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Passive Cooling

To read James Dulley's columns which are related to the various section topics below, click on the three-digit column code link(s) listed after the section heading.

Being inside a house without air conditioning on a warm day can be uncomfortable unless the house's design includes features that facilitate passive cooling. This type of house, which uses passive cooling techniques, can remain comfortably cool on hot summer days without adding to the electric bill. Passive cooling uses non-mechanical methods of reducing heat gain by increasing air circulation and decreasing solar radiation absorbed by the building. Today, these techniques are being perfected and widely used as people seek ways to save energy.

Design Considerations for New Construction

A number of passive cooling strategies can be integrated into the design of a new home. The climate and topography or a specific site will help determine which passive cooling techniques should be incorporated into the design of a building. The following are guidelines to consider:

1) Place the house where it will benefit from cool summer breezes while not exposing it to cold winter wind.

2) Take note of the prominent physical features surrounding the proposed site - bodies of water, hills, groups of trees, amount of pavement - as well as the site's microclimate.

3) A passive solar house with a long, well-glazed southern exposure, a sparsely-glazed northern side, and relatively small eastern and western exposures will stay cool in the summer and effectively collect the winter sun's heat. Limited eastern and western window exposures are especially important because during the summer they receive twice as much solar contact as the southern wall and three times as much as the northern wall.

4) Indoor room orientation is also crucial. Main living areas should be placed to protect them from direct sunshine in the summer.

Fortunately, incorporating passive solar cooling techniques can offer impressive energy savings for almost any home. The rest of this fact sheet discusses various cooling options for homes being planned, or for those already built.

Weatherize for Summer (410, 448, 937)

The basic principle of passive cooling is to keep the heat outside. Conservation measures, such as insulation, weatherstripping, and caulking help seal and protect a house against the winter cold and summer heat. A good place to start is in the attic, since it is a major source of heat infiltration. By adequately insulating and ventilating the attic, the upper floors of a house will be well protected from the sun beating down on the roof. In warm climates, adding a radiant barrier in the attic may be a cost-effective way to reduce heat gain (see section on radiant barriers). Many books available from libraries and bookstores describe other conservation measures.

Since dark colors absorb radiation, passive solar cooling practices emphasize the use of lighter colors. Using a light colored paint on the outside of a house reduces heat gain. In addition, white and light colored roofing materials help prevent attic or upper floor overheating. While both these measures can help reduce summer cooling costs, if re-roofing or repainting is not yet necessary for the house, it is probably not cost-effective to immediately do either one.

Reduce Internal Sources of Heat

During warm weather, use appliances such as ovens, dishwashers, and dryers sparingly. They emit a great deal of heat into the living space. If possible, use them in the early morning or evening hours when the heat they give off can be better tolerated. Also, since most of the energy light bulbs use is given off as heat, use them only when necessary; be sure to take advantage of the extra summer daylight to illuminate the house. Finally, when it's time to buy a new appliance, make sure it is energy efficient.

Shade Your Home

Shading is a cooling technique with which most people are familiar. Effectively shading a house can reduce indoor temperatures by as much as 20 degrees F. The main ways to create useful shade are through planting vegetation and placing architectural obstructions (e.g. overhangs and awnings) between the sun's light and the house.

Vegetation (701, 765, 793, 819)

Using vegetation to shade a house does not always provide immediate relief from the sun's heat, since it takes many years for new trees, bushes, vines, etc., to grow to the size required to protect the house. Many homeowners, however, feel the wait is worthwhile since a well placed plant can deliver effective, cool shade, as well as add to the aesthetic value of the property.

Plants are effective for cooling because they absorb the heat. Since leaves are generally dark and coarse and thus reflect very little light, they make ideal solar radiation controllers. Photosynthesis is the way plants convert light into nourishment. During photosynthesis, a process known as evapo-transpiration occurs, in which large amounts of water vapor must escape through the leaves. The water vapor that plants emit cools the air passing by it, under it, or over it.

The landscape design you create should use plants that are native to the area and that survive in the climate with minimal care. Low ground covers, such as grass, small plants (e.g., pachysandra), and bushes should be your first consideration. This type of vegetation can significantly reduce temperatures around a house through evapo-transpiration and the absorption of heat. In fact, a grass covered lawn is usually 10 degrees F cooler than the bare ground in the summer.

