Our home, built 2010-11, has a double-stud exterior. Wall insulation is dense-packed cellulose to R40 or so, attic floor is loose-blow cellulose to R60. Then there is 4" foam under the basement slab and on the foundation walls. Windows are a mix of triple-pane casement and fixed glass (avg U=0.17). Only the doors have just double-pane glass.The house is extremely tight; ventilation to provide fresh air and keep interior humidity down in winter is provided by heat recovery ventilator (HRV - basically an air-to-air heat exchanger).
Heat for the house is by ground source heat pump ("geothermal"), with capacity just two tons, for about 4K sqft conditioned space. When it's around zero outside, the system still keeps the place at temperature in just first stage, at about 75% of capacity. Power for the system costs about $6-700 for the whole heating season. In summer, the heat pump is reversed for AC; worst-case conditions call for less than one ton of capacity.
I did what I could to determine bump in construction cost over "just to code" construction. After the framing was up, I asked the lead framer how much extra time he thought the double framed exterior took. He thought about a day and a half. Overall, the cost bump for "superinsulated" was on the order of 5%, which was what others have indicated.
For more reading on the subject of building to much better than code, go to greenbuildingadvisor.com and do a search on "pretty good house."
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