From Ken Kern to Popular Science there’s been a lot of talk about the importance of proper siting of your home in relation to the sun (so-called passive design). I agree with them completely. The first half of this article explains the movement of the earth around the sun and on its own axis and how that affects you and your home. (It will also help you to tell time and direction by the sun.) The second half gives some specifics on how to design to best use the sun. Most of these ideas are useful only in new construction but should be of some value to owners of existing homes, especially if they’re considering solar heating.
Okay, so the earth goes around the sun once a year and rotates on its own axis every day; what else do you need to know? The crucial detail is the tilt of the earth’s axis with respect to the plane of the earth’s orbit around the sun. Most of the time the north pole is not perpendicular to that plane. In fact, only two days each year is the earth “straight up-and-down.” They are called the equinox (March and September 21st) because day and night are each exactly twelve hours. On the equinox, at noon here, the sun is 54° above the horizon. In December, the north pole tilts away from the sun, making our days shorter and the sun lower — 31° at noon. Conversely, in summer the north pole tilts toward the sun and at noon in mid-June the altitude is 77°. Thus, from winter to summer the position of the sun at noon rises 46°. This movement can be used to heat your home in the winter and cool it in summer.
Knowing that the sun passes through the southeastern sky and its noontime altitude moves up and down with the seasons, you have the basics of passive design. All you need to do now is place your windows facing south and provide for shading in the summer. The “Solar Angles” diagram shows how the low winter sunlight passes under the overhang, through the windows, but is blocked by the overhang in the summer. Roughly, every foot of overhang on the south side will shade down two feet during the summer months. So if you have a window going to the floor eight feet below the roof you’ll need a four foot overhang to completely shade the window.
The other figure shows the impact of building orientation on summer temperatures. These data were gathered in newly constructed identical apartments in Davis, California. As shown, the temperature in the first floor apartments which faced north and south varied from 62° to 75° where as an identical first floor apartment facing east and west experienced temperatures from 63° to 92°. So try to orient your house so that the long axis runs east to west. In this climate, the optimum shape for your structure is a rectangle whose long axis is 1.7 times the short. That orientation and shape minimizes the summer heat gain which is most on the west side. It also gives plenty of South face to soak up that winter sun.
The heavy white walls, grape arbors, and narrow streets in Greek villages are good examples of passive adaptation to climate. All this is done to temper the intense summer sun. Let the deciduous grape vines allow the winter sun to pour in. Just as they and many other cultures do, we need to get to know the sun and love it, not try to overpower it with all sorts of mechanical devices and fossil fuels.
A very good book on passive design is Solar Energy and Shelter Design by Bruce Anderson. Using its figures, I’ve calculated that every square foot of south-facing double-glass exposed to the sun collects a net (including night losses) of about 100,000 BTU’s per heating season. At present prices, that’s about $1 of heat per season. In one house I designed, the south windows alone will provide about a third of the heating needs. Once all that heat is in the house, a major problem, surprisingly enough, becomes what to do with it. If the structure is lightweight and well insulated, it may very well overheat on sunny, but cold winter days. That’s when some thermal mass is needed in the building to absorb the heat and give it back later on. Water, stone, and masonry, in that order, are the best materials for this purpose. They readily absorb heat, store it, and give it back later when the air temperature is lower than that of the massive material.
Briefly then, you should try to put as much double glass on the south side as possible. Ideally, you would then put some massive materials inside the house on the south side where the winter sun will strike them directly. Once the heat is in the house, the best way to keep it there is to place some sort of insulation (like polystyrene) over the windows on the inside at night. Windows on the north side of the house should be minimal but if they are needed, put them on the east first, then on the north and west. Of course all of these hints will help to produce an energy efficient house, but not necessarily a nice one. Many tradeoffs are required, but from a passive heating point of view these should be prime design objectives.
If most of the windows are on the south, and well shaded during the summer, that alone will keep your house much cooler in summer than most. It is not uncommon for windows to account for one half of the air-conditioning load in a typical building. Shading the windows can drastically reduce that load as I explained in the article in issue 22. Further passive cooling can be achieved by using these same massive materials that stored heat in the winter. With your south windows completely shaded in the summer, the sun won’t strike the concrete floor, or water bed, or water barrel, or brick planter, or whatever you’re using for thermal storage. So, the mass stays relatively cool during the day absorbing heat from the air; at night, open the house up to cool the thermal mass to get ready for the next day. This operation will substantially lower daytime but slightly increase nighttime temperatures in the house. So, if you’re never home during the day the mass may not be advantageous for you in the summer, but its value in the winter will more than compensate. Windows and vents should be placed to allow the hot air to escape in the summer, drawing in cool air preferably from the north side. The greatest volume of air flow can be achieved when the inlets and outlets are the same size.
A fellow in New Mexico has built some great passively-designed houses. They are shaped in a half-circle with the flat side facing south. The entire south side is glass while the rest of the walls are very thick adobe with few windows. The depth of the house from the south wall is designed to enable the winter sun to hit the entire floor area. Moveable insulation covers the windows on winter nights.
A French architect has inverted what is called a Trombé wall. This is the same story — lots of south glass, but he adds a blackened block wall inside the house just behind the glass. Openings at the bottom of the wall draw cold air along the floor of the house. The air then passes between the wall and the glass where it is heated (the heating makes it draw like a chimney), then exhausted back into the house through openings at the top of the wall. During the summer a vent above the windows in the outside wall is opened. This allows the heated air to escape outside which causes cool air to enter the house from vents low on the north wall.
The last design I want to mention is not truly passive because some mechanical work is required to make it operate, but not much. I’m referring to the skytherm design by Harold Hay in California. As with most passive design, the concept is elegantly simple. The roof is flat and covered with ponds of water. Insulating covers can be moved over the ponds or shoved aside to allow the sun to strike the blackened ponds.
In the winter the water is uncovered during the day to allow the water to heat up. At night, the covers are put in place to keep the heat in. The roof itself can be concrete or metal. The warm water on the top warms the roof which is also the ceiling (no insulation). So all winter long the ceiling radiates heat into every corner of the house. The ponds can hold enough heat to last through several cloudy days.
Summer operation is exactly opposite. The covers are left on the ponds during the day and removed at night. This means the ceiling is cool all summer. Hay has reported that the temperature in one of his houses varied less than 1° in six months of operation without auxiliary equipment! Very few mechanical systems could do that.
In our humid climate we would not get the fine cooling effect they get in these houses in California. Also, mildew might grow very well on the ceiling. However, winter heating would be great and if you stop the mildew growth, the system could only help in the summer.
The essence of passive design is recognizing what the sun is doing in various seasons and how you can best use it to keep your home comfortable. Further cognizance of terrain, prevailing winds, and vegetation can allow your home to exist in harmony with natural forces. After you’ve done everything you can to integrate the house with the environment and insulate it well then you might consider an active solar energy system with rooftop panels, at least for hot water.
The diagram and figure mentioned in the article are available as a PDF only. Click here to download.