Ventilation is the least expensive and most energy-efficient way to cool buildings. Ventilation works best when combined with methods to avoid heat buildup in your home. In some cases, natural ventilation will suffice for cooling, although it usually needs to be supplemented with spot ventilation, ceiling fans and window fans. For large homes, homeowners might want to investigate whole house fans.
Ventilation is ineffective in hot, humid climates where temperature swings between day and night are small. In these climates, attic ventilation can help to reduce your use of air conditioning. 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.
Principles of Heating and Cooling
Understanding how heat is transferred from the outdoors into your home and from your home to your body is important for understanding the challenge of keeping your house cool. Understanding the processes that help keep your body cool is important in understanding cooling strategies for your home.
Principles of Heat Transfer
Heat is transferred to and from objects—such as you and your home — via three processes: conduction, radiation, and convection.
Conduction is heat traveling through a solid material. On hot days, heat is conducted into your home through the roof, walls, and windows. Heat-reflecting roofs, insulation, and energy efficient windows will help to reduce that heat conduction.
Radiation is heat traveling in the form of visible and non-visible light. Sunlight is an obvious source of heat for homes. In addition, low-wavelength, non-visible infrared radiation can carry heat directly from warm objects to cooler objects. Infrared radiation is why you can feel the heat of a hot burner element on a stovetop, even from across the room. Older windows will allow infrared radiation coming from warm objects outside to radiate into your home; shades can help to block this radiation. Newer windows have low-e coatings that block infrared radiation. Infrared radiation will also carry the heat of your walls and ceiling directly to your body.
Convection is another means for the heat from your walls and ceiling to reach you. Hot air naturally rises, carrying heat away from your walls and causing it to circulate throughout your home. As the hot air circulates past your skin (and you breathe it in), it warms you.
Cooling Your Body
Your body can cool down through three processes: convection, radiation, and perspiration. Ventilation enhances all these processes. You can also cool your body via conduction—some car seats now feature cooling elements, for instance—but this is not generally practical for use in your home.
Convection occurs when heat is carried away from your body via moving air. If the surrounding air is cooler than your skin, the air will absorb your heat and rise. As the warmed air rises around you, cooler air moves in to take its place and absorb more of your warmth. The faster this convecting air moves, the cooler you feel.
Radiation occurs when heat radiates across the space between you and the objects in your home. If objects are warmer than you are, heat will travel toward you. Removing heat through ventilation reduces the temperature of the ceiling, walls, and furnishings. The cooler your surroundings, the more you will radiate heat to the objects, rather than the other way around.
Perspiration can be uncomfortable, and many people would prefer to stay cool without it. However, during hot weather and physical exercise, perspiration is the body's powerful cooling mechanism. As moisture leaves your skin pores, it carries a lot of heat with it, cooling your body. If a breeze (ventilation) passes over your skin, that moisture will evaporate more quickly, and you'll be even cooler.
Avoiding Heat Buildup
To avoid heat buildup in your home, plan ahead by landscaping your lot to shade your house. If you replace your roof, use a light-colored material to help it reflect heat. Insulate your house to at least the recommended levels to help keep out the heat, and consider using a radiant barrier.
On hot days, whenever outdoor temperatures are higher than the temperature inside your house, close tightly all the windows and exterior doors. Also install window shades or other window treatments and close the shades. Shades will help block out not only direct sunlight, but also radiated heat from the outdoors, and insulated shades will reduce the conduction of heat into your home through your windows.
Cooking can be a major source of heat within a home. On hot days, avoid using the oven; cook on the stovetop, or better yet, use only a microwave oven. For stovetop or oven cooking, use the spot ventilation of your oven hood to help remove the heat from the house (this will suck some hot outside air into your home, so don't overdo it). Outdoor grilling is a great way to avoid cooking indoors, and of course, going out to eat or ordering take-out work as well.
Bathing, washing laundry, and other activities can also pump heat into your home. When you shower or take a bath, use the spot ventilation of a bathroom fan to remove the heat and humidity from your home. Your laundry room might also benefit from spot ventilation. If you use an electric dryer, be sure it's vented to the outside (for safety, gas dryers should ALWAYS be vented to the outside). If you live in an older home with a sump that your laundry drains to, drain the sump after running any loads in hot water (or better yet, avoid using hot water for your laundry).
Finally, avoid any activities that generate a lot of heat, such as running a computer, burning open flames, running a dishwasher, and using hot devices such as curling irons or hair dryers. Even stereos and televisions will add some heat to your home.
Natural ventilation relies on the wind and the "chimney effect" to keep a home cool. Natural ventilation works best in climates with cool nights and regular breezes.
The wind will naturally ventilate your home by entering or leaving windows, depending on their orientation to the wind. When wind blows against your home, air is forced into your windows on the side facing into the wind, while a natural vacuum effect tends to draw air out of windows on the leeward (downwind) side. In coastal climates, many seaside buildings are designed with large ocean-facing windows to take advantage of cooling sea breezes. For drier climates, natural ventilation involves avoiding heat buildup during the day and ventilating at night.
The chimney effect relies on convection and occurs when cool air enters a home on the first floor or basement, absorbs heat in the room, rises, and exits through upstairs windows. This creates a partial vacuum, which pulls more air in through lower-level windows. The effect works best in open-air designs with cathedral ceilings and windows located near the top of the house, in clerestories, or in operable skylights.
Passive solar homes are often designed to take advantage of convection to distribute heat evenly through the home. These homes are often amenable to natural ventilation by ventilating them near the top.
