he was doing a redo job about 5 years after the house was built, the tyvec crumpled in his hands like dust.

Yeah I see your point. also very interesting about the Tyvec.. Once that tape or the tyvec goes, there is NOTHING stopping water from coming in. Then everything goes to pot!

I'm sure the tape itself is tough- it's the ADHESIVE that's the potential mechanical weak point decades out. (And if you think you can tell how the adhesive will hold up merely by handling it you're a prophet! :-) ) I'm sure adhesives are much better now than they were 50 years ago, but I'd have more confidence in redundant metal fasteners like staples & nails. (Lots of centuries old houses built with nails seem to still be hangin' in there. :-) )

I'm not quite buying the crumbling Tyvek story- I'd be very curious to know the detailed history. (Was it even Tyvek? Which type?) Lot's of web references to "...a contractor friend of my sister-in-law" kind of stuff, but not ONE web image. I'd think there would be PLENTY of pictures taken by the insurance adjusters and class-action tort lawyers if it were real! (Their competition would surely pay for that sort of photographic documenation too, eh?) Tyvek housewraps have been in wide use for ~30 years now, literally MILLIONS of homes- if there's a generic disintegration problem I'm pretty sure it would have been duly noted and in the courts by now.

From personal experience, I've opened up houses 5, 10, 20 years later and found the housewrap looking like it had been installed yesterday (yes, even Tyvek.) And I've yet to see anything that looked like material degradation. Perhaps some mis-applied tape here & there that wasn't fully adhered (helps if the surfaces are clean & dry when you tape it, eh? ;-) ), but nothing like crumbling, embrittlement, staining, etc. Reads a bit like a fish story, or urban myth, or (best-case) perhaps a bad batch of polymer(?). (Not likely- it's pretty standard well-understood & very stable chemistry.)

The only real problems I've found tangentially related to housewraps is where they fail to protect the sheathing from damage due to wetting when the siding is applied in direct contact to the housewrap (no gap to allow it to behave as a drain-plane.) In those cases the sheathing & or siding suffer mold damage. While the stuff is hydrophobic, it is by no means WATERPROOF in a trapped-water condition. (If it were, it would be a vapor barrier.) The furring strips are a must. (I would assume this to be true with Huber ZipSystem as well.) It's considerably more permeable than 15lb felt, and thus far less water-PROOF. If mis-applied it won't quite do it's "stuff" as far as protecting the sheathing from exterior wetting- it needs at least a LITTLE bit of drain-gap between the housewrap & siding to work correctly as a drain-plane (not that it's universally applied this way...)

Full-width house wrap held in place with mechanical fasteners (like staples), and seams taped &/or mechanically held with furring strips will (almost unquestionably) have very GOOD century-performance. But whether it's "necessary" or "worth it" is another matter. I'm sure it'll outlast the (still performing) heavy craft-paper air barriers used in 1920s houses (often woven between the studs, or between the sheathing & studs, with standard lapped felt between the sheathing & siding.) Modern housewraps are more moisture tolerant & hygrophobic as well. But is the tape adhesive as durable? Time will tell...

FWIW: The craft paper & horsehair laminate air barrier/insulation in my ca. 1923 home was still in pretty good shape as of 2007, except for a few places on studwalls adjacent to a dirt-floored crawlspace, where it was just beginning to deteriorate, somewhat brittle & crumbly along the foundation sill. It made retro-insulating the cavities a total PITA though, since it was stud-woven cutting the cavities in half diagonally, and wouldn't rip under insulation-blower-pressure alone. It had to be mechanically ripped & removed (a painstaking labor-intensive process to manage through a 2.5" dense-packing hole. :-( )

I don't think I'm a prophet, but after having to remove some of that tape after it was applied onto a roof and melted in place for a few days by the sun, I have a great deal of faith in it. If I didn't there is no way I would ever spend $30 a roll on it.

This was the contractor that I worked for for three years. The housewrap was tyvec brand, and it was installed by the same contractor. The contractor was not telling me this story to try to degrade tyvec or get attention, he was telling the story about how pissed off he was because the housewrap was falling off of this multi-million dollar home. There was no sort of suing going on, mainly because the typical homeowner couldn't tell if their whole housewrap magically dissapeared overnight. The only way to tell anything is to rip the siding off, and the only reason he found this out is because he had to take a couple pieces of siding off. The tyvec had hardiplank concrete siding applied right over it, no furring strips, so it could have been the mold problem you spoke of. That being said, I have never (NEVER) seen a house in my area where they used furring strips over tyvec that was sitting right on plywood sheathing. (Not that it is a bad idea, it just isn't done around here. Besides, if not using furring strips screws the stuff up in just a few years, it was too fragile to begin with. I like things to be built like a tank. For all I know it could have been just a bad batch, or some wierd circumstance or reaction that caused it, but I'm not going to roll the dice on my house.

