The term passive solar refers to controlling the thermal (heat) energy the sun exerts on our homes and buildings every day of the year. Basically, it is the process of capturing this free heat energy in the cold months, and deflecting it in the warm months. Your cat or dog knows how to take advantage of passive solar. In the winter, our pets are often found soaking up the sun, luxuriating in the warmth radiating through the windows. In the dog days of summer, we often find them chilling on the cold ceramic shaded areas of the home. They are taking advantage of passive solar technology.
The practice of using passive solar in our homes goes back thousands of years but has largely been forgotten. As the Greek philosopher Aeschylus wrote more than 2000 years ago: “Only primitives & barbarians lack knowledge of houses turned to face the Winter sun.” What would the ancient Greeks think of us now, where virtually every subdivision built in Canada in the past 40 years completely disregards solar exposure as a significant design component. I think people tend to be more concerned with granite counter tops and stainless kitchens, than with energy efficiency.
Hopefully that’s changing and we are seeing more interest in energy efficiencies as energy prices continue to rise and environmental issues become ever more pressing. Most passive solar designed homes are built from scratch utilizing building orientation, materials and special design features to take full advantage of the free energy, but there are still some things you can do to retrofit an existing home.
The basics of passive solar design are quite simple. In the northern hemisphere where we live here in Ontario, the sun is much lower on the horizon in the cold months, and higher in the sky during the summer months. In order to capture the heat in the winter, and deflect it in the summer, any windows facing the sun should be shaded in the summer and fully exposed in the winter. In a passive designed home this is done with the use of overhangs or awnings (see illustration).
Back when central air conditioning was a luxury most could not afford, our parents and grandparents knew to keep the windows shaded in the summer. Thick heavy curtains were drawn over the windows creating a thermal barrier to the suns rays. Deciduous trees were also strategically planted to provide shading in the summer and allow sun to stream through in the winter. Today, we can still do the same and substantially reduce our a/c costs. In fact there are several ways we can retrofit our existing homes to take advantage of passive solar without having to rebuild.
Seal the Leaks
Passive solar retrofitting begins not by adding south facing windows, though this is an important measure, but by weatherizing and insulating. This is critical to efficiently heating any home, solar or not. Be sure to conduct an energy audit or hire a professional auditor to do the job. Not long ago the government would pay for all or part of this audit but those programs have not been renewed. A great source for assistance and advice is Green Communities Canada. Check them out for help with greening your home. An energy auditor will inspect your home and perform a blower-door test, which tells how leaky a building is and identifies all cracks and crevices in a building envelope. In order to reduce your heating and a/c load the leaks need to be sealed to make the building closer to airtight.
Leaks in the building envelope can be sealed with caulk or foam. Those around doors and windows are typically sealed with weather-stripping. Once again, you can do the work yourself or hire a professional energy retrofitter.
After you’ve sealed the leaks in the building envelope, it may be time to improve your home’s insulation, depending on what the energy audit revealed. Be sure to insulate the walls, ceilings, and the foundation. Based on new recommendations and rising fuel costs for heating and cooling, ceiling insulation should be boosted to R-50 or R-60, if possible. Wall insulation should be increased to R-30, although that’s not often possible in existing homes or businesses—there’s just not enough space with 2-by-4 construction. Insulation under floors over unconditioned space (i.e., crawl spaces) should be boosted to R-25 in most climates. Remember, insulation is just as important in summer months as it is in winter months.
Install insulated window shades over all windows to hold heat in at night and block unwanted sun in the summer—and don’t forget to use them! Insulated window shades come in several varieties. Cellular shades, which look like honeycombs, are an excellent choice and are widely available online and through numerous stores—even many building supply stores carry them. Warm Window shades, which consist of a layer of heat-reflecting metalized polyester film sandwiched between layers of polyester, also are effective. You can purchase the materials and sew them yourself, or hire a seamstress to make the curtains for you.
Sealing a home or business, then adding insulation, can reduce heating (and cooling) costs dramatically—by 10 to 50%, depending on how leaky and under insulated your home or business is. These measures help retain the hard-gained solar heat you’re about to invite into the building with your passive solar retrofit.
South-facing windows live a difficult life. They are exposed to lots of sunlight, and choosing frame materials that resist expansion and contraction is smart. Wood-framed windows with an exterior cladding of aluminum to protect against the weather are generally good performers, as well as all-fiberglass units. Avoid aluminum-framed windows unless the manufacturer has insulated the frame, since un-insulated metal frames conduct heat out of the building in the winter and into the building during the summer. Buy windows with warm edge spacers, which consist of foam inserted between the panes of glass along the perimeter of the window. They reduce heat loss around the perimeter of a window, greatly decreasing heat loss in the winter and heat gain in the summer. Warm edge-spacers also reduce condensation, which damages wood sills and sashes (the wood that holds the panes of glass in place).
Thermal mass is another vital and complimentary component of passive solar design. A material that has thermal mass is one that has the capacity to absorb, store and release the sun’s heat energy. Think of it like a solar battery. It stores the suns energy and allows you to access this energy long after the sun has gone down. Its density and levels of thermal conductivity help to keep the internal temperature of a building stable. Objects that have thermal mass have inherent qualities for both heating and cooling.
Materials with thermal mass (such as concrete, marble, ceramic tile and brick) are typically used in the floor or inside walls of a passive solar structure and located near the solar glazing (southern facing windows) to allow the sun’s energy to shine directly on them. In this manner, they can store and release the sun’s heat energy.
Objects with thermal mass will also keep a building cool. Have you ever felt the coolness of shaded marble on a hot summer day? If dense objects with thermal mass are kept out of the sun, they will tend to stay cool and thus, help to keep the building cool. The summer sun’s heat energy can and should be blocked from entering the building and hitting the thermal mass, by an overhang or other type of device or entryway.
In the evening, when it is cooler outside, a passive solar building can be opened up to absorb the cooler evening and night temperatures within its mass. The dense material can cool and will absorb heat the following day.
Another retrofit idea is to install a Trombe wall, named after French engineer Felix Trombe. A Trombe wall, simply put, is a very thick, south-facing wall (or North facing wall if in the Southern Hemisphere), which is painted black and made of a material that absorbs a lot of heat. A pane of glass or plastic glazing, installed a few inches in front of the wall, helps hold in the heat. The Trombe wall heats up slowly during the day, then as it cools gradually during the night, it gives off its heat inside the building.
Trombe walls are a great passive (as in no mechanical parts) way of providing heat to a room or space. They are often easily built from readily available materials and very reliable. An extra benefit is that the heat is radiated in the infra red wavelength, which is more penetrating and pleasant than the traditional convective forced air heating systems.
The simplest form of Trombe wall consists of a glass pane held against a wall with an air space behind it. Connecting this air space with the inner room are two vents, one at the top and one at the bottom of the air space. During the day the sun first heats the air in this space, then the solid wall behind. Once the air is heated it rises and enters into the room, giving it additional heat. Also the rising air pulls in cooler air from the room below to also be heated. In most installs a valve system is used to stop the system from pulling in cool air after the sun has gone down and the thermal mass has cooled down.
These are a few of the ways to take advantage of passive solar in your existing home and if you are considering building a passive solar home from scratch there many designers and builders that specialize in this type of home. There are also many sites dedicated to this type of home design and they can help you plan your dream home. By combining passive solar with solar PV, wind power and water management systems, you can truly be independent and tread very lightly upon the environment. If anyone reading this has had experience with this type of home design, we would love to hear from you.