PrintEnergy Savings by Naturally Cooling Your Home
Keeping cool indoors when it is hot outdoors is a problem. The sun beating down on the homes causes indoor temperatures to rise to uncomfortable levels. Air conditioning provides some relief. But the initial costs of installing an air conditioner and the electricity costs to run it can be high. In addition, conventional air conditioners use refrigerants made of chlorine compounds, suspected contributors to the depletion of the ozone layer and global warming. But there are alternatives to air conditioning.
An alternative way to maintain a cool house or reduce air-conditioning use is natural (or passive) cooling. Passive cooling uses non-mechanical methods to maintain a comfortable indoor temperature.
Specific methods to prevent heat gain include:
- reflecting heat (i.e., sunlight) away from your house;
- blocking the heat;
- removing built-up heat
- reducing or eliminating heat-generating sources in your home.
Reflecting Heat Away
Dull, dark-colored home exteriors absorb 70 to 90 percent of the radiant energy from the sun that strikes the home's surfaces. Some of this absorbed energy is then transferred into a home by way of conduction, resulting in heat gain. In contrast, light-colored surfaces effectively reflect most of the heat away from a home.
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Reflecting Heat Away
The following are passive methods for reflecting unwanted heat energy away from your home.
1) The roof, made out of traditional roofing materials, allows about 1/3 of unwanted heat that builds up in the home. Unlike most light-colored surfaces, even white asphalt and fiberglass absorb 70 percent of the solar radiation. One solution is to apply a reflective coating to your existing roof. Two standard roofing coatings are marketed primarily for mobile homes and recreational vehicles. Both are waterproof and have reflective properties.
2) Wall color is not as important as roof color, but it does affect heat gain somewhat. White exterior walls absorb less heat than dark walls. And light, bright walls increase the longevity of siding, particularly on the east, west, and south sides of the house.
3) Windows permit about 40 percent of the unwanted heat that builds up in the home. Reflective window coatings are plastic sheets treated with dyes or thin layers of metal. Besides keeping your house cooler, these reflective coatings cut glare and reduce fading of furniture, draperies, and carpeting. Two main types of coatings include sun-control films and combination films.
- Sun control films are best for windows in warmer climates because they can reflect as much as 80 percent of the incoming sunlight. Many of these films are tinted, however, and tend to reduce light transmission as much as they reduce heat, thereby darkening the room.
- Combination films are best for climates that have both hot and cold seasons. They allow some light into a room but they also let some heat in and prevent interior heat from escaping.
Blocking the Heat
Two excellent methods to block heat are insulation and shading.
Insulation helps keep your home comfortable and saves money on mechanical cooling systems such as air conditioners and electric fans.
- Shading devices block the sun's rays, absorb or reflect the solar heat, and reduce indoor temperatures by as much as 20° F. Shading can be provided by trees and other vegetation or exterior or interior shades.
- Exterior shades are generally more effective than interior shades at controlling heat gain because they block sunlight before it enters windows. Exterior shading devices include awnings, louvers, shutters, rolling shutters and shades, and solar screens.
- When deciding which devices to use and where to use them, consider whether you are willing to open and close them daily or just put them up for the hottest season. You also want to know how they will affect ventilation.
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Blocking the Heat
The following are passive methods for blocking unwanted heat energy from your home.
- Low ground cover such as grass, small plants, and bushes can also be very effective in cooling. A grass-covered lawn is usually 10 degrees F (6 degrees C) cooler than bare ground in the summer. If you are in an arid or semiarid climate, consider native ground covers that require little water.
- Deciduous trees that lose their leaves in the fall help cut cooling energy costs the most. When selectively placed around a house, they provide excellent protection from the summer sun and permit winter sunlight to reach and warm your house. The height, growth rate, branch spread, and shape are all factors to consider in choosing a tree. Vines are a quick way to provide shading and cooling. Grown on trellises, vines can shade windows or the whole side of a house.
- Solar screens resemble standard window screens except they keep direct sunlight from entering the window, cut glare, and block light without blocking the view or eliminating air flow. They also provide privacy by restricting the view of the interior from outside your house. Solar screens come in a variety of colors and screening materials to compliment any home. Although do-it-yourself kits are available, these screens will not last as long as professionally built screens.
- The attic is a good place to start insulating because it is a major source of heat gain. Adequately insulating the attic protects the upper floors of a house. Recommended attic insulation levels depend on where you live and the type of heating system you use. For most climates, you want a minimum of R-30. In climates with extremely cold winters, you may want as much as R-49.
