Keeping cool indoors when it's hot outdoors is a problem... especially in Arizona summers! 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 a new 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 just air conditioning alone. This information provides some common sense suggestions and low-cost retrofit options to help you "keep your cool"- and save on electricity.
Staying Cool
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, 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.
Roofs
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%.
Walls
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.
Windows
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
Insulation
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
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.
Stay tuned for Part 2 of this blog series on How to Naturally Cool Your Home... coming soon!
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