There are many reasons you might plan to purchase new windows: your existing ones may be worn out and not close or open effectively; you may have condensation issues or the space between the panes may be cloudy. Whatever the reason, it pays to know how to find the right windows for your openings to the world.
Plan to look at various materials (wood, vinyl) and styles (double hung, casements and more) and various brands. You also need to pay attention to the label on the windows. The standardized label is required by the National Fenestration Rating Council (NFRC) so that consumers can be sure that the windows installed will perform as expected.
These standardized labels contain valuable details that can help you summarize the window’s features and also explain how your chosen windows will handle sun, heat, light, air leakage and even condensation.
The various parts of the label aren’t easily understood, so here is a translation of each of the components you may see (not all are required):
Product Information. This includes a description of the window including the manufacturer’s name; the frame material; lists whether it has single-, double- or triple pane glass; mentions any gas between the panes (argon or krypton); notes if there is a low-E coating; and identifies the operating style (sliding, double-hung, casement or awning).
U-Factor or U-Value. This number measures how well the window will keep heat during the winter (or cool air in the summer) from escaping from inside the house. The range is 0.20-1.20, with lower numbers being better. The U-Value number depends on material quality, type of materials used, design and engineering, and quality of construction; these recommendations are based on where you live.
Solar Heat Gain Coefficient. The lower the number (range is 0-1), the less heat allowed in. This is important during the summer air-conditioning season, especially on west-facing walls where you may receive direct sunlight on hot afternoons. However, in reverse, a higher number, especially on south-facing walls, can help naturally heat your home during cold months.
Additional Performance Ratings
While the three ratings above are required, you won’t see all of the following options on every window. Some of these may be important to look for in your particular situation.
- Visible Transmittance. This number shows how well the window allows daylight in, potentially allowing you to turn on fewer indoor lights during daytime hours. The range is 0-1; higher numbers indicate that the window allows in more light. Be aware that other factors, such as buying a triple-pane window, will reduce the amount of light entering your home, and that it’s important to balance the amount of daylight let in with the U-Value and Solar Heat Gain Coefficient.
- Air Leakage. This number indicates how drafty the window and frame are. The range is 0.1-0.3. If you live in a windy area, opt for windows with a low number.
- Condensation Resistance. This number shows the amount of resistance to the formation of condensation. This number doesn’t predict the amount of condensation, but it does provide a way to compare the potential between various windows. This number will range between 1 and 100, with the higher numbers showing a better resistance to condensation.
- Design Pressure (DP). This shows how much pressure a window can take before breaking. Higher ratings mean stronger windows: an important option in areas where hurricanes occur. Ranges are from DP 30-50.
Finding one or both of these certifications on the windows you are considering indicates quality and energy efficiency.
- Window and Door Manufacturers Association (WDMA), Hallmark Certified. This certification indicates that the manufacturer of the window has followed WDMA performance standards.
- Energy Star certified. This certification from the U.S. Department of Energy uses only the window's U-Value and Solar Heat Gain Coefficient for ratings, as these are considered to be the best measure of a window’s performance.
For a look at a label, view this slide show, How to Read a Window Label from Better Homes and Gardens.