Appreciate your comments on the 2 alternative wall sections that we're considering for a new home in zone 5 - cold climate in OH (see attachment sketch). These are being looked at as an alternative to 2" of exterior rigid foam outside of OSB sheathing. Option 1 - Utilizes the fairly new 1 1/2" Huber R Zip product (R6.6 http://www.zipsystem.com/zip-system/rsheat.aspx) to provide a thermal break right up against standard 2x6 wall construction with studs @16"OC followed by closed cell foam and a blown insulation product in the interior (see Manville hybrid insulation bulletin http://www.specjm.com/files/pdf/IST10-004.pdf). This would also necessitate the use of some additional shear wall bracing. Option 2: Obtain the thermal break by using a 2x6 plate for the walls with offset staggered 2x4 studs at 16"OC on both sides with a good exterior OSB sheathing such as Advantech? In this manner, the shear wall strength comes from the OSB sheathing and also allows for a 2" layer of closed cell spray foam right up against the sheathing up to the inner staggered wall studs. Balance of the insulation would again be a blown in fiberglass or cellulose product to fill the wall. Any pro's or con's that you can see? In the community we're looking to build at, they require things like a 42" masonry water table of either brick or synthetic stone all around the house, with either more of the same running up some of the exterior faces in the front or using something like fiber cement or stucco. In either case, the masonry cladding gets attached via ties/lath directly to the wall studs, right? Seems like it's much simpler with either of the above 2 options as compared to exterior rigid foam especially regarding details around windows, etc. The drainage plane product, in lieu of furring strips, would be a product like Benjamin Obdyke's Hydrogap (http://www.benjaminobdyke.com/visitor/product/key/hydrogap) which creates a 1mm drainage space. Seems like it would work well with either of the wall construction methods above. Appreciate your comments.

The whole wall-R of option 1 with a 25% framing-fraction works out to ~R21 (despite a center-cavity R north of R30.)  A 20% framing fraction would deliver ~R22.

The whole-wall R of option 2 with works out to ~R19-R20.  Even though there's a comparable or better thermal break on the studs and an additional ~R0.5 from the sheathing, there's no thermal break on the stud plates which cuts into performance. With option 1 the plates AND the band-joists have the R6.6 thermal break on the exterior, which means you could use cheap stuff insulate the interior of the band joists and foundation sills as well (although I'd still use 2" of  2lb foam, since the truck is already on site.)

Still it's not a huge difference in overall thermal performance.

If you moved the 2" of  2lb foam to the exterior of OSB/ply sheating as the exterior insulation (which would also be the drain-plane & weather-barrier, and used Spider or cellulose in the 2x6 cavity you'd be at ~R26 whole-wall, and you'd be able to achieve a better air seal.  To have something to hang the siding on, use chunks of 2" XPS as spacers 16" o.c. for furring that gets through-screwed to the structural sheathing/studs prior to spraying the foam. The furring then behaves as a depth-guide for the foam installer as well.  It's 25-30% more foam as well as more fiber, but it puts all of the structural wood completely inside the moisture boundary of the assembly.  With R12 on the exterior you would have huge margin on wintertime dew-point at the sheathing in climate zone 5, eliminating the need for an interior vapor retarder.

You can hang siding off of XPS? Is that what you are saying?

No- you hang the sideing off the furring. The small blocks of XPS are used as consistent depth spacers to keep the furring 2" away from the siding prior to spraying the foam. The timber screws holding the furring go through the furring + XPS + siding into the studs.

For 1x furring and 2" foam depth you'll need 3.5" minimum length pancake head heavy duty timber or SIP screw. For 2x furring bump the length to 4.5" min.

Torx/star type drive recommended". See:

http://www.homedepot.com/h_d1/N-5yc1v/R-202268258/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=FastenMaster+HeadLok&storeId=10051

Using 2x furring results in flatter siding and more bite-depth available for the siding fasteners, but adds 3/4" to the total wall thickness.

Ok, I read it incorrectly. However, with 1x furring, 2" foam and sheathing, 3.5" screws will barely nick the stud.

Posted By NFC on 14 Jun 2012 04:55 PM
Ok, I read it incorrectly. However, with 1x furring, 2" foam and sheathing, 3.5" screws will barely nick the stud.

Presumably the sheathing is already properly nailed to the stud to keep from pulling.  As long as the timber screws have at least 3/4" of bite into wood the retention is good enough to handle the loading of the siding, but clearly longer can only be better.

I haven't seen any shorter than 4" anyway (but they may exist), which would give you more than enough.   For really thick foam the moment arms get longer and the fastener spacing may need to tighten up if you used a very heavy siding, but for 2" foam you could even get away with 24" o.c. spacing even with stucco or stone, but 16" o.c. gives you a flatter plane to work with at a very marginal uptick in thermal bridging from fasteners. See:

http://www.greenbuildingadvisor.com...athed-wall

Not built into the structural specs, (but very real when working with 2lb foam) is the fact that the foam itself adds signficant rigidity to the sheathing, and forms a continuous bond between the furring & sheathing giving it even more structural capacity for carrying the siding.

After adopting Dana1's excellent suggestions go to 24" OC framing to keep the framing fraction under or near 20% and use r23 mineral wool bats instead of blown cellulose for slightly lower material costs and whole wall r is just short of r30 !

Mea culpa! Looking over other notes & materials you need 1.5" penetration in to wood on the furring screws, not 3/4"!

So for 1x furrng + 2"foam you'd really need 4.5" screws min, 5" min if 2x furring.

R23 rock wool batts are only ~R22 center-cavity at the installed 5.5" thickness (they're allowed to label the R at the uncompressed ~6" thickness, which is higher R than at 5.5".) Allowing R12 for the 2" of foam and R1 for structural sheathing + gypsum, with a 20% framing fraction you're hair under ~R27, well shy of R30. Some foams might hit R14 at 2" when initially installed, but count on more than R12 performance long term. With cellulose you'd be at R26.

Batts always have gaps & compressions- as a rule blown fiber does a better job of filling in, with fewer voids and less chance of convective loops within the cavity robbing performance. Uless perfectly installed the batts will underperform a blown-fiber despite a slightly higher R/inch.