You've probably been complaining lately about the freezing rain, piles of snow, and cold, whipping wind. It's been hard to keep your home or business warm this winter, and the heating bills have been shockingly high with the record oil and natural gas prices.

One of the best solutions to heating woes is to replace your old windows with some of the many super-efficient windows on the market.

"In 1990 alone, the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings cost the United States $20 billion (one-fourth of all the energy used for space heating and cooling)," said Gregg D. Ander in the Whole Building Design Guide.

But windows have gotten a lot more efficient since then.

The advanced windows manufactured with low-emissivity coatings (more about this later) typically cost about 10-15 percent more than regular windows, but they reduce energy loss by as much as 30-50 percent.

But what if you live in a Southern state and aren't concerned about heating your home?

Today's windows can be modified to insulate in cold climates, while letting in heat energy and light from the sun. In warm climates, these windows can reflect heat energy yet allow the sun's natural light to enter. (This daylighting saves on electricity.)

How do you know if you need new windows?

It's a good idea to hire a contractor to do an energy audit in your home. If the audit shows serious leaks around your windows, it's probably time to invest in new ones.

Another sign that your existing windows are failing is if you find a lot of condensation on them. Condensation forms because your windows are too cold (not insulated well enough). This moisture can cause mold to grow, warp the window sash and frame, and ruin paint.

Also, technologically advanced windows can slow the fading of photographs and furniture from direct sunlight because they reflect ultraviolet rays.

Energy-efficient windows make great financial sense in new buildings, as it could mean that smaller (and less expensive) heating and cooling systems are needed.

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If your home isn't new, you'll save about $125 to $425 if you replace single-pane windows and $25 to $110 if you replace double-pane, clear glass windows. The savings are greatest if you live in a climate that requires significant heating or air conditioning.

It typically takes two to ten years to recoup the extra money spent on efficient windows in energy savings. (Heating and cooling bill reductions can be up to 15 percent.)

How do you buy windows?

The key standards-setting organization for efficient windows is ENERGY STAR, administered by the U.S. Department of Energy and Environmental Protection Agency.

ENERGY STAR lists manufacturers that build qualified windows based on testing by the National Fenestration Rating Council. ENERGY STAR rates windows according to their U-Factor and Solar Heat Gain Coefficient (SHGC).

The U-Factor measures the amount of heat that transfers through a window. A low U-Factor means the window insulates well.

The SHGC measures the amount of solar heat admitted by a window. In the SHGC ratio, 0 means no solar heat is transmitted through the window and 1 means all of the sun's heat energy is transmitted through the window.

The average U-Factor ranges from 1.3 (for aluminum frame single-glazed windows) to about 0.2 (for multi-paned windows with low-emissivity, or low-E, coatings and insulated frames).

In order to be an ENERGY STAR product, a window installed in the Northern climate zone must have a low U-Factor (0.35 or less) and any SHGC. This is because windows are needed during winter that retain furnace heat in a building, while letting in the sun's heat.

In the Southern climate zone, ENERGY STAR windows must have a high U-Factor (0.65 or less) and low SHGC (0.40 or less), so they prevent heat from entering the structure but don't retain heat indoors.

(Windows installed in Central states must have U-Factor and SHGC numbers between these extremes.)

The most efficient windows have insulating gas filler (like argon or krypton -- careful Superman!) amid the multiple panes. They also have low-E coatings and robust frames.

Low-E coatings are made of transparent metallic oxides that reflect up to 90 percent of long-wave heat energy, while letting shorter wave, visible light pass through. They work equally well in warm and cold climates. In the Northern zone, the coatings reflect heat energy back into the house.

Wood and vinyl frames are far more energy efficient than aluminum ones.

Next Steps

Of course, you'll want to find a reliable and experienced contractor to install your windows. The manufacturer's warranty on the windows might be voided if they aren't installed correctly.

Interview several contractors about what types of energy-efficient windows are right for your home. You should ask for customer references, as well as information on their contractor's license and insurance. And get a written contract before any work commences.

Unfortunately, the 10 percent federal tax credit for installing ENERGY STAR-qualified windows expired at the end of 2007. Congressional leadership hopes to pass legislation in 2008 to reinstate the credit. In the meantime, some states and local utilities offer rebates on these windows.

But do you really need to replace your windows?

The National Trust for Historic Preservation strongly recommends against replacing windows in old structures. [Check out its magazine, Preservation]

National Trust-affiliated preservationists argue most energy loss is through under-insulated attics and walls, not windows.

Donovan Rypkema, a preservation economist with PlaceEconomics, believes replacing rather than repairing building components is unsustainable.

He cites "embodied energy" as the reason.

"Embodied energy is the total expenditure of energy involved in the creation of the building and its constituent materials," Rypkema said. "When we throw away an historic building, we simultaneously throw away the embodied energy incorporated into that building."

Economists can now equate how much embodied energy is in an existing building: 5 to 15 gallons of gasoline per square foot.

By repairing windows, you keep them out of landfills and avoid new products like vinyl, which don't biodegrade.

Plus, there are many ways to make your existing windows more efficient (and possibly as good as new).

The easiest fixes are to weatherstrip and caulk window sills and sashes.

Use drapes, shades, blinds and shutters to block heat transfer on winter nights and summer days. Awnings can be beneficial in warm climates.

Also, storm windows can reduce heating and cooling costs. They don't provide insulation, but they do block air flow around primary windows (which lessens air leaks).

Plant evergreen trees on the north and west sides of your home (to block winter storm winds) and deciduous trees on the south and west sides (to block the summer sun).

And remember that your attic, dryer vents and exterior doors may be bigger air leak culprits than your windows. An energy audit will inform you.

[See the U.S. Department of Energy's guide]

But whether you replace or repair your windows, it's important to know what a key role they play in an energy-efficient building.