NEW YORK (This Old House) - Nearly 30 years have passed since the first oil crisis gave Americans an indelible lesson in energy deprivation.

Yet many homeowners still don't realize how much energy seeps out of their houses every day despite the steps they might have taken. According to experts, many homes -- including new ones -- act more like sieves than like sealed buildings.

"What we've learned about basic energy efficiency isn't readily available to homeowners, builders and contractors," says Dave Brook, an extension agent specializing in energy for Oregon State University.

The reason is clear enough: Because much of that knowledge was developed for low-income housing as part of the federal Weatherization Assistance Program, it hasn't yet reached the mainstream housing industry. Nevertheless, it includes a number of findings that affect all homes.

For example, because hot air rises, most heat lost in a building goes right through the roof. What causes that heat loss? Leaks in attic floors are the culprit, lowering the R-value of attic insulation and draining 30 to 50 percent of a home's heating energy.

And while leaks around windows and doors let out far less energy than you probably thought, gaps in forced-air ducts can cut home heating and cooling efficiency 40 percent.

Fortunately, making your home more energy efficient isn't rocket science. A couple of weekends sealing the attic and furnace ducting using materials that cost less than $50 on average will slash up to 30 percent off your energy bill.

To save energy immediately, begin by sealing the gaps that lead from your living areas to the attic. Some of these gaps accommodate wiring and pipes, while others result from poor craftsmanship and the normal settling of the building.

But all of them serve as passageways for heated air to escape.

That's because houses act like big chimneys. Warm air rises to top of the building, increasing air pressure near the ceiling. The difference between that pressure and the lower pressure outside on a cold day drives the warm air through any crack, crevice or gap it can find. The high pressure at the top also creates low pressure near the bottom of the house, which pulls cold air in through openings around the foundation or slab.

Energy experts call this the stack effect. The larger the spread between inside and outside temperatures, the greater the pressure differences and the stronger its pull. However, if you have mold or condensation problems in your home during winter, don't do any sealing until you've tackled the moisture situation.

What insulation can't do. An insulated attic isn't necessarily a sealed attic. Insulating materials are designed to slow down heat loss through solid materials rather than to stop airflow. Insulation works with weatherizing to create a thermal boundary between the inside and outside of your home. Unfortunately, most homeowners pay attention solely to the insulation part of the equation.

"Half the money people pay for insulation is often lost due to leaks," says Michael Lamb, a home-energy specialist for the Energy Efficiency and Renewable Energy Clearinghouse, a McLean, Virginia-based information network sponsored by the U.S. Department of Energy (DOE). "If homeowners sealed the attic floor before insulating, they would save a lot more energy."

Filling the gaps. Spotting the holes and gaps you need to seal is easy in an uninsulated attic. Lay planks across joists and stay on them so you don't step through the ceiling. Then check for gaps around anything that comes through the floor. Examples include the tops of light fixtures, pipes, wiring, the chimney and heating and cooling ducts. Also check for gaps around the top plates of interior partitions.

If your attic is insulated, you'll need to roll back batts to get at the gaps. Wear pants and long sleeves, gloves, eye protection, and a dust mask. Gray or black smudges in the insulation signal air leaks. If your attic is insulated with loose-fill insulation, which can't be peeled back, you might want to call a professional weatherization contractor to locate the leaks. Then seal as many of them as possible.

• Instead of insulation, use latex caulk to fill gaps up to about 3/8 inch wide. For holes up to about 1 inch wide, use expanding urethane foam (it comes in a can). Be careful -- the foam is hard to get off of clothes and hands. A new latex sealant from DAP ($3.50 per 12-oz. can) cleans up with soap and water.
• For larger holes, create a plug from a piece of drywall. Cut it to fill the hole, push it into place and then seal the edges with urethane foam. Or, use fiberglass insulation stuffed into plastic bags.
• Seal gaps around chimneys and stove flues with a sheet-metal collar and caulk.
• Insulate and apply weather stripping around the edges of the hatch or door that leads to the attic.
• On cathedral ceilings, apply caulk in spots where drywall meets exposed beams.

