Passive cooling is natural air conditioning. By reducing a home’s internal and external heat gains, with clever techniques like properly positioned shade trees, houses can be completely cooled without using a mechanical air conditioner.
Evergreens on the south side of homes block the low-angled winter sun and increase passive cooling effect.
Photo courtesy Dan Chiras
Well-illustrated, and highly accessible, The Homeowner’s Guide to Renewable Energy by Dan Chiras (New Society Publishers, 2011) is an essential resource for anyone wanting to enter the renewable energy field, whether their goal is a lower monthly utility bill or complete energy independence. In this excerpt from Chapter 7, you will learn how passive cooling works, how to reduce internal heat gain, and how to use natural shade trees to promote cooling.
You can purchase this book from the MOTHER EARTH NEWS store: The Homeowner’s Guide to Renewable Energy.
Passive cooling is a key element of a larger strategy known as natural conditioning — heating, cooling, ventilating and lighting a building naturally, that is, without mechanical or electronic devices and without outside energy.
Like passive solar heating, passive cooling may require some backup from time to time. The goal, however, is to reduce our reliance on mechanical cooling and ventilation systems and the outside energy needed to run them. In the process, we slash our energy bills, increase our energy independence and dramatically reduce our impact on the environment, the life-support system of the planet and, lest we forget, the source of all our wealth.
Passive cooling taps into natural forces, such as cool breezes, shade and cool nighttime air, as well as ordinary building components, such as insulation, overhangs and energy-efficient windows. As noted in Chapter 4, many of the steps taken to heat a home passively also contribute to passive cooling. When building a new home, for instance, the simple act of orienting a building to true south increases wintertime passive solar gain while greatly reducing summertime heat gain. The net effect of this simple measure is that the house stays warm in the winter and cool in the summer, naturally. Other passive heating strategies like sealing leaks in the building envelope, insulating well, installing energy-efficient windows and building with sufficient overhangs also enhance year-round comfort. But there’s a lot more you can do to passively cool a new home, and there’s much you can do to an existing home to reduce its reliance on mechanical cooling and the costly environmentally damaging fuels that power it.
Moreover, it doesn’t matter where you live. Passive cooling techniques work well in all climate zones, from hot, humid regions like the midwestern and southeastern United States to hot, arid climates like the western and southwestern United States, although humid climates pose greater challenges, as you shall soon see.
Most homes in the United States are cooled by air conditioners powered by electricity derived from the combustion of coal or nuclear fission. A fair amount of natural gas and a tiny amount of oil is also used each year to generate electricity. Passive cooling could help reduce our dependence on nuclear energy and fossil fuels and reduce the ever-growing strain on North America’s already-taxed electrical supply system. This, in turn, will help to reduce the potential for annoying, costly and sometimes highly disruptive summer brownouts and blackouts.
Passive cooling relies on numerous seemingly insignificant measures that, when combined, dramatically reduce cooling loads in a building. These measures can be grouped in four general categories: (1) reducing internal heat gain, (2) reducing external heat gain, (3) purging built-up heat and (4) cooling people directly.
All homes contain numerous internal sources of heat. These, in turn, result in a phenomenon called internal heat gain. Common sources of internal heat gain include people, pets, electronic devices and lights, which all generate varying amounts of heat. Even small transformers for answering machines produce a small amount of waste heat. The most significant sources of heat include conventional stoves and ovens, clothes dryers, dishwashers, water heaters, incandescent lights, aquarium lights and heaters, television sets and computers. Even microwave ovens produce waste heat, though much less than conventional stoves and ovens.
When added up, these numerous small and seemingly insignificant internal heat sources collectively generate a substantial amount of heat. In the winter, internal heat sources help keep buildings warmer; in fact, in some super-smart, energy-efficient office buildings, internal heat sources from copy machines, computers, lights, people and other sources satisfy much of the wintertime heat load. At times, internal heat sources are an asset.
In the summer, however, internal heat sources become a liability; they make our homes and offices warmer and less comfortable, and they contribute to higher fuel bills. Although ridding a home of internal heat sources could increase heating costs, cooling costs often outweigh heating costs. It therefore makes economic sense in the long run to eliminate internal heat sources.
Eliminating internal heat gain is the first step to passively cooling your home. You can start by systematically locating all sources of internal heat gain in your home. To begin, take out a piece of paper and make a list of all appliances, electronic devices and lights in your home. Which ones are used most frequently? Star those items and focus your attention on them.
