I just heard back from a local foam installer (seem reputable). He proposed 3" closed cell foam in the wall (R20.4 at low 90's% efficiency), and 4" directly in the cathedral (vaulted) ceiling (R29.2 at mid 90's% efficiency). I asked about these, "in my opinion", very minimal thicknesses in order to meet code, and the potential for ice damming. He argued that a completely sealed envelope will not cause any moist air to escape, hence no ice damming. He also believed that 4" would not cause any snow to melt either (said that cabins up north only have 3" without issues) What I like about this idea: the best air tightness possible (?). I don't have my drawings ready yet, so he could only give me a rough cost, but although this is more expensive than standard 2*6 with Roxul batts, is is significantly better and likely "only" a couple grand more, if I include in our own saved labour. In the grand scheme, I'm more than ok with that. in a single 2*4 wall, electrical would have to be installed first (ok, whatever), but the insulation behind the boxes would be very thin, and the wires would likely be IN the foam. How would that work in 20 years when for some reason we'd need to re-wire the place? Another big question: is the proposed thickness enough??? It also does not solve the thermal barrier issue. Would it make sense to increase the thickness a bit in order to cover the 2*4, then do another "interior" wall to install the electrical? (likely another stupid question, but could you also add the ventilation (HRV ducts) in this space?) Another option might be to do 2*6, filled 3.5-4"(?) with foam, then leave the rest open for electrical to be installed after. This leaves a full bridge though, but perhaps that's not that important?? For the ceilings we aren't limited by thickness at all, and can go with whatever. I'm just not sure what would make the most sense. Any additional thickness would drive the costs up to a point where it might not be financially viable anymore. Any input at all would be highly appreciated! Thanks

24" oc is not a "cheaper" solution, as in not as good of quality house...its simply a different and SLIGHTLY more energy efficient solution, depending on the details. True AF wants 1-2"+ of XPS sheathing, using either osb or cross strapping for shear bracing. Determining your ideal exterior insulation thickness depends on your climate zone, and how you want to handle interior wall condensation. Drywall clips, properly sized headers, etc are all part of the equation that the builder needs to do. AF is not intended to be mixed with 16" spacing. AF, using 2x6 walls, is structurally sound, assuming it is done correctly. Trusses align with 2x6, which align with floor structure, which carry the full point loads to your foundation. It does reduce the thermal bridging slightly in the walls, as well as replace some of the wood with more insulation. However IMO all houses in at least cool and cold climates should be using exterior XPS over whatever structural surface is in place, whether its osb over studs or no osb. I read these forums and too many people get caught up in R value and adding more insulation (actually most do not care about efficiency at all..."i am building efficient- 2x6 walls with batts! please........). Problem with that is, insulation is not an air barrier. Any air infiltration through your walls will give you an r-0 no matter how much you stuff in there. Design the shell to stop infiltration first, then worry about insulation. Affective products are sealed and taped exterior XPS, ZIP sheathing, osb sheathing with seamed primed and taped under a house wrap, and caulk. lots of caulk. Sill and head plates, studs to sheathing, outlets, etc. The spray foam guys like to tell you their product stops infiltration. While it does to a certain degree, it doesn't stop it at the sill and head plates, in or around headers, not to mention its very expensive. You can add exterior insulation which will give you more benefits for half the price. Focus on the shell first, insulation between studs 2nd. (however when you do, use a spray product, such as cellulose or fiberglass instead of batts) Back on topic, if done correctly AF can be just as strong and stable as 16 oc spacing. However do the math. Most cases, the $ in lumber savings is only a few hundred, like others said, the costs are offset in thicker drywall and more insulation. Trimming and attachement points are easier with 16" oc spacing, but thinking ahead in the design allows you to place blocking in walls for cabinets, closets, etc.

"Have you thought of changing the wall studs to 2x6" rather than 2x4"?" I don't wish to destroy the wall on the far side. I have a large map wallpapered to the other side. I could build out with a false wall for some additional distance, but I don't see that as necessary since I can do at least 3-1/2"x14" (almost equivalent to a 8" round duct). Plus, I found a standard "Straight Register Boot" to transition to 8" diameter. I believe 3-1/2x14" equates to about .05 friction loss for five feet). I just don't know what 2200-2500 FPM will equate to in sound level at the hood. I foresee cutting a 4x15 opening (or bigger), at the back of the hood. I am re-reading rjpjnk's post and I remind you I only have five straight feet till it feeds straight into the fan venturi. I found this is an engineering handbook: Duct velocity should be between 1500 and 4000 FPM  Hood velocities (not less than 50 FPM over face area between hood and cooking surface)  Wall Type - 80 CFM/ft2  Island Type - 125 CFM/ft2  Extend hood beyond cook surface 0.4 x distance between hood and cooking surface Filters  Select ï¬lter velocity between 100 - 400 FPM  Determine number of ï¬lters required from a manufacturerÂs data (usually 2 CFM exhaust for each sq. in. of ï¬lter area maximum)  Install filters at 45 - 60°to horizontal, never horizontal Using this as a guide I think an average 400 CFM for general-purpose venting @ 1500 FPM means that I am well covered with a variable speed 1000 CFM external fan and 8" duct area. I'm guessing that at 1500 FPM it will be pretty quiet. The old Vent-a-Hood units are supposed to deliver 300 CFM each and I usually only use one. They are super noisy at near my ear level and 18 inches away. The air passes through a 3-1/2" x 4-1/2" rectangular opening! That's small and seems to equate to almost 3000 FPM! I guess it's no wonder they're so noisy... Am I missing anything? Bad calculations or asumptions?

You're looking at the table for 10 ft tall studs. For studs heights above 10 ft, TABLE R602.3.1 limits the height of 2x4's @ 16" o.c. to 12 ft. assuming the following: the studs support a roof only; wind speed will not exceed 100 MPH; snow load not exceeding 25 psf; fb not less than 1,310 psi determined by multiplying the AF&PA NDS tabular base design value by the repetitive use factor, and by the size factor for all species except southern pine; E not less than 1.6 × 106 psi, tributary dimensions for floors and roofs not exceeding 6 feet; maximum span for floors and roof not exceeding 12 feet; eaves not over 2 feet in dimension and cover with exterior sheathing. Also, Utility, Standard, Stud and No. 3 lumber grades of any species are not permitted for any stud regulated by the table. In my experience, it is common to see a poor quality and incorrect grade 2x4s on a job site; it is difficult to monitor the grades unless you are on the site every day; and I would not assume the weather would always cooperate. If I were the architect, I would specify 2x6s for an 11 ft wall regardless of the other variables or what the building code says since it would be my stamp on the drawings.