Decided to make a new thread for this one. So my contractor and architect are more comfortable with 2x6 construction with sprayed cellulose than foam sheathing and it is significantly cheaper. I read somewhere that thin foam strips can be used to thermally break the studs. Anyone have experience with this as far as cost, ease of build, or any problems that can arise when screwing the studs to the plywood. Am i right in assuming its stud, then .5 inch of foam, then plywood, and then tyvek wrap? I also want to make this house very tight airwise-I understand this will be more difficult without foam sheathing. Should I foam around windows and doors- tape the plywood joint(if that even works) and rely on the mass of cellulose for the rest to prevent air infiltration and then rely on the door blower tests to reveal any weak spots? In case you are wondering Im on the edge of zone 3 and 4. Memphis, TN. Thanks again for ya'lls expertise. It is much obliged. Doc

I would use thicker (1" ?) rigid iso foam if you go with that design. I'd also consider taping the Tyvek and a layer of Membrain on the inside.

You want the plywood / OSB attached directly to the studs for shear strength. If it is standing off the studs by 1/2" or more because of foam between it and the studs, it will just bend the nails / screws rather than provide shear reinforcement. If you want a thermal break using foam, you either need to put it outside the plywood / OSB or in the inside between the studs and drywall.  The standard construction method is to put the foam on the outside.  If you use two layers of 1/2" foam with staggered vertical seams, you can make it pretty tight without taping or the need for house wrap.

What arkie6 said- if you're using thin strips as the thermal break, put it between the stud & drywall or you'll risk structural issues. But on the warm edge of zone 4 it's probably still worth putting full coverage 1" foil faced polyiso on the exterior instead, which would bring the whole-wall R up to something like R20, since it thermally breaks everything, including the band joist and subfloor edge, which would be missed using a strip-approach.

To use the sheathing as the primary air-barrier, first, lay a bead of caulk or construction adhesive under the bottom studwall plate and between the upper stud plates, and between the sheathing & studs as it goes up. If the framing and sheathing is already up you can still do a pretty good job of it by caulking the stud/sheathing corners inside each stud bay, at the seam of the top stud plates, and where the bottom plate meets the subfloor. Use something extra-sticky with long-term pliability such as acoustic sealant caulk, or low-expansion 1-part spray foam. Using construction adhesive or low-expansion foam between framing and sheathing effectively glues it all together making it measurably stronger than a nailed-only approach. Caulk would seal well, but wouldn't add to load capacities.

If using exterior rigid foam, staggering the seams of the foam with that of the sheathing is still worth it, as is taping the seams.

In your climate zone 3/4" of exterior iso would be enough to be able to skip interior side vapor retarders.

If using the strips on the stud edges you could also get there with a 3/8" rainscreen gap between the sheathing and siding, which gives the sheathing massive drying capacity to the exterior, eliminating the need for interior vapor retarders more powerful than standard latex paints. Ripping 3/8" or half-inch OSB into furring for mounting the siding works- put the housewrap or felt between the furring and sheathing. Allow for free air-movement and drainage at both top and bottom of this thin cavity, but put in some screening at to keep the rainscreen cavites from becoming critter-condos.

If you want to stick with your foam position and your loads require it, you can make up for the reduction in shear strength with diagonal metal strapping.

Great replies. So ideally 1 inch foam overlapping tyvek seams and taped on outside if the budget can afford it. Otherwise 1 inch foam on inner studs between drywall and stud. Mondo caulking of the tyvek paper. And realize a loss at some thermal breaks.
If I have brick and synthetic stone siding with lots of brick ties for earthquake integrity, a 1.5 to 2.5 inch gap then tyvek then wall, will I be okay for moisture and vapor or is that going to be a problem?

And are the penetrating brick ties reducing the efficacy of the tyvek and air tightness?

With a brick cladding there's even more reason to put the foam on the exterior of the sheathing, facing the brick, since the foam is a fairly good vapor retarder. Otherwise the sheathing is subjected to fairly high moisture drives in summer when the sun is cooking the water out of dew/rain wetted masonry. If the contractor balks and it's only half-inch, it makes a difference! Foil faced iso is VERY low permeance- a true vapor barrier.

Fasteners that penetrate the sheathing aren't a big deal- nails & screws are still pretty tight plugs for the holes they create (if they're loose enough to leak much air they aren't doing their primarystructural function!) Tears in Tyvek can add up- it isn't really a great primary air-barrier. It's real function is as a drain-plane. As much as the marketing fluff goes on about air-tightness, housewrap is a pretty poor air barrier unless detailed perfectly and protected from incidental damage during construction, which is tough to do. Detailing the less tear-prone sheathing as the primary air barrier is more robust.

Rigid foam facing the cavity behind the brick is pretty standard in residential construction in my neighborhood, and has been a standard for commercial construction for at least a couple of decades. I'm surprised your contractor is shying away from it.

Was reading a green building advisor blog and saw this product mentioned. Zipline. http://www.zipsystem.com/zip-system/rsheat.aspx Anybody have experience with it. I dont know if the increase in cost would save enough in labor to justify it. It also has the foam directly against the studs so the shear strength would be decreased. And the barrier seems permeable which wouldnt be good in my brick clad case. Anyway just wanted to get your thoughts.
Thanks

The Huber ZIP-R approach is pretty good (and sometimes a cost-saver on new construction in applications where you're only looking for R6 of thermal break) but would be a crazy-expensive approach for retrofit air sealing & insulation compared to adding foam over existing sheathing. The insulating value of the bonded inch of polyiso is about the same as putting an inch of XPS on the exterior.

The total permeance of Zip-R is specified at less than 1 perm. (By comparison, #15 felt is 2-5 perms depending on humidity- the higher the moisture, the higher the permeance.) But since the OSB is part of the vapor retarder, and the facer on the iso is more vapor retardent than the OSB, it leaves ist somewhat vulnerable to the high moisture drives of the brick, ergo not the most resiliant stackup for a brick cladding.

Any particular brand of foil faced polyiso ya'll like? What about sealing tape? I found DOW brand at lowes for $16 for 1 inch 4x8. Estimating 125 panels to cover 4000 square feet of wall and didnt subtract doors or windows. Unsure if different brands have different R values and or reduction with time. Thanks

Most foil faced iso is about the same as any other- they all have aged R values of about R6/inch and even you found a higher density version comes in at R6.5 the extra R0.5 isn't worth paying extra for. There are also 1" foil-faced EPS products out there of differening densities rated R3.8-R4.2, but it's worth it to stick with R6 iso even though it WILL cost a bit more.

At 125 panels you might want to price it out at a couple of different places, including distributors who cater mostly contractors, but it's a commodity item and it's unlikely you'll get it to get it or very much under 7.5-8 cents per square foot per R. (At $16 for a 32 square foot sheet you're at 8.3cents/R-foot.) Still, $100 is $100, and you can sometimes get it for closer to $15/sheet, and you can shop by phone.

Any 2" FSK tape designed for sealing heating/cooling ducts has an adhesive that tolerates a large temperature & humidity ranges for decades without degradation (and sticks better to foil facers than the galvanized or painted steel on which it is most often used.) Box stores near me carry the ~2" versions of Nashua brand "Extreme Weather" and "Cold Weather" FSK tape either of which would be fine, but there are others. In your application and location as long as the adhesive temperature range handles an operation temp spanning 0F - 140F you're good, and most are good to at least -20F to 225F or even wider. But read the operating temp spec. for anything you use, just in case.