Houses leak most of their heat through the windows. Seals and weather-stripping on cheap and badly installed windows allow heat-robbing drafts, and regular glass is not a very good insulator. But the initial cost of quality energy-efficient windows can give one pause. Is it really worth the extra money to get windows that can reduce your house's energy consumption by as much as twenty-five percent?

The US Federal Government certainly wants to encourage you to install energy-saving windows by offering you as a home owner a tax credit to cover 10 per cent of the windows' price up to a limit of $500 per year. So you can see that this is effectively a discount on the purchase price making energy-saving windows and doors much more affordable - and don't forget to factor in the reduced energy bills you'll enjoy too. This particular tax credit is called the Existing Home Tax Credit for Fenestration and covers windows installed from 31st December 2005 to 1st January 2008.

There's an organization that has come up with a rating system to work out the energy efficiency of windows. It is the National Fenestration Rating Council or NFRC. Its' rating systems are the U-Factor which determines how good your windows are at retaining the heat in your home (the lower this rating, the better) and the Solar Heat Gain Coefficient which measures how the windows help to keep the house cool (again, the lower the rating the better). If you want to qualify for the Federal Government tax credit you need to install products that have been rated by this organization. You'll need to submit a copy of your receipt as proof of purchase along with the product's NFRC sticker.

Here are some pointers to help you determine which energy-efficient doors and windows would be worth purchasing.

First, most insulating windows are made with two, or sometimes three, panes of glass. The manufacturer often fills the space between the panes with an insulating inert gas such as argon or krypton.

Next, window manufacturers can achieve lower U-Ratings by glazing the glass to various degrees with a special film or a reflective metallic coating. Some glazing even reduces transmission of damaging ultra-violet rays. In addition to insulating your house, this kind of glazing can protect your furniture and drapes from deteriorating due to UV exposure. If installing a West window, try to get glass with the lowest Solar Heat Gain Coefficient possible --- rooms with West exposure are notorious for overheating in the afternoon, and this type of window will help.

Another very important consideration is the window seal. It's imperative that any window you are contemplating buying has a tight seal. You can check this by noting the resistence felt when opening and closing the window in the showroom. The higher the resistence, the better the seal.

So, although replacing those old, drafty windows may look to be an expensive proposition in the short-term, remember to factor in the government tax break of ten per cent and ten to twenty five per cent savings on energy costs, and they might not look quite so expensive after all.