My concern with your option is more structural than insulative. The OSB/ZIP applied directly to the studs creates a structural shear wall that strengthens and stiffens the structure significantly. By placing the insulation between the studs and OSB, you loose the shear rating on the wall, and make the structure much less stable.

OSB will work fine as the sheathing (on the studs as Wes points out). The advantage of ZIP is the ability to tape the seams to get a good air barrier, but you can use OSB and go over it with a layer of Tyvek or similar, and tape the seams on that. The Tyvek then becomes your air barrier layer. We also tape the seams on the XPS to maintain that air barrier, then strapping over that and your siding for an R-30 wall. You can also get to R-40 pretty easily by substituting 2 layers of polyiso (Tuff-R, Thermax) in place of the 2" XPS.

apart from the structural reasons there is another reason for which the foam is better to the outside of the sheating: it keeps the sheating warmer and there is less chance of condensation on the sheating. with the foam-sheating-studs ensamble - if water gets between sheating and foam you're not going to be happy with the result especially with XPS and ZIP due to the lower permeability of both materials. check building science website for more details.

good luck,
Adi

A question for Bob, (and ayone else).
Do you normally allow the added insulation to cover the floor joist system as well as the walls? and do you offset the foundation to maintain the finsihed exteroir wall plane as it passes over this transition? Just curious about the detailing.

is XPS over ZIP way overkill then? The price of ZIP sheathing adds about $1k to the project over standard osb, but subtract the Tyvek price of $400 and its only a $600 difference.

I still havent ruled out the 6" urethane SIPs.  I am going for a direct gain solar design, DIY, and time is pretty critical as we will be renting some place.

I am also thinking about using Superior walls as the basement also for speed.  As a bonus, they came in about $1000 BELOW a 8" poured wall without even adding in insulation and stud furring.

foam over frame si not overkill. it is actually very good for minimising the thermal bridge and lowering the condensation potential inside the wall. how much foam, what kind of foam and how to detail it is the main problem. if you use foam on the exterior of the sheating than you can detail that as your air barrier so the ZIP is not a necessity anymore.if you go with foam - whatever thickness of foam you decide to use split the thickness in 2 and offset the joints.

have a look at this for cost comparison:

http://energetechs.com/wp-content/uploads/2009/12/Wall-assemblies-cost-analysis.pdf

adi

adi43d: I got a virus threat detected notice from AVG when I tried to look at that file...

Wes: We run the exterior foam down to just below the foundation & foam that joint. The foamed wall sticks out beyondthe foundation. I like the idea of insualting the exterior of the foundation, but I'm not yet sold (after 35 years) of having exteior foam prey to insects. Yes, I know, we've alway been good about stuccoing the wall, but I've seen too many poor installations where ants (or ?) have tunneled their way in, so I figure if I built it right & the owner eventually adds on or breaks the stucco bond.... One less thing to worry about & its easy & inexpensive to insulate the inside of the foundatiuon to R30 or 40.
Bob

I've read on this forum of using OSB sheathing, and then priming all of the seams about 1-1/2" or so on each side. The primer on the OSB will allow you to tape the seams of the OSB and it will stick very well. This would be a cost effective option vs. ZIP sheathing. You would also prime the rim joist/floor system seams and tape as well.

Wes,
I highly suggest the exterior foam board on the walls to also extend down over the floor system.
Depending on what foundation you go with, would determine if the foundation needs to be offset to allow for the foam thickness.
- An ICF basement, the exterior foam board on the framing could be flush with the outside ICF wall.

- A typical poured concrete or block basement, could be designed to allow the foam board, on the framing of the house, to also extend down over the basement wall... basically the wood framing of the house could be flush with the concrete wall as the foam board would be covering the entire outer surface of the wood and concrete. As others pointed out about ants, termites (in certain climates), etc.
OR the foundation could be offset if you wanted the foam over the framing only, and not over the foundation... the foam board would be flush with the foundation.

- A Superior Walls precast insulated concrete basement, the exterior foam on the framing could extend over the floor system and be flush with the foundation.


Jere

http://www.buildingscience.com/documents/guides-and-manuals/gm-guide-insulating-sheathing/view?topic=doctypes/guides-and-manuals