I would like to extend our growing season as much as possible without adding a huge cost in supplemental heat. I envision a 3 stage gardening process for homestead and hobby gardeners in south central Alaska. Starting seeds indoors under grow lights in late January to early February then moving the seedlings to a small heated greenhouse in late March to April and that means having a super insulated greenhouse with twin wall or triple wall polycarbonate windows and an insulated foundation with a radiant floor heating system if possible. Then moving plants out to an uninsulated hoop tunnel or polycarbonate covered greenhouse for final growing in late May. I think my Alaska Grow Bucket system would adapt to this process very well.
I think the most efficient design for our cold climate is a South facing shed roof lean-to style with an R-40 or higher insulated North wall. There is no advantage to having windows on the North side and the heat loss is just too great. Twin wall polycarbonate panels would probably work with the additional layer of corrugated polycarbonate over the roof for added snow load strength. Ventilation is also very important and I would recommend solar window openers and would have several windows on the low side and several on the insulated wall near the peak for convection flow - along with a thermostat controlled vent fan in one end wall.
The greenhouse that I built only has R-12 insulated 2x4 walls and is only covered with a single layer of plastic film. It loses too much heat at night even with a propane wall heater to be effective in March and April - but the heat gain during daylight is more then adequate.
I found another design on the web and I really like the idea of an attached storage shed on the North side.