As I have discussed in previous posts, windows with low U-value (or high R-value, the inverse of U-value) and high solar heat gain coefficient are necessary for passive solar gains. Two other features of good solar design—solar orientation and thermal mass—are the topics of this post.
If we pay attention to the sun’s movement, we can readily see why our TerraHaus designers placed windows where they did. The goal is to maximize solar gain in the winter and minimize solar gain in the summer.
We can describe the sun’s position in two ways: the altitude above the horizontal and the direction, measured in compass bearings or azimuths. As shown in the diagrams below, the sun in the northern hemisphere moves from sunrise to sunset through the south rather than directly overhead. In the winter, the sun’s altitude remains low and the sun comes up in the southeast and sets in the southwest. In the summer the sun reaches a higher altitude, and it rises and sets much further north of east and west than in winter. (Click to enlarge.)
Usefully, this means that in the summer, the roof and the east and west walls receive more solar gain than the south wall. Minimizing glazing on the roof, and the east and west walls, therefore, will reduce overheating. A generous roof overhang on the south can further reduce direct sun on the south windows.
In the winter, the roof, east and west walls receive very little direct sun, and the south wall receives the highest level of direct sun. High amounts of glazing on the south wall–with minimal glazing on the north, east and west walls—makes good sense.
Our landscaping will also make a difference over time. Shade from trees on the east and west walls will block excess solar radiation in the early morning and late afternoon without blocking winter gain. What about all those diagrams from 1970s passive solar books showing deciduous trees shading the south sides of homes in summer, dropping their leaves in the winter? According to the Chicago Urban Climate Study even the branches of deciduous trees cast so much shade in the winter that deciduous trees on the south sides of homes raise heating costs more than they lower summer cooling costs.
All pretty simple, eh? Well, not quite. Unfortunately, our solstices—when we get our highest and lowest sun altitudes, June 21 and December 21–do not correspond to our highest heating and cooling needs. In practice in Maine, this means that it is easy to get too much sun in October and less than we’d like in March. We may find ourselves tilting the windows open and pulling the shades in October.
Thermal mass is an important component of the solar heating plan for TerraHaus. Thermal mass has been compared to a thermal flywheel, evening out the air temperature of a building.
The concrete slab and kitchen island of TerraHaus absorb heat during the day, keeping the house from overheating. (Thermal mass releases heat during the day too, but on a net basis it absorbs more than it releases.) At night, this heat is released at a rate related to the drop in temperature, counteracting the loss of solar gain. Passive solar designers use various formulas to optimize the ratio of glazing to thermal mass.
While it was not available for purchase in the small amount we could have used for TerraHaus, we are interested in trying a new product when it becomes more available: phase change drywall. One current version incorporates cells of paraffin which absorbs high amounts of heat as it melts and releases this heat as it cools at night. Passive solar homes can use this on wall surfaces that receive direct sunlight.
Doug Fox, Director, Center for Sustainability and Global Change