- Weatherization measures such as insulating, weather-stripping, and caulking help seal and protect your house against the summer heat, in addition to keeping out the winter cold. Although unintentional infiltration of outside air is not a major contributor to inside temperature, it is still a good idea to keep it out. Outside air can infiltrate your home around poorly sealed doors, windows, electrical outlets, and trough openings in foundations and exterior walls.
- Draperies and curtains made of tightly woven, light-colored, opaque fabrics reflect more of the sun's rays then 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 the draperies' insulation when it is either hot or cold outside.
- Landscaping is a natural and beautiful way to shade your home and block the sun. A well-placed tree, bush or vine can deliver effective shade and add to the aesthetic value of your property. When designing the landscaping, use of plants that are native to the local area survive with minimal care.
- Shutters are movable wooden or metal coverings that, when closed, keep sunlight out. Shutters are either solid or slatted with fixed or adjustable slats. Besides reducing heat gain, they can provide privacy and security. Some shutters help insulate windows when it is cold outside.
- Awnings are very effective because they block direct sunlight. They are usually made of fabric or metal and are attached above the window and extend down and out. A properly installed awning can reduce heat gain up to 65 percent on southern windows and 77 percent on eastern windows. A light-colored awning does double duty by also reflecting sunlight.
- Wall insulation is not as important for cooling as attic insulation because outdoor temperatures are not as hot as attic temperatures. Also, floor insulation has little or no effect on cooling.
- Venetian blinds, although 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 reflective finishes. To be effective, the reflective surfaces must face the outdoors.
- Besides providing shade, trees and vines create a cool microclimate that dramatically reduces the temperature (by as much as 9 degrees F [5 degrees C]) in the surrounding area. During photosynthesis, large amounts of water vapor escape through the leaves, cooling the passing air. And the generally dark and coarse leaves absorb solar radiation.
Removing Built-Up Heat
Nothing feels better on a hot day than a cool breeze. Encouraging cool air to enter your house forces warm air out, keeping your house comfortably cool. However, this strategy only works when the inside temperature is higher than the outside temperature.
Natural ventilation maintains indoor temperatures close to outdoor temperatures and helps remove heat from your home. But only ventilate during the coolest parts of the day or night, and seal off your house from the hot sun and air during the hottest parts of the day.
The climate you live in determines the best ventilation strategy.
| Climate | Strategy |
|---|---|
| Cool nights and very hot days | Let night air cool the house. A well-insulated house will gain only 1° F (0.6° C) per hour if the outside temperature is 85° to 90° F (29° to 32° C). By the time the interior heats up, the outside air should be cooler and can be allowed indoors. |
| Daytime breezes | Open windows on the side from where the breeze is coming and on the opposite side of the house. Keep interior doors open to encourage whole-house ventilation. If your location lacks consistent breezes, create them by opening windows at the lowest and highest points in your house. This natural "thermosiphoning," or "chimney," effect can be taken a step further by adding a clerestory or a vented skylight. |
| Hot and humid where temperature swings between day and night are small | Ventilate when humidity is not excessive. Ventilating your attic greatly reduces the amount of accumulated heat, which eventually works its way into the main part of your house. Ventilated attics are about 30° F (16° C) cooler than unventilated attics. Properly sized and placed louvers and roof vents help prevent moisture buildup and overheating in your attic. |
Reducing Heat-Generating Sources
Often-overlooked sources of interior heat gain are lights and household appliances, such as ovens, dishwashers, and dryers.
| Heat generating light or appliance | Reducing Heat |
|---|---|
| Incandescent Lamps | Use only when necessary and take advantage of daylight to illuminate house. Also consider switching to fluorescent lamps, which use 75% less energy than incandescent lamps and emit 90% less heat for the same amount of light. |
| Kitchen Appliances | Use in the morning or late evening when extra heat can be tolerated. Consider cooking on an outside grill or using the microwave oven, which does not generate as much heat and uses less energy than a gas or electric range. |
| Laundry Appliances | Seal off laundry room and water heater from rest of room. Purchase new energy-efficient appliances that generate less heat and use less energy. Look for the Energy Guide label indicating the annual estimated cost for operating the appliance or a standardized efficiency ratio, and use this information to select the most efficient model for your needs. |
See Resources document for more information on energy-efficient lighting and appliances.