Natural ventilation can be enhanced or diminished through landscaping. Depending on the house design and wind direction, a windbreakólike a fence, hedge, or row of trees that blocks the windócan force air either into or away from nearby windows.
Ceiling Fans and Other Circulating Fans
Circulating fans include ceiling fans, table fans, floor fans, and fans mounted to poles or walls. These fans create a wind chill effect that will make you more comfortable in your home, even if it's also cooled by natural ventilation or air conditioning. Ceiling fans are considered the most effective of these types of fans, since they effectively circulate the air in a room to create a draft throughout the room.
If you use air conditioning, a ceiling fan will allow you to raise the thermostat setting about 4°F with no reduction in comfort. In temperate climates, or during moderately hot weather, ceiling fans may allow you to avoid using your air conditioner altogether. Install a fan in each room that needs to be cooled during hot weather.
Ceiling fans are only appropriate in rooms with ceilings at least eight feet high. Fans work best when the blades are 7–9 feet above the floor and 10–12 inches below the ceiling. Fans should be installed so their blades are no closer than 8 inches from the ceiling and 18 inches from the walls.
Larger ceiling fans can move more air than smaller fans. A 36- or 44-inch diameter fan will cool rooms up to 225 square feet, while fans that are 52 inches or more should be used in larger rooms. Multiple fans work best in rooms longer than 18 feet. Small- and medium-sized fans will provide efficient cooling in a 4- to 6-foot diameter area, while larger fans are effective up to 10 feet.
A larger blade will also provide comparable cooling at a lower velocity than a smaller blade. This may be important in areas where loose papers or other objects will be disturbed by a strong breeze. The fan should also be fitted to the aesthetics of the roomóa large fan may appear overpowering in a small room.
A more expensive fan that operates quietly and smoothly will probably offer more trouble-free service than cheaper units. Check the noise ratings, and, if possible, listen to your fan in operation before you buy it.
When buying window fans, look for the ENERGY STAR® label. Fans that earn the label move air 20% more efficiently, on average, than standard models.
Window fans use little energy and can provide effective cooling in many climates. Window fans are best used in windows facing away from the prevailing wind and exhausting hot air from your home. To cool as much of your home as possible, tightly close windows near the fan and open windows in rooms far from the fan, preferably on the windward side of your home. Windows near cooler, shaded outdoor areas provide the best intake air.
In multi-level houses, the fan should be located on the upper level, if possible, and the open windows should be located on a lower level. If that's not practical, you may want to independently ventilate each level of your home with separate fans.
Depending on the layout of your home, you might want to use several window fans working together to pull the air through your home. For instance, fans in several upstairs bedrooms will assure that each bedroom is cooled, and will work together to pull air in through the rest of your home.
Whole House Fans
Whole house cooling using a whole house fan can substitute for an air conditioner most of the year in most climates. Whole house fans combined with ceiling fans and other circulating fans provide acceptable summer comfort for many families, even in hot weather. In addition to whole house fans, the ducts of your central heating and cooling system can be modified to provide whole house cooling.
How Whole House Fans Work
The whole house fan pulls air in from open windows and exhausts it through the attic and roof. It provides good attic ventilation in addition to whole house cooling. Whole house fans should provide houses with 30–60 air changes per hour (varies with climate, floor plan, etc.ócheck with a professional to determine what is appropriate for your home). The air-change rate you will choose depends on your climate and how much you will depend on the whole house fan for cooling.
Sizing a Whole House Fan
Whole house fans are sized in cubic feet per minute (cfm) of cooling power. To determine the size you'll need, first calculate the volume of your house in cubic feet. To do that, multiply the square footage of the floor area you want to cool by the height from floor to ceiling. Take that volume and multiply by 30 to 60 air changes per hour (depending on the power you need). Then, divide by 60 minutes to get the cubic feet per minute of capacity your house requires.
30 Changes Per Hour
(Square feet______ x room height______) x 30 / 60 = cfm required _________.
60 Changes Per Hour
(Square feet______ x room height______) x 60/ 60 = cfm required _________.
Installing and Using a Whole House Fan
Installing a whole house fan is tricky and should be done by a professional. An experienced professional should perform your attic measurements and install your dedicated circuit wiring and, if needed, your new attic vents.
Attic ventilation will usually need to be increased to exhaust the fan's air outdoors. You'll need 2–4 times the normal area of attic vents, or about one square foot of net free area for every 750 cubic feet per minute of fan capacity. The net free area of a vent takes into account the resistance offered by its louvers and insect screens. More vent area is better for optimal whole house fan performance.
If your fan doesn't come with a tight-sealing winter cover, you should either buy one or build one. If you switch between air conditioning and cooling with a whole house fan as the summer weather changes, build a tightly sealed, hinged door for the fan opening that is easy to open and close when switching cooling methods.
Be cautious when operating these large exhaust fans. Open windows throughout the house to prevent a powerful and concentrated suction in one location. If enough ventilation isn't provided, the fans can cause a backdraft in your furnace, water heater or gas-fired dryer, pulling combustion products such as carbon monoxide into your living space.
Drawbacks of Whole House Fans
Whole house fans can be noisy, especially if improperly installed. In general, a large-capacity fan running at low speed makes less noise than a small fan operating at high speed. All whole house fans should be installed with rubber or felt gaskets to dampen noise. You can set a multi-speed fan to a lower speed when noise is a problem.
Using Your Duct System as a Whole House Fan
You may be able to use the heating and air conditioning ducts in your home as a means of whole house ventilation. This would involve installing an intake duct to pull air into an attic-mounted system that directs the air into your heating and cooling ducts. A damper would control exhaust air from your home into the attic. Check with a local professional to find out if this option is right for you.