Once you've air-sealed the attic, be sure insulation meets DOE standard. The standard for most of the U.S. is R-38. Call the Energy Efficiency and Renewable Energy clearinghouse (800-363-3732) for the recommendation for your area.

If you need to add more insulation on top of the old, use unfaced batts or loose fill. Owens Corning and Johns Manville make a batt encased in plastic for easy handling.

You can also reduce heat loss by insulating over light fixtures in the rooms below your attic — provided you replace the standard recessed versions with fixtures rated "IC," for insulated ceiling. To prevent airflow, buy IC fixtures that also have an air- and moisture-tight housing. They're available from most major lighting companies. Halo also offers an airtight retrofit collar for its IC fixtures.

Leaky ductwork in a forced-air heating and cooling system creates several problems. A supply duct that leaks into an attic or crawl space pours cooled or heated air — and the money you paid for it — into the void. Leaky return ducts pull hot or cold attic or crawl space air into the system. Indeed, a duct that runs through the attic can pull in 140°F air in summer, making the cooling system work that much harder. It also pulls in dust, moisture, mold, and other contaminates.

Start by reconnecting any ducts that have fallen apart. Then hunt for holes in supply ducts by feeling for the air as it leaks out and seeing if a tissue clings to return ducts as air is sucked in. Use duct mastic (available in cans or caulking tubes) to seal small gaps. For larger ones, reinforce the mastic with fiberglass mesh tape. You can also use UL-181 aluminum tape — essentially professional duct tape. Just don't use the cloth variety labeled duct tape, which really isn't for ducts.

Return and supply ducts should also be pressure-balanced for forced-air systems to work efficiently. Leaks upset that balance, and can drive heated or cooled air out of the house or pull outside air in. Unfortunately, sealing only some of the leaks can do the same thing. Have the system inspected by a pro when you're done to be sure you didn't miss any.

After the ducts are sealed, be sure any that run through unconditioned crawl spaces, basements, or attics also are insulated. Insulating long runs of ductwork is best left to a contractor. But you can handle short runs yourself with foil-faced fiberglass duct insulation. Cover all sides and secure the insulation with a cable tie.

Note: Have a pro perform a backdraft test before and after you work on the ducts.

If your energy bill still seems too high, get a professional energy audit. Be sure it includes a blower-door test. Without it, the contractor can only guess at your energy problems. Essentially a large fan, a blower door pulls most of the air out of the house to pinpoint outside air leaking through holes and cracks. The technician locates the gaps, measures their size, and provides options for sealing them.

A blower-door test costs about $100, though some contractors will do it for free if you ultimately choose them to do the sealing work. But it's hard to find a company that performs this type of test. Contact your local utility, state energy office, weatherization contractors, and home inspectors for leads on finding someone in your area.

Finally, don't seal the foundation completely. A good weatherization contractor will seal it just enough to stop serious leaks without cutting off the air needed for combustion appliances, like furnaces, water heaters, fireplaces, ranges, and dryers.

Once you've sealed the yawning chasms throughout your home, go after the details.

• Replace single-pane windows with low-e units rated R-3 (also listed as U.40) or higher, says Michael Lamb, a specialist for the Energy Efficiency and Renewable Energy Clearinghouse, an information network sponsored by the U.S. Department of Energy. "You'll cut 20 percent or more off your heating or cooling bills," Lamb explains.
• While leaks around windows bleed relatively little energy (except in exposed, windy areas), seal any obvious gaps. You'll find the largest ones between the window frame and the rough opening in the framing of your home. Use expanding foam (sold in cans) for best results.
• Invest in a set-back thermostat. You can slice your energy bill up to 15 percent simply by setting the temperature back 10°F for an eight-hour period.
• Install wall insulation. When properly installed, cellulose and lightweight foam products reduce heat loss and air leaks.