You probably think of an air conditioner as something that puts cool air in your home, but what it really does is remove heat and moisture from the air in your home. So an understanding of how heat gets in your home is the key to choosing the most cost-effective ways to cut summer utility bills while staying cool and comfortable.

According to analysis by the Florida Solar Energy Center, the main sources of heat gain in homes in summer, in order of greatest to smallest, are:

• Windows (solar heat gain through glass)
• Duct system (air leaks and absorbed heat)
• Roof and attic
• Internally generated heat (appliances, electronics, cooking, bathing, etc.)
• Air infiltration (outside air leaking into the home)
• Walls

Surprised? Many people assume just the opposite. Although the amount of heat gain from each source varies among houses and lifestyles, the top five tend to offer the greatest opportunity to save money and stay cool in our hot, humid climate.

1. Shade windows. Sun control strategies can provide the greatest bang for the buck. An exterior shading strategy should be used for any glass that receives direct sunshine or heat radiating from pavement. Interior window treatments like blinds help, but are not nearly as effective as exterior or glass solar control.

Solar screens, or shade screens, are an inexpensive, do-it-yourself strategy that can block up to 70% of solar heat while preserving the view. The screening fabric is easy to install in aluminum screen frames with a spline. The energy savings can surpass the cost in one or two summers.

Solar films (heat control film applied to the interior side of glass) are available with a range of properties to fit your needs. Look for a solar heat gain coefficient (SHGC) under 0.4 (meaning it admits 40% and blocks 60% of radiant heat) and a visible light transmittance (VT) of 0.5 or greater. The lower the SHGC, the better; the higher the VT, the better. Do it yourself kits are available, but it does take a bit of finesse to avoid air bubbles.

If you plan to install new windows, be sure to choose units that are Energy Star certified for your location. Energy Star windows for the southern region will have an SHGC of 0.25 or lower!

Landscaping is great way to shade both glass and walls as well as add value to your home. Awnings are another good option with aesthetic benefits, but more costly.

2. Test and improve ductwork. If your home is typical, your ductwork might be losing 30% or more of the cooling you pay for!That’s because ducts in most homes in the south are located in the hottest place on earth (the attic) and tend to be quite leaky. Leaky ducts mean you are both air conditioning the outdoors and causing your home to draw in hot, humid outdoor air to make up for the leaked air.

It’s a great investment to have your ducts leak-tested by a trained professional with specialized equipment, then have all leaks sealed with mastic (not duct tape).It’s also helpful to add duct insulation, when feasible, and improve its layout to correct tight bends and crimps.

3. Block attic heat. If you have a vented attic, it can become much hotter than the outdoor temperature. First, find and seal air leaks in your ceilings, around chimneys and any other bypasses. That can also reduce dust in your home. If space permits, increase attic floor insulation to R-38.

If your air conditioner and ducts are in the attic, a radiant barrier system can reduce the heat-up of your ductwork and the attic by blocking radiant heat from the hot roof. This can be a do-it-yourself project by stapling reinforced foil radiant barrier to rafters, shiny side down.

Powered roof vents are not recommended. They not only use energy, but can also create a suction in the attic that pulls air from the living space into the attic. That can result in higher energy bills because the attic is being cooled with your conditioned air!

4. Produce less heat indoors. In general, each three kwh of energy saved in the home can reduce the need for cooling by an additional kwh. So you save energy and money two ways.

Leaving computers, TVs, lights and even ceiling fans on add heat needlessly. Ceiling fans can save energy by keeping you cooler at higher thermostat settings, but they waste energy if you leave them on in unoccupied rooms. Turning everything off when not needed is free. If that’s a difficult habit to enforce, install and use timers or motion sensors.

When replacing appliances, look for the EnergyStar label. Investing in high efficiency pays off. Refrigerators and freezers are especially important since they run (and give off heat) continuously. Replace incandescent light bulbs with LED’s or compact fluorescent lamps (CFLs). They have a higher price tag, but use about 1/4 the electricity, produce 1/4 the heat and last 10-20 times longer -- so you save money in the long run and stay cooler.

Think about ways to reduce your heat-making activities. Cook with a microwave oven and outdoor grill. Wait for full loads to run the dishwasher. Use less hot water for laundry, showers, etc.

5. Cool more efficiently. Last but not least, get your air conditioning system professionally cleaned and serviced to keep it running as efficiently as possible. Change the air filter as recommended on its label, because a dirty filter that restricts air flow causes a big loss of efficiency.

When it’s time to replace your air conditioner, invest in a Seasonal Energy Efficiency Ratio (SEER) of 14-16 or an Energy Star qualified model. Make sure it is right-sized. Bigger is not better. An oversized A/C cools the space too quickly, resulting in short cycles that do not dehumidify adequately. An oversized unit will also cost more to buy, more to operate and will not last as long.