Shading: First Step Toward Natural Cooling
In 1960, 12 percent of all American homes had air conditioners. By the late 1980s that figure was 64 percent and Americans were spending about $10 billion to operate them. Air conditioners were installed in 77 percent of the single-family homes built in 1989. Before refrigeration, comfort was a matter of siting and design to take advantage of natural cooling forces. But now, builders simply turn the plans over to a mechanical contractor, who superimposes cooling equipment.
The monetary cost (equipment, labor and energy) of the modern approach is climbing rapidly. The environmental cost of CFC refrigerants is now being felt by the refrigeration industry. Consumers will feel the pinch soon.
To fight these rising costs, designers and builders can look to the "pre-refrigeration" era for natural cooling ideas. Climate is a key factor that will affect your design decisions. In many climates, the right combination of these natural methods, properly implemented, can provide cooling equivalent to mechanical air conditioning. At the very least, natural cooling allows you to install smaller cooling equipment that will run fewer hours and consume less energy.
Natural cooling techniques work in one of three ways:
- Shading blocks the sun before it can get into the building.
- Building features prevent heat from reaching the occupied spaces. These include attic ventilation, radiant barriers and insulation.
- Ventilation moves air through the building to remove excess heat and make the occupants feel more comfortable.
This article outlines only the shading options.
Shading is like putting a hat on your building. While there's no dispute about the general benefit of shading, nailing down actual savings can be difficult. Energy savings range anywhere from 10 to 50 percent.
Place the highest priority on the surfaces that receive the most summer heat. That's usually the east and west. The best shading occurs before the sun's heat reaches the building. Shading options in order of effectiveness are: trees and shrubs, trellises, overhangs, awnings, shade screens, window coatings and interior shades.
With natural cooling, you can't just turn the whole problem over to a subcontractor. Instead, you'll need to combine a number of methods, including site design, shading, ventilation and insulation. Shading should be your first line of defense against excessive heat gain.
Trees offer excellent natural cooling.They throw shade over the walls and roof. They also will shade driveways, sidewalks and patios that can bounce heat to the building. Since big trees give more shade than little ones, devise a site plan that preserves as many existing trees as possible. Then plant new trees immediately after construction.
Trees provide a cooling bonus. To keep themselves cool, trees pump water from the ground into their leaves. As this water evaporates from the surface of the leaves, it cools the tree. This "evaporative cooling" cools the surrounding area, too.
Deciduous trees are best for south yards, because their canopies are broad and dense. When the leaves fall in the winter, many deciduous trees allow solar heat to reach the building. Evergreens can work well for north and northwest yards.
The closer a tree is to the building, the more hours of shade it will give. To be effective, trees should be planted between 5 and 20 feet from the building.
Shrubs offer less shading, but they have several other advantages. They usually cost less, reach mature size more quickly and require less space. Shrubs can shade walls and windows without blocking roof-mounted solar panels.
Trellises are permanent structures that partially shade the outside of a building. Clinging vines growing over the trellis add more shade and evaporative cooling. A special trellis to shade air conditioners, heat pumps and evaporative coolers improves the equipment's performance. Be sure not to restrict air flow to the equipment.
Fast growing vines create shade quickly, while trees can take years to provide useful shade. Deciduous vines, such as grape and Wisteria lose their leaves in winter allowing the sun's heat to strike the building. Trellises and climbing plants are a design solution that's attractive and flexible.
Outside shade screens prevent sun from entering a window. Put these only on windows exposed to direct sunlight. These devices are often called "sun screens," "shade cloths" or "solar shields." The screens&emdash;made from aluminum or plastic&emdash;are lightweight, durable and easy to install. Unlike insect screens, shade screens are specially made to block a certain amount of the sun's energy&emdash;usually between 50 and 90 percent of the energy striking the outside of the window. The term "shading coefficient" describes the amount of heat that penetrates the screen&emdash;lower numbers mean less energy gets through. While you can see through a shade screen, the view is obscured.
Awnings work like the visors on baseball caps by blocking high-angle sunlight. On buildings, awnings can cover individual windows or sections of outside walls. They are most effective on the south side of the building.
Awnings come in a variety of shapes, sizes and colors to match many building designs. Some awnings stay in a fixed position. Others can be rolled up in the winter to allow low-angle sun to reach the building. No matter how they work, awnings are an easy way to add shading to an existing building. Awnings have the disadvantage of blocking the view from the top half of the window.
Most homes have a built-in shading device. Overhangs block the high-angle, summer sun, but allow the lower winter sun to strike the building. As shading devices, overhangs have several limitations. For example, a roof overhang on a two story house will not shade a first story window. Nor will overhangs provide relief for east and west windows. Fixed overhangs will always be a compromise, since the sun's angle is the same in spring and autumn. You might want solar gain in March, but not in September. (For more information on overhangs, see The Passive Solar Energy Book by Ed Mazria.)
Most window manufacturers offer optional heat-reflecting coatings that block heat gain, but allow penetration of natural light. Windows that receive large amounts of direct or reflected sunlight are good candidates for window coatings. This is a great choice for west-facing view windows.
For most residential applications low-emissivity (low-e) coatings are sufficient. They can cut heat gain up to 25 percent without changing the window's appearance. You may think of low-e as a wintertime heat saver that cuts heat loss from a building. Low-e coatings also reduce summer heat gain. Manufacturers often configure low-e coatings for either heating or cooling climates. This is sometimes called northern and southern low-e. If you build in the north, but want to reduce heat gain for west-facing glass, you may need to special order the proper low-e arrangement. When selecting glazing, you should start by looking at two important specifications: U-value is the rate of heat transmission (lower is better) and the solar heat gain coefficient (lower means less heat gain). Also consider visible light transmittance (higher is better).
Interior window shades, such as roller shades, blinds and drapes, can reduce heat gain. However, interior shades don't block sunlight as well as exterior shades. Interior shades work in three ways. They reflect sunlight back out the window before it can turn into heat. They block the movement of hot air from the area around the window into the room. They insulate the room from the hot surfaces of the window glass and frame. To give you the most benefit, interior shades should:
- have a light colored surface on the side that faces the window;
- fit tightly to prevent air movement into the room;
- be made of an insulating material;
- cover the whole window.
Your shades do not need all these features, but the more they have the better they'll work. Roller shades work best, followed by venetian blinds. Curtains help only a little.
This article appeared in Energy Source Builder #34 August 1994