Our house was built to be thermally efficient, and was featured in a case study by Larry Kinney entitled "Clean Mountain Living" in the Boulder Green Building Journal, Winter 2007. Here are some of the features it already had when we bought it, as documented in that article:
Thicker-than-usual walls and ceiling with extra insulation. "The wall framing is 2 x 6 studs on 24-inch centers with 2 x 4s installed horizontally at 24-inch intervals on their flat sides to form 11⁄2-inch- thick strapping. The wall sheathing consists of oriented strand board (OSB) on the corners (for shear strength) and 1⁄2-inch thick polyisocyanurate (Tuff RTM) on the rest of the walls. This produces 7-inch stud cavities, which are insulated with sprayed cellulose. The exterior finish is a fireproof cementitious board siding (Hardie Plank) nailed through an air barrier. The interior finish consists of 1⁄2-inch gypsum board with hypoallergenic joint compound painted with a low volatile organic compound (VOC) white paint. The result is a total wall R-value of 30." "The gypsum board ceiling and vapor barrier is covered by 16 inches of blown cellulose, for a total ceiling R-value of 60."
High-performance windows, optimized for their location. "There are 66 square feet of windows on the south side of the house, which have an R-value of 3.2 and a solar heat gain coefficient of 0.70. The 78 square feet of windows on the other three sides have an R-value of 9.1. Because these windows are substantially shaded from the sun, the builder chose glazing with a solar heat gain coefficient of 0.31, so that they contribute little to solar heating but have high insulation values." The windows also have cellular shades that can easily be drawn at night, to dramatically reduce the heat lost through the windows.
No drafts. "All penetrations through the floor, walls, and ceiling were carefully sealed using state-of-the-art materials and techniques [at the time (2003)]. As a consequence, post-construction blower door testing revealed an envelope so tight that the blower door couldn’t measure the leaks." This is much tighter than most houses. Average natural air changes per hour for our house=0.07. This feature makes the house very snug, but causes indoor air quality issues that we have had to manage, as you can imagine (more on this later).
According to the thermal analysis in the article, the heat loss in the house is primarily through the windows, though there is also some through the walls and the floor, which is a standard slab-on-grade with 4 inches of foam insulation and radon barrier underneath. The estimate for how much propane is needed to heat the house via a single fireplace matches our experience pretty closely -- a bit less than one 500-gal tank of propane per year. In practice, we have found that lowering the shades at night makes the heat loss through the windows barely noticeable, but the floor is uncomfortably cold, radiating "coolth." Clearly, it is too large a thermal mass for the heat gain we get through the windows and from the propane fireplace to raise its temperature above the air temperature, which we keep at 65 degrees in the winter.
We are grateful we were able to start with a leg up on heating our house with very little fuel, because it was already small and well-insulated. Read on for more information about the steps we are taking to reduce the fuel needed to heat the house even further!