Trees (819)

Deciduous trees selectively placed around a house provide excellent protection from the summer sun while permitting winter sun to reach the house (see "Caution" section below for possible winter shading problems). The height, growth rate, branch spread, and shape are factors to consider in choosing a tree. Trees that drop their leaves at the start of the heating season and sprout their leaves at the start of the cooling season help cut cooling costs the most. Placement of the trees is of great importance. Since the summer sun is at a low angle in the morning and late afternoon, trees shade most effectively when placed on the northeast/southeast and on the northwest/southwest sides of a house. The mid-day sun is almost directly overhead and can be blocked by overhangs, though additional shading may be necessary.

Vines (765)

For a quicker way to provide shading with vegetation, the growth of vines on trellises can provide good shade as well as cool air through evapo-transpiration. Vines have a number of excellent qualities: there is a variety for just about every soil type and hardiness zone; they grow to an effective density in just two or three years; they require very little space and will grow in almost any configuration; and they provide color, fragrance and privacy, all in addition to their cooling capability. As they grow, vines can climb trellises designed to shade windows or whole sides of houses. The trellises should be set far enough away from the house to allow air to circulate around the vines (so as not to trap hot air) and to keep the vines from attaching to the house's facade, which could damage the exterior of the house. Vines such as Dutchman's Pipe, Boston Ivy, and the different varieties of clematis could be used successfully. Ask your local nursery what vine would be best suited to your climate and needs.

Warnings About Using Vegetation

While plants are a great way to keep a house cool in the summer, there are several things to watch for in using them. If the house is designed to gather heat from the winter sun, place the trees on the east and west sides so they will not cast shadows on the south side of the house in the winter. Even deciduous tress (trees that lose their leaves for the winter) should not be placed on the south side since in the winter the bare branches can significantly reduce the amount of sun reaching the house. Do not place shrubs or a dense line of evergreen trees where they will block the flow of cool summer air. Do not place shrubs so close to walls that air cannot circulate around or through them. Doing so may trap heat in the bushes and make the air around the house even warmer. Be careful not to plant trees or large bushes where their roots may damage septic tanks, underground wires, or the house's foundation. And, as stated earlier, make sure the plants can withstand the local weather extremes.

Shading Devices

Another effective way to control solar heat gain in the summer is by using overhangs, porches, awnings, or interior shades. Exterior shades are generally much more effective than interior shades because they do not allow sunlight to reach the windows. The particular needs of a home should be the first consideration in choosing a shading device.

Exterior Shades (889, 883)

There are six main types of exterior shading devices: roof overhangs, awnings, shutters, louvers, blinds, and solar screens.

Roof Overhangs are especially effective for the south side of a house, since they block out the high summer sun while permitting the winter sun to warm the house. To calculate the needed size of the overhang so that it blocks the sun in the summer but admits it in the winter, determine the summer and winter path of the sun across the sky.

Awnings are attached above and extend down and away from a window, effectively blocking direct sunlight. A well installed awning can reduce heat gain up to 65% on southern windows and 77% on eastern windows. To help increase the reflection of heat, the awning should be a light color. Solid-surface awnings may require an opening between the top of the awning and the side of the house to vent hot air that accumulates. If winter heat gain is desired, awnings should be easy to remove for winter storage.

Shutters are wooden or metal coverings which, when closed, cover a window and prevent sunlight from entering a house. Shutters also can be placed on the inside of the window. Besides blocking out the summer heat, shutters provide privacy, security, and some insulate windows in the winter.

Louvers have slats like shutters, but instead of having to move an entire shutter, just the slats are closed or opened to prevent or allow sunlight to enter the room. The slats can be vertical or horizontal, and, depending on the design, are controlled from inside or outside the home.

Exterior Rolled Blinds also have a series of slats that are lowered along a track when shading is desired. The lower the shade is pulled, the further closed the slats become; when fully extended the blind allows no light to enter.

Solar Screens resemble standard window screen except they prevent sunlight from entering the window. One can choose reflective or nonreflective screening, both of which reduce the amount of the sun's heat that enters the house by 50%, while not blocking the view or eliminating air flow through the window.

Interior Shades (970, 563, 617)

While interior shading is not as effective as exterior shading, it is still worthwhile if none of the above mentioned techniques are possible. There are numerous ways to block the sun's heat from inside a house. The following is a description of several strategies.

Draperies and Curtains made of tightly-woven, light-colored, opaque fabrics reflect more of the sun's rays than they let through. The tighter the curtain is against the wall around the window, the better it will prevent heat gain. Two layers of draperies improve the effectiveness of draperies for both summer and winter.

Venetian Blinds, while not as effective as draperies, can be adjusted to let in some light and air, while reflecting the sun's heat. Some newer blinds are coated with more reflective finishes.

