Simply put, a passive solar house is a house that heats and cools itself without the aid of a furnace or air conditioner and has no solar electric system. Think of a frigid, but sunny, day when your car had been parked in the sun and when you got in you were pleasantly surprised that it was toasty warm inside?
That is passive solar heating at work. Have you ever sat in the shade of a big tree next to a lake on a sweltering day and felt a cooling breeze come in across the water and bring some relief from the heat? That is passive cooling, a corollary of passive solar heating, at work. And as you might have guessed, opening the window blinds to brighten a room is an example of passive solar lighting.
Passive solar house design is the art and science of designing a house to take advantage of the natural heating and cooling the environment provides. And of course, the sun is the generator of it all. Since no one has figured out how to charge us for the use of the sun, the solar energy that radiates from it is free.
The costs of solar energy come from the costs of the equipment we need to buy and the things we have to do to make use of that free energy.
When you mention solar energy, people often think of solar panels or photovoltaics. These are the familiar panels of shiny silicon wafers that convert sunlight into electricity. These are also the most expensive ways to make use of the sun’s free power. True passive solar house design does not use photovoltaics.
Instead, a passive solar house makes use of the sun through proper orientation, well designed shading and roof overhangs, window placement and orientation, non-mechanical heat storage, and convective air currents.
It is hard to make a house 100% passive solar in most climates. Sooner or later the law of diminishing returns will take over and the cost of implementing methods for heating and cooling will be so high, the payback time frame will be too long to make the initial investment prudent.
But there are many things you can do with the design of a house to utilize the benefits of passive solar heating and cooling that basically cost nothing. And your partially passive solar house does not have to look like something out of Star Wars. It can match your image of “home” and can actually be quite traditional in style.
A lot of passive solar house design may seem elementary. But these common sense principles have sadly been forgotten in house design and home building. Houses have been plopped down on subdivision lots across the United States and the world with no regard for where the sun rises and sets. We have forgotten the lessons learned over thousands of years of home building before mechanical systems were invented.
Orientation is the first principle of passive solar house design. This is the positioning of the house relative to the points of the compass and the sun. In the northern hemisphere, the sun will be on the south side of your house during the majority of the day and the north side of your house will be shaded.
So the heat is to the south and the cool is to the north. Naturally, all of the house elements that are supposed to collect heat from the sun need to be on the south side of the house and any cooling ventilation for the hot summer months needs to come into the house from the shady, north side.
If you did nothing more than rotate a standard house plan so that the side with most of the windows faced south, you would reduce the heating expense considerably. Add more glass and you gain even more heat. Of course, there could be too much solar gain and the rooms could become unlivable, especially in summer.
Since the sun is lower in the sky in winter (in the northern hemisphere) and higher in the sky in summer, it is possible to design a roof overhang that is big enough to shade the window from the high summer sun, when you don’t want the heat, and short enough to allow the low winter sun to stream in.
The overhang can even be adjustable with the season to maximize the effect. Sun angles vary with the latitude from region to region. Sun charts are available to help with calculating the ideal orientation, overhang size, and placement for your locale.
When the sun goes down in the evening, the solar heating stops. To increase the passive solar house design benefits, your house should have some method for accepting excess heat, a way to store it, and a way to circulate that heat throughout the house later. Sun rooms or sun spaces like Trombe walls can do this.
Trombe walls are used in many passive solar houses and consist of a wall of glass, a narrow air space, and a high-mass wall, often built of masonry and painted black to maximize solar absorption. These walls have vents at the top and bottom of both the glass wall and the masonry wall.
Trombe walls face the midday sun. The sun streams into the air space and heats the masonry wall, actually overheating it in the sunniest part of the day. When heat is needed in the house, vents are opened and the air is allowed to circulate in. When heat is not needed, the excess solar heat in the Trombe wall is absorbed and stored by the high-mass wall.
If you are fortunate enough to have a site that slopes down toward the midday sun, Trombe walls are ideal to place along the basement wall that is exposed to the sun. Otherwise, your Trombe wall will compete with your house windows for space on the south or sun-facing wall.
Incidentally, just to prove that there is nothing new under the sun (sorry for the pun) the Trombe wall was invented and patented in 1881 by Edward Morse. But it was popularized in 1964 by French engineer Felix Trombe and architect Jacques Michel. I guess Trombe had the better PR agent.
