Understanding consumer ratings for replacement windows

Improved energy efficiency is one of the top reasons homeowners and businesses replace their windows. The British Fenestration Rating Council (BFRC) runs a scheme called Window Energy Ratings. The BFRC rates each window using a simple A-E guide whereby 'A' is the most energy efficient and 'E' the least efficient.

A BFRC rated window has a label. The label provides:

1. The overall A-E rating.
2. The energy rating - This appears in kilowatt hours per square metre per year. The lower the figure, the better the energy efficiency.
3. The window U-value (thermal transmittance) - This is the measurement of the amount of heat transfer through a material such as glass. The value appears in watts per square meter per year and should be as low as possible.
4. Air penetration or effective air leakage (the L value) - This relates to the heat loss caused by air penetration. Measurement is the same as the U value. The lower the figure, the better the heat conservation.
5. The solar factor - This measures how much of the sun's heat comes into the home through the window. The lower the figure, the less solar heat the window lets in.

While the BFRC label is an effective way to gauge energy efficiency, manufacturers are not required to use it. However, we recommend that you shop for windows that include this label to guarantee the level of efficiency you need.

The one thing the label will not detail is how durable the window is. This is a key reason to look for an experienced installer when you're ready to purchase windows. An installer can provide a clearer estimate for how long a window will last.

Choosing the right U-value

Of the five BFRC ratings, the U-value is the most important. In order to decide how low the U-value needs to be for your energy-efficient windows, you should consider your particular home situation.

For instance, if you plan to stay in your home for only a few years and you live in an area where the winters don't get exceptionally cold, you don't need to spend extra to get the lowest U-value. Instead, you can purchase double-glazed windows with a U-value of 1.8 and still get lower energy costs.

If you intend to live in your home for at least 10 years and you face expensive heating bills, however, it's worth spending more on windows with a Uvalue of 1.1. Triple-glazed windows with a very low Uvalue are priced at a premium, but you'll see the difference in your heating costs almost immediately.

Metal oxide coated windows for improved efficiency

Coating windows with metal oxide is one way manufacturers can greatly improve the U-value. The metal oxide prevents heat loss and is transparent, and windows can be coated on one side of the glass or both. It's an inexpensive way to improve the insulating qualities of your windows while providing protection from harmful UV rays.

A step up from standard glazed windows is low-emissivity (or "Low-E") windows. These windows fill the space between inner and outer panes with inert gasses such as argon, krypton, and carbon dioxide. Low-E windows provide great insulation and clear visibility year-round, but are also the most expensive.