One of our goals in building this house was to be able to live in something easy and inexpensive to heat---an important consideration in Maine. This naturally led us to try to incorporate passive-solar features.
The basic features of passive-solar house design include south-facing orientation, overhangs/shading, thermal mass, sufficient insulation, and energy-efficient windows. Now that the house is coming together, we can see that we have been more successful in incorporating some of these features than others.
We sited the house really well. To take advantage of the sun's heat, a house should be longer than wide, with the long axis east-west, and a long south-facing wall, fitted with lots of window glass situated to absorb the heat from the low winter sun.
Now that the windows are in, we think that we have under-glazed. Since the south wall fronts on a contiguous great-room space that is fairly shallow, we were afraid of overheating, so we were cautious about glazing. We also neglected to consider that the installed windows would actually lose a full two inches in each direction to the frame, so that the actual glass area is 4 inches narrower and 4 inches shorter than we had planned on each window.
On the other hand, we paid really close attention to the angle of the sun and where it hit the south wall in the winter and the summer, and put our glazing high enough to catch the winter sun and avoid most of the summer sun. Our 2.5-foot roof overhangs, also a key passive-solar feature, are integral to making this happen.
Regarding thermal mass: our external walls, with 18 inches of mortar and wood, supply a huge amount of ambient thermal mass. What we are missing is thermal mass specifically located to absorb and moderate the sun's rays. We can still add some in the future, but for now we have decided to forgo extra thermal mass in our internal walls and floors. (We had also decided against a concrete pad foundation, which is a feature of many passive-solar designs.)
Insulation: Our 18" walls contain an average of 8 inches of sawdust insulation. However the value of cordwood lies in the combination of insulation and thermal mass, even more so than in log homes. Nevertheless, I would guess that the insulation alone probably meets the standard for walls, even without considering the mass factor. Our ceiling has standard materials (batts and rigid foam) but at R35, is a somewhat lower R-value than the new super-efficient standards.
Energy-efficient windows: here's where we could have used a bit more education. We thought we had done well, and I was excited at the prospect of new, "Low-e" windows that reduce heat loss through the glass. However, we didn't realize that the windows that were sold to us as energy-efficient were actually "LoE-2" (the "2" is superscripted so you don't really notice it). LoE-2 windows, as it turns out, are manufactured and offered as the "standard" by the major window manufacturers as an attempt to meet the energy efficiency needs of all 50 states. In addition to reducing heat loss through the glass, they also limit solar heat gain by blocking passage of infrared and some ultraviolet rays---a feature helpful in hot-weather climates, but detrimental to a passive-solar design.
So, in the end, our house may end up functioning more as what Dan Chiras terms a "sun-tempered" design than a true passive-solar spacing-heating system. Even so, according to Chiras, a sun-tempered design can take care of 20 to 30 percent of the annual heating load. We'll report back after we've lived there for a while.