Sun spaces, such as sun rooms can be utilized as solar heat collecting and storage devices in a passive solar house, while also providing additional living space. But to be effective, you will have to be willing to let the sun room “overheat” in the middle of a sunny day.
The floor should be some sort of high-mass material like tile on concrete with insulation below the concrete. And the room should have a way of extracting the heat it has collected and circulating it throughout the house. A sun space works on the same principles as a Trombe wall.
In a truly “passive” passive solar house, the air circulation should work on the principle of convective currents and not utilize any electric fans. A convective current is nothing more than hot air rising and cold air falling. In our Trombe wall or sun space, the heated air rises to the top and that is the point at which the venting should be placed to take the warm air into the house.
Practicality dictates that convective currents are not always the most effective way to circulate the air. The small amount of electricity it takes to run a couple of fans will provide much better circulation. And if you combine your passive solar design with a small bit of active solar design in the form of the fans, you could run those fans with a photovoltaic panel and battery array and still not have to buy any electricity to do the job.
Want to create a cooling draft and let your passive solar house ventilate and cool naturally? High windows that are operable can greatly improve natural ventilation and provide passive solar cooling. Your house will be warmed by the sun during the day. Because of those convective currents the air that is high up in the house is warmer. Opening a high window can allow this warm air to escape.
When it leaves, it pulls in outside air to replace it. This is where the shady north side plays a roll in passive solar design. If there are open windows positioned low on the shady, cooler side of the house, this cooler air will be drawn in. To make this passive cooling work best, open the north windows only partially so the air comes in evenly. Operable skylights can also be used in this way.
With your Trombe wall or sunroom design, you can induce a draft by opening the high vents or windows in them to let the hot air out and open your shaded windows to allow cooler air to enter.
As you might have realized, a passive solar house behaves best if it is only one room deep. Every living space should have exposure on the south and the north. In cold northern climates, most of the glass should be placed in the south wall with properly designed shading.
The north side should have fewer windows and be well insulated and sealed from the winter winds. Each room can then control its own warming and ventilating and the need to circulate air to other rooms is reduced or eliminated.
In warm summer climates, most of the passive solar house should be flipped. Here you will want to maximize solar induced cooling. Most of the windows should be on the north side of the house to reduce solar heat gain. The south-facing walls should be shaded by long overhangs to keep them from warming in the midday sun. Ventilation should still occur on the north face where the air is cooler.
One method of cooling houses in hot climates is centuries old. Solar chimneys have been used in homes since Roman times. A solar chimney is a tall vertical shaft located on the sunny side of the house and it must extend above the roofline. The shaft is painted black to absorb the solar heat or may have a glass face on the south side with the interior of the shaft painted black.
When the air in the shaft is warmed by the sun, it rises, just as it does in a fireplace chimney and is vented out through vents on the side of the chimney away from the prevailing winds. This induces a convective current. The hot air rises and cool air is drawn in low through vents or windows in the wall on the shaded side of the house. A cooling breeze is created within the house even on perfectly calm days. No fan or electricity is needed.
To enhance the cooling effect of a solar chimney, a passive geothermal heat exchanging pipe system can be installed. This is a pipe that is buried several feet underground and connects the house to the outdoors. As the warm air rises in the solar chimney, air is sucked into the house through the heat exchanging pipes. The ground is cool a few feet down even in the hottest climates. As the outside air passes through the pipes, it is cooled by the soil around it.
By the time it reaches the house it is much cooler than when it was outside above the ground. One more enhancement would be to connect the solar chimney to the unconditioned attic space. The induced draft would aid in ventilating and cooling the attic, thus reducing the temperature of the rooms below. A system like this can effectively cool a passive solar house even on very hot days.
But keep in mind that all passive cooling systems are more effective in regions with low humidity and less effective in muggy, humid locales.
Incorporating passive solar design into a traditional house can lower operating costs dramatically. Proper solar orientation, well placed windows and glass doors, properly designed overhangs, consideration of natural ventilation, and the inclusion of a Trombe wall or sun room designed to act as a sun space are things that can be done without dramatically changing the style, look or feel of your home. Your dream house can still match your vision of “home.”
But remember, every passive solar house must have the best insulation available. All of your efforts will be fruitless if your walls, roof, and floor leak heat and air. Your windows must be high quality and tight. And your other mechanical systems need to be efficient and properly designed.
It’s not likely that you can heat and cool your house with passive systems alone. Think of your house as a total system and you’ll get the results and low utility bill you want.