To elaborate on thermal bridging since it was brought up again... there are several ways to calculate it, but to keep it as simple as possible, here is how it affects your total wall's insulation. First thing, if you put in r20 rated insulation product, you are certainly not getting an r20 wall for cavity walls. Other products such as ICF and SIPs (some SIPs even have thermal bridges) are nearly thermal bridge free and have what is called continuous insulation. My home is an ICF house from the footings to the roof, and there are no thermal bridges anywhere until the roof line. This gives me an r28 continuous insulation. However with wood frame structures, you have a framing reduction to take into account. Most new homes, studs, heads and plates take up anywhere from 18-25% of the wall area. (lets keep the numbers whole and rounded for easy math) Say you select blown fiberglass in a 2x6 wall, gives you roughly r20. You have 1000sqft of wall surface. That r20 is only for the stud bays, and every 16" roughly you only have an r1 due to the stud (framing factor) So you have 800 sqft of wall at r20 and 200 sqft of wall at r1. . Your "r20" wall is performing at an r16 on average with the studs acting like heat sinks. Add just 1" of XPS foam to the exterior (r5). Your r20 turns into r25, and your r1 turns into r6. Now 200 sqft of your wall is an r6 vs an r1 and reduces the heat sink effect.The materials to add that 1" XPS would be roughly 500-600 for that 1000 sqft wall. The upgrade from blown fiberglass to open cell foam is typically more than that. For adding exterior foam, details are pretty simply up to about 1.5" thick. After that, the details change and become more costly. Luckily for you, in zone 4. 1.5 is about as thick as you need to go. 1" would be a good start, and your details really do not change. I have detailed 1" foam and windows 1 of 2 ways. Either install the sheathing and foam and then the window. Or with 1" foam, most window flanges are 1 1/8"+ wide, so you can install the window and then install the foam. This gives them about 1/8"+ lap for a caulk joint for trim to hide the foam. The trim stands proud of the window with this detail obviously. 1.5" adds another step. I like to detail a frame out around the windows and butt the foam into it. So frame the wall as normal, install sheathing, then rip a 2x4 in half to give them 1.5x1.75 furring to frame out around the window RO. Install the foam, butt into the furring around the window, and then set the window, nailing into the furring. Everything planes out, and quite simple. Obviously this can also be done with 1" foam too, its just not necessary. There are lots of details floating around out there. Check out green building advisor as well. You can also checkout my blog for more details and nerdy building information. Again, a very untechincal explaination, but you get the idea hopefully. Here is a link that might be useful: home building blog

so even though the walls & ceilings are not sheetrocked the attic is decked? interesting. normally, when insulating attic floor with foam the foam is sprayed against back side of the sheetrock ceilings. since your attic floor is decked, spraying against the decking from inside the hosue would be how you'd do it. so to do that..first the perimeter of the ceiling would be sealed. you want to seal the top plates, wire, pipe penetrations. then fill the ceiling joist bay with foam. to get the full depth of the joist you'd over fill the joist bay, then trim off excess prior to installing sheetrock ceiling. this would be option three. and the one I'd chose recommend. everyone...house is North Carolina. ducts in crawlspace, so equipment is either in closet inside living space, or in crawlspace. if the roof is high, and not used for ducts then it is more cost effective to seal & insulate attic floor. OP, check into what foam is used in your area. we use open cell in hot humid climates. are you sure the ceiling joists are 2x10? if so, that gives you 9.5" for insulation with open cell..that would be about R-36 depending upon R-value per inch of product. shop foam insulators wisely. you want a full joist bay fill, not an 'average' fill. what they mean by 'average' is that if you add up low spots & high spots of foam it 'averages' out to xxx. you want full depth everywhere. excess is cut off so that sheetrock install is smooth. ask to see jobs of the foam companies in progress. ask to talk to former customers. once completed..blower door test the install to make sure it has stopped air movement between attic space above into living space below. I gotta ask....why floor the attic so early in process? and where are heating system & water heaters? tell us locations, what size & what fuel type please. how will attic above foamed floor of attic be vented? do you have recessed lights? what provision has been made for recessed lights & insulation plan? are recessed lights Insulation Contact Air Tight ICAT?? knowing the answers to these questions may change answers to previous questions. best of luck

Spray foam is the easiest way to seal rim joist cavities. I've never understood the point of insulating between the basement and the first floor-- it is code required here and no one can tell me why, There's typically little, if any, temperature differential, it's all inside the house's conditioned envelope. Rigid foam on the basement walls would be my second step, with, as Worthy says, the top half being more important than the bottom half-- sealing the rims would be first because that's typically a source of a lot of air infiltration. In a small house it's an easy (and fun) DIY project with the spray kits available at the big box stores.

"I was thinking the one big advantage of 2x6 here is that the framing portion goes from R3 to R5 - a rather significant increase. I would also guess that 2x4 construction would be laughed out of Minnesota." Yes, but also remember that a stud transmits heat about twice as well as stationary air so every stud that is connected to both the inside (sheetrock) and outside (sheathing) acts as a thermal bridge that sucks heat out of the house. If a cavity is fully insulated then a 2x6 stud will transmit less heat out than a 2x4 stud will but it will still suck heat out. However, if the cavities are not full then the exposed sides of those studs become cold sinks. They suck heat out of the air space between the sheetrock and foam insulation. Heat in the room goes through the sheetrock in to the air cavity and then in to the side of the stud and out of the house. In effect bypassing the foam insulation. When calculating whole wall r-value (by first calculating whole wall u-value) most houses use a 'framing factor' of 25%. These bits of the wall have a higher u-factor because they transmit heat out much better than the insulated portions. However, with bits of stud sticking out acting as cold sinks the framing factor will be higher. Rather than a 2" width of exposed stud we now have about 6" of exposed stud (2" on each side plus 2" on the face). So all of these studs will create a much higher framing factor and so much higher u-value and much lower whole wall r-value. So, doing a very quick calc on one of our walls... I came up with an actual framing factor of 32% using the traditional face method. When adding in the exposed sides of studs if we only did 3" of closed cell I get a framing factor of 63% (many of the studs that calculate in to the framing factor, such as jacks, are sandwiched between other studs so do not have the exposed side). So while the r-value for the cavities is r-21, the whole wall r-value for a 32% framing factor is r-11 and the whole wall r-value including the exposed sides of the studs is r-6. R-6 is quite bad if that is indeed what it is. So in this case, if there will only be 3" of insulation in the cavity, a 2x4 wall will actually be more energy efficient.