As far as the zip style wax impregnated plywood, we threw some of it in the mud and left it there throughout a job (almost a year) and when we pulled it out it was still just as solid as when we threw it in.

aard,
Your story sounds like an experience I had many years ago (late 70's). Aspenite (OSB) had just come into our part of the world and I built a dog house out of it. Almost a year later, I found a small piece laying in a ditch near my driveway. It had gone through many inches of rain, two large (for KY) snow falls and months of summer dry weather. When I cut the piece up, it was just as solid as the day I bought it. That's the day I started specing OSB in lieu of plywood on all my jobs.

Interesting, I contact Huber the maker of the ZIP sheathing. In a quick short email back from them about furing strips and using Hardi Plank siding over it. They told me this...


The following wall coverings can be applied directly to the ZIP System wall.
• Vinyl and aluminum siding
• Cementious plank or lapped siding
• Wood siding and shakes
• Brick or Stone (1” minimum air space for stacked installation)
The following wall coverings should have one additional layer of protection (i.e. felt or housewrap) applied to
the ZIP System wall.
• Hardcoat (traditional) Stucco
• Natural or Manufactured Stone (adhered application)

So it appears the Hardi Plank is recommended to go directly on top with no furing strips needed.

As far as the Zip sheathing - what kind of pricing are we looking at on it? I know the Advantech subfloor is quite pricey compared to OSB and even plywood. I'm going to spec Advantech anyways - now I'm curious about the Zip wall?

Roughly around $15.99 per sheet and about $34 a roll for tape from what ive seen.

Two comments.
1) The price is over twice as much as standard 7/16" OSB, not counting the cost of the tape.
Is it really worth the difference?
2) While Huber may not worry about furring strips, I would. Not just for the sheathing, but more so for the siding. While vinyl siding is applied loose enough for air movement behind it without furring, any siding that is installed tightly to the sheathing will have a moisture buildup. I am not sure of the long-term effects of excess moisture on cementious siding, but what about its effect on the paint finish? And if you install wood siding without some kind of drainage plane, you can lose up to half its usable life expectancy.

I used the Zip system on my walls and roof of a house I am building now.  The pannels seem much stiffer and generaly tougher  then commodity OSB.    The roof  zip is much less slick then normal osb, Especialy with a bit of dew on the pannels.  I liked it to work with.  Now I don't completely trust it on a roof without an underlayment so I used gaf breathable roof underlayment under the shingles. 

On the walls I used Zip Pannel, then glued 1" polyiso, then 3/4"  horisontal furring strips with kerfs for drainage then vertical siding
To me it seems like a better product then OSB

I have a couple of pannels I left outside in the mud one face up, one down.  so far they are weathering fine.  Better the exterior plywood or regualr OSB

Cheers,
Eric

Curious!  Hardieplank is cement-based and quite porous, so it could become a considerable moisture reservoir post-wetting, wicking onto the housewrap wherever it is in direct contact.  There may be some chemistry compatiblity issues between fairly-alkaline cement-based materials in contact with housewraps too (probably made worse if kept damp.) 

From a hygric, chemical, and vapor drive point of view I'd think Hardie materials would have much in common with masonry veneer or stucco, more so than the wood materials they're mimicking the look of.  The installation instructions do call out a water-resistant housewrap (and Hardie makes their own version, which would presumably be material-compatible.) The also call out that it should have at least a 1"-2" clearance to horizontal flashing at the bottom at porch decks, specify that the butt joints & flashing NOT be caulked.  (This would reduce, but not eliminate the inward vapor-drive issues when the sun hits wetted siding.) They don't specify a drain gap between the siding and the housewrap, however (but perhaps they SHOULD!?!)  The recommended clearances vary somewhat by region:

http://www.jameshardie.com/homeowner/products_siding_hardieplankLapSiding.py?openTab=jsnavLink4

Gaps between drain-plane and siding have been around for quite awhile, and not just in masonry veneer, cavity walls, & stucco.  In my ca. 1923 wood framed bungalow they used strips of lath (the same lath as the interior plaster) between the 10" cedar clapboards and the overlap felt stapled to the plank sheathing.  It may be an irrelevant detail in some climates, but regardless of the weatherized surface material, providing some gap seems like it would provide some insurance against trapped-moisture issue, with very little downside. Making sure the drain-plane can actually DRAIN seems like a no-brainer anywhere that gets a significant amount of precipitation.