When your list is completed, jot down a strategy or two for reducing or eliminating the heat generated by each item on your list. For example, next to lights you could write “replace with compact fluorescent light bulbs.” Next to the stove and oven, you can write “use the microwave during the summer more than the stove.” Microwaves use much less energy to cook food and thus produce less waste heat. You could also consider cooking more meals outdoors on a grill or perhaps on a solar oven. Or, you could eat meals that require less cook time, for example, salads with chicken or boiled eggs for protein. Next to the water heater, write “install an insulated water heater blanket.” Next to the clothes dryer, you could write “hang clothes on a solar clothes dryer” (commonly referred to as a clothesline).
As you work through the list, you will find that most heat-reducing solutions are fairly easy and inexpensive. Some may require more costly fixes — like replacing energy-inefficient appliances.
Replacing an old inefficient water heater with a tankless model, especially a model without a pilot light, will reduce internal heat gain and save loads of money over time. Such a costly solution often makes good sense, especially if the appliance needs replacement anyway.
Strategies to reduce internal heat gain are, for the most part, relatively painless and economical. Unfortunately, the most significant source of heat in most homes and businesses during the summer is external heat gain — heat that enters buildings from the outside.
Heat enters our homes in a variety of ways. For instance, light striking the roof of a house is absorbed by the shingles and converted to heat. Much of this heat is conducted through the roof into the attic. It then passes through the insulation or, more likely, through the wood framing of the ceiling and radiates into our homes. (Attics are typically 30° to 40°F [17° to 22°C] hotter than ambient air in the summer.) In homes built without attics, heat passes directly from the roof through the ceiling cavity into the living space.
Sunlight striking windows and walls also heats up our homes. Warm air surrounding our homes also contributes to external heat gain. Warm air is generated when sunlight strikes driveways, streets, sidewalks, patios and decks. Warm air heats up the walls and windows of a building, much the same way a pie is heated in an oven. External heat is then conducted through walls. Warm air can also enter buildings through open doors and windows and through the multitude of cracks in the building envelope, as explained in Chapter 2.
Stopping the flow of heat into a building from the outside requires a new set of tools. One of the most important is to cool the external environment by providing shade, for example, by planting shade trees.
Shade trees cool in two ways. They block sunlight directly, reducing heat absorption by roofs, walls, driveways, walkways and street pavement. They also cool evaporatively. Just like perspiration that cools our bodies on a hot summer day, evaporation from the surface of leaves removes heat from the environment, creating a cooler atmosphere. This happens because evaporation requires energy to move water molecules from within a leaf into the air surrounding it. That energy is supplied by heat. (Scientists call this the latent heat of vaporization, just in case you wondered.)
Don’t plant tiny seedlings to save a few bucks. Young trees are less likely to survive, and they will take many years to reach full size. You may be in the retirement home — or in the nearby cemetery — before they begin producing significant shade. Bite the bullet and purchase large shade trees or, perhaps, fast-growing shade trees like cottonless cottonwoods. (Consult your local nursery for advice on the hardiest, fastest-growing shade trees.)
I recommend planting deciduous as well as non-deciduous trees (evergreens) on the north, west and east sides of a home to provide shade. If you are going to retrofit your home for passive solar to provide space heat, keep the south side tree-free. If you want to plant trees on the south side, be sure to minimize the number and then plant only high-crowned deciduous trees; they’ll lose their leaves in the fall which will still allow some solar gain. Certain species, like hickory, grow long tall trunks with few branches and leaves near ground level. Instead, their branches and leaves are concentrated near the top of the tree. Thus, they can provide shade in the summer, and won’t block as much winter sunlight as many species of tree. (Consult with a local nursery on the best type of tree.) Whatever you do, don’t plant evergreen trees on the south side of your home. Evergreens dramatically reduce, and even eliminate, the potential for passive solar gain.
Shade can also be provided by structures such as an arbor. An arbor is a wooden frame for growing grapes, ivy or other forms of vegetation (even squash or cucumbers) in the spring and summer. The vegetation may provide edible fruit and flowers or habitat for birds and butterflies, while blocking sunlight. Shade provided by an arbor reduces heat gain through the windows and walls of a home. In addition, the vegetation adorning an arbor also cools the air around a home (via evaporation), reducing external heat gain. Arbors create lovely spots for you to relax on a hot summer day, too. Vines can also be grown on trellises or directly on the exterior walls of a home. Ivy adds a touch of beauty to a home, and also shades the walls and provides evaporative cooling. However, ivy can damage walls, especially masonry walls.
This excerpt has been reprinted with permission from The Homeowner’s Guide to Renewable Energy: Achieving Energy Independence Through Solar, Wind, Biomass and Hydropower by Dan Chiras, published by New Society Publishers, 2011.
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