Keeping cool indoors when it is hot outdoors is a problem. The sun beating down on our 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. This information provides some common sense suggestions and low-cost retrofit options to help you "keep your cool"- and save electricity.
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.
The most effective method to cool your home is to keep the heat from building up in the first place. The primary source of heat buildup (i.e., gain) is sunlight absorbed by your house through the roof, walls, and windows. Secondary sources are heat generating appliances in the home and air leakage. Specific methods to prevent heat gain include reflecting heat (i.e., sunlight) away from your house, blocking the heat, removing built up heat, and reducing or eliminating heat generating sources in your home.
Reflecting Heat Away
The most effective method to cool your home is to keep the heat from building up in the first place. Dull, dark colored home exteriors absorb 70% to 90% of the radiant energy from the sun that strikes the home's surfaces. Some of this absorbed energy is then transferred into your home by way of conduction, resulting in heat gain. In contrast, light colored surfaces effectively reflect most of the heat away from your home.
Installing a radiant barrier
Radiant barriers are easy to install. It does not matter which way the shiny surface faces - up or down. But you must install it on the underside of your roof - not horizontally over the ceiling, and the barrier must face an airspace.
For your own comfort while in the attic, install the radiant barrier on a cool, cloudy day. Use plywood walk boards or wooden planks over the ceiling joists for support. Caution: Do not step between the ceiling joists, or you may fall through the ceiling.
Staple the foil to the bottom or side of the rafters, draping it from rafter to rafter. Do not worry about a tight fit or small tears in the fabric; radiant transfer is not affected by air movement. The staples should be no more than 2 to 3 inches (5 to 8 centimeters) apart to prevent air circulation from loosening or detaching the radiant barrier. Use a caulking gun to apply a thin bead of construction adhesive to the rafters along the seams of the foil barrier. This will make the installation permanent.
About a third of the unwanted heat that builds up in your home comes in through the roof. This is hard to control with traditional roofing materials. For example, unlike most light colored surfaces, even white asphalt and fiberglass shingles absorb 70% of the solar radiation. One good solution is to apply a reflective coating to your existing roof. Two standard roofing coatings are available at your local hardware store or lumberyard. They have both waterproof and reflective properties and are marketed primarily for mobile homes and recreational vehicles. One coating is white latex that you can apply over many common roofing materials, such as asphalt and fiberglass shingles, tar paper, and metal.
A second coating is asphalt based and contains glass fibers and aluminum particles. You can apply it to most metal and asphalt roofs. Because it has a tacky surface, it attracts dust, which reduces its reflective somewhat.
Another way to reflect heat is to install a radiant barrier on the underside of your roof. A radiant barrier is simply a sheet of aluminum foil with a paper backing. When installed correctly, a radiant barrier can reduce heat gains through your ceiling by about 25%.
Wall color is not as important as roof color, but 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.
Roughly 40% of the unwanted heat that builds up in your home comes in through windows. Reflective window coatings are one way to reflect heat away from your home. These 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 warmer climates because they can reflect as much as 80% 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 allow some light into a room but they also let some heat in and prevent interior heat from escaping. These films are best for climates that have both hot and cold seasons. Investigate the different film options carefully to select the film that best meets your needs. Note: do not place reflective coatings on south facing windows if you want to take advantage of heat gain during the winter. The coatings are applied to the interior surface of the window. Although you can apply the films yourself, it is a good idea to have a professional install the coatings, particularly if you have several large windows. This will ensure a more durable installation and a more aesthetically pleasing look.
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 and absorb or reflect the solar heat.
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. 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.
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.
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 through openings in foundations and exterior walls. Thorough caulking and weather stripping will control most of these air leaks.
Shading your home can reduce indoor temperatures by as much as 20°f (11°c). Effective shading can be provided by trees and other vegetation and exterior or interior shades.
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 your landscaping, use plants native to your area that survive with minimal care. Trees that lose their leaves in the fall (i.e., deciduous) 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. Ask your local nursery which vine is best suited to your climate and needs.
Besides providing shade, trees and vines create a cool microclimate that dramatically reduces the temperature (by as much as (9°f [5°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. You might also consider low ground cover such as grass, small plants, and bushes. A grass-covered lawn is usually 10°f (6°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.
Planning Your Planting
Placement of vegetation is important when landscaping your home. The following are suggestions to help you gain the most from vegetation.
- Plant trees on the northeast-southeast and the northwest-southwest sides of your house. Unless you live in a climate where it is hot year round, do not plant trees directly to the south. Even the bare branches of mature deciduous trees can significantly reduce the amount of sun reaching your house in the winter.
- Plant trees and shrubs so they can direct breezes. Do not place a dense line of evergreen trees where they will block the flow of cool air around or through them.
- Set trellises away from your house to allow air to circulate and keep the vines from attaching to your house's facade and damaging its exterior. Placing vegetation too close to your house can trap heat and make the air around your house even warmer.
- Do not plant trees or large bushes where their roots can damage septic tanks, sewer lines, underground wires, or your house's foundation.
- Make sure the plants you choose can withstand local weather extremes.
Both exterior and interior shades control heat gain. Exterior shades are generally more effective than interior shades because they block sunlight before it enters windows. 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.
Exterior shading devices include awnings, lovers, shutters, rolling shutters and shades, and solar screens. Awnings are very effective because the 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% on southern windows and 77% on eastern windows. A light colored awning does double duty by also reflecting sunlight.
Opaque roller shades are effective when fully drawn but also block light and restrict air flow.
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 close to outdoor temperatures and helps remove heat from your home. But only ventilated 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. In areas with cool nights and very hot days, let the night air in to cool your house. By the time the interior heats up, and the outside air should be cooler and can be allowed indoors.
Reducing Heat-Generating Sources
Often overlooked sources of interior heat gain are lights and household appliances, such as ovens, dishwashers, and dryers. Because most of the energy that incandescent lamps use is given off as heat, use them only when necessary. Take advantage of daylight to illuminate your house, and consider switching to compact fluorescent lamps. These use about 75% less energy than incandescent lamps, and emit 90% less heat for the same amount of light.
New, energy efficient appliances generate less heat and use less energy.
Using any or all of these strategies will help keep you cool. Even if you use air conditioning, many of these strategies, may not be enough. Sometimes you need to supplement natural cooling with mechanical devices. Fans and evaporative coolers can supplement your cooling strategies and cost less to install and run than air conditioners.
Ceiling fans make you feel cooler. Their effect is equivalent to lowering the air temperature by about 4°f (2°c). Evaporative coolers use about one-fourth the energy of conventional air conditioners.
Cooling Strategies Checklist
Cooling strategies to consider:
- lighten roof and exterior wall color
- replace/coat roof with bright white or shiny material
- install a radiant barrier
- add reflective coatings to windows
- insulate attic and walls
- caulk and weather strip to seal air leaks
- add shade trees, bushes, or vines
- add exterior awnings and shades
- add interior drapes and shades
- ventilate attic
- increase natural ventilation
- isolate heat-generating appliances
- replace heat-generating appliances
- replace light bulbs with energy-efficient fluorescent's
courtesy of The Energy Efficiency and Renewable Energy Clearinghouse (EREC) You may also contact your state and local energy offices as well as your local solar energy association for regional-specific information on natural cooling.