Roller Shades can be effective when fully drawn, but will prevent natural light and air from entering.

Reflective Window Coatings are plastic sheets treated with dyes or thin layers of metal to reduce heat entering the window. There are two main types of coating: sun-control films and combination films. Sun-control films are best for warmer climates (where cooling accounts for the largest part of the energy bill), since they can reflect as much as 80 percent of the incoming sunshine. Many of these films are tinted and therefore darken a room. Combination films allow some light and heat to enter a room while preventing interior heat from escaping. These films are best for colder climates. It is important to investigate the different possibilities carefully to select the film that best meets your energy and aesthetic needs. Do not place reflective coatings on south-facing windows if winter heat gain is desired.


Nothing feels better on a hot summer day than a cool breeze. By encouraging cool air to enter a house and allowing warm air to escape, a house can be kept comfortably cool during much of the summer.

Ventilation strategies vary according to climate. In areas with cool nights and very hot days, the cooler night air should be allowed to enter the house to cool it down. During the day the house should be sealed off from the hot sun and air. A well insulated house will gain only 1 degree F per hour if the temperature is 80 degrees F to 90 degrees F; by the time the house heats up, the outside air should be cooling and can be brought indoors. In climates with daytime breezes, steps should be taken to let the cool air in while preventing the sun's rays from entering the house. In hot and humid climates where there is a small temperature swing between day and night, ventilation should be encouraged. If an area lacks consistent breezes, an artificial breeze can be created by opening a window at the lowest point in the house and one at the highest point; a natural thermosiphoning of air should increase indoor comfort. If a breeze cannot be generated, a fan will help you to feel cool, while using much less electricity than an air conditioner.

The thermosiphoning or stack effect can be taken one step further by including a cupola, clerestory, or vented skylight in the design of a house. There has been some interest in thermal chimneys which consist of a box, the south side of which is glass, with the opposite, south-facing surface painted black. The box is connected to an opening at the highest point of the house. As the sun heats up the black side, the surrounding warm air begins to rise rapidly, which draws out the warm air from the house and draws in cool air from the carefully chosen opening at the lowest, coolest point of the house. While it sounds practical, in reality thermal chimneys have been found not to work as well as expected. Though you may not want to install a thermal chimney, it is still important to make sure that the attic is adequately vented to prevent overheating.

Other Methods

Radiant Barriers (411)

When the summer sun beats down on a house, the surface temperature of the roof can rise to 160 degrees F. Much of that heat conducts through the entire roof, down into the insulation and then into the room below. This heat transfer chain can be broken if a shiny surfaced material, such as builder's foil, is interposed between the underside of the roof and the insulation. The material acts as a barrier to radiant heat, reflecting it back toward the roof and preventing the attic from overheating. Studies indicate that radiant barriers cut cooling costs 10% to 15% in houses with R-19 ceiling insulation. Installed by a professional, a roof radiant barrier will pay for itself in 5 to 10 years.

Earth Cooling Tubes

This cooling technique involves burying long pipes underground, connecting one end to the house and the other end to the surface outside. Hot outside air is drawn through the pipes, where it gives up some of its heat to the surrounding soil and enters the house as cool air. While this concept is theoretically sound, the practical application of this concept is not favorable. Studies have found that when air is circulated through the tubes continuously, the heat absorbing capacity of the earth can be depleted in a few weeks. Furthermore, there has been some concern that potentially harmful biological growth can occur in the tubes, and that in some areas, the tubes may carry radon gas into the home. If you are considering the use of earth cooling tubes, read all available information to see if they are appropriate for your location and budget.

Earth Contact

Earth bermed and underground homes are not only protected from the harsh winter weather, they can also be insulated from the heat of summer. However, in climates with short winters and very warm summers, the ground surrounding the house may become warm long before summer is over and no longer provide cooling benefits.

Roof Ponds

Rood ponds are effective in areas such as the Southwest which have hot days and cool, clear nights. Most roof ponds are made by placing bags of water over a steel or concrete roof deck. During the day, movable insulation is placed over the water bags which absorb the heat from inside the house. When the sun sets, the insulation is removed, and the water bags radiate their collected heat to the cooler night sky. Roof pond systems are most practical when they are incorporated in the original design of the house.

Evaporative Cooling

In hot, dry climates, evaporative cooling provides relief from the heat. Evaporative cooling can be done in many ways, such as spraying the lawn or the roof, or installing a small pond near the house. Whatever the method, the goal is to use the heat absorbing action caused by the phase change of water to water vapor to cool the air surrounding the house.