J
I am in NE Iowa, just north of the zone 5 line, in zone 5. I have had to educate a few builders in our area so far of framing other then "Standard" as you stated. What sort of performance and goals are you looking for for your home? I can do my best to help you out/make suggestions.

Whallyden, we are also in central Iowa and used double stud construction. We have two 2x4 walls with an air gap in between. The total wall cavity is 9 1/4 inches. This space was then insulated with blown in fiberglass (Spyder) insulation. Our insulation contractor also did an awesome job with caulking/air sealing before blowing the fiberglass in. The rim joists and corners were filled with spray foam and caulking was used in smaller areas that were not foamed. We have a total approximate R39 in our walls with this system.

We had investigated alternate systems before starting construction and were also interested in ICF, SIPS, etc., however very few contractors were familiar with these methods. By using double studs, we were able to use an "old school" contractor with minimal learning curve needed on their part. They had built one double stud house in the past, but that had been in 1976!

After being in the house since August, we are very happy with our decision. Our utility bills have been extremely reasonable for a 3700 square foot house. We used an hybrid heating/cooling system with an air to air heat pump and back up propane furnace. Our average heating bill this mild winter has been $35 per month.

I'm in South Central IN. Finding a builder experienced in non-standard construction has been a joke! We had planned to use SIPS, but just recently changed our minds to using ICF.....mainly because we finally found a builder who specializes in ICF houses. However, he is 75 miles away. Which means his subs are 75 miles away. Not looking forward to paying for their fuel surcharges!! We're still looking for a closer match, but we're not holding out much hope. We're convinced that a tight envelope is the only way to build a house these days, so we're sticking to our guns and will keep searching!

Good luck to you!

The ones here that haven't had much luck finding someone that builds ICF or SIPS, you might want to go to greenbuildingtalk.com and ask in the forums if a contractor is near you. With these new types of building, it is hard to find someone. I know several people that found a contractor this way.

Good idea, robin0919! I just did a post over there- hopefully I'll find who I'm looking for! Thanks for the suggestion~

Thank-you for the replies. @lzerarc -- I'm interested in persuing the "60/40/30/10 rule". The sticking point with builders is getting an exterior wall to r40. I had been hoping to get close with 2x6 24" oc framing, dense pack cellulose, and 2" of rigid foam. However, few builders have experience with rigid foam in exeterior walls or 24" oc framing.

I had hoped to use cement board siding -- does anyone know how much rigid foam I can use?

Whally
2x6 framing with 2" of foam (I will use XPS in this example) does not get you an r40. Here is an example:
dense pack cellulose has r of approximately 3.9, so lets say r22 for the 2x6 walls. 24" oc spacing will yield a slightly better framing factor. Typically the reduction is 20%, so lets use 15% here instead. r22 reduced by a 15% framing factor gets you around an r18-19 wall. Add 2" of XPS (r10) and you are sitting at just under an r30 wall.
Going with 3" of XPS gets you around r35. However anything over 1" thick of foam with the use of Hardi board will require exterior strapping, which is a good idea over foam anyway to give you a drainage plane. The strapping is not expensive, however the labor and long screws required can start to add up. Going thicker then 1.5" then also starts to muddy window flashing details. You need to have a good builder and a good architect familiar with these types of designs to produce the drawings required. Thicker foams 2-3" can still easily be done, the screws get longer and labor increases. XPS foam is not overly cheap, and going 2-3" on the home will then most likely pass a true r35-40 double stud wall assembly.
Getting to r40 is easily done with the double stud approach, and as stated above, requires little "training" on the contractors part.
Urethane SIP panels is another way to get a super tight shell and high r value. A 6" urethane panel will hit an r40.

And I need to add...r value is not always everything. Air sealing is the first priority. If you can get a super tight shell (say 1 air change or less) then you are doing very well. Typical "code" homes are around 3-5 air changes. Passive Haus is .6, which is considered extremely tight. Very high attention to detail and design needs to be applied to hit these levels. After you are air sealed, then you can focus on r value and thermal bridging. Reducing thermal bridging will further increase the performance of your insulation. In my example above with 2x6 framing...if you eliminate thermal bridging then that 15% framing factor is now eliminated as well.

Air sealing is the first priority. If you can get a super tight shell (say 1 air change or less) then you are doing very well.

Canada's R2000 program recommends that a double stud wall have the vapor barrier on the inside surface of the outside wall thus allowing the vapor barrier to be free from puncture. All the wiring etc is placed in the interior wall.

Secondly, the thermal bridging issue can be diminished by incorporated an air space between the two walls and filling the space with rigid insulation.

You should substitute "air barrier" for "vapor barrier" in your description of the Canadian system. The wall would still be able to dry to the outside through the vapor permeable air barrier and moisture would not condense on the exterior barrier and cause problems.

You should substitute "air barrier" for "vapor barrier" in your description of the Canadian system. The wall would still be able to dry to the outside through the vapor permeable air barrier and moisture would not condense on the exterior barrier and cause problems.

In a cold climate, if a vapor barrier were located on either side of the exterior sheathing, sufficient rigid insulation would need to be placed on the exterior face of that sheathing to prevent condensation at the vapor barrier.

Ren
If you place enough exterior rigid on the exterior to raise the dew point at the sheathing plane, an interior vapor barrier needs to be avoided. A vapor permeable product, such as latex paint on the walls should be used. With 2" of XPS (which would be the required amount of r to raise the dew point in zone 5) you need to allow drying to the interior, thus omitting the vb. However as you stated, an air barrier needs to be included in either situation. I prefer to place it on the exterior plane using Huber ZIP sheathing. ZIP combined with the correct amount of exterior XPS creates a very tight, efficient shell.