Understanding Window Specifications

What is the U-Factor?

The U-Factor measures how much heat is transferred through a complete window. The lower the U-Factor, the better the window's insulating properties. Typically, double-pane, white vinyl windows will have a U-Factor between 0.25 and 1.24. Triple-pane windows can have a U-Factor as low as 0.15, which is why some windows qualify for Energy Star rebates. The optimal U-Factor for your location depends on your climate. In the Northeast, there are two zones: a very cold zone (Northern) and North-Central. New Jersey falls in the latter zone, where a U-Factor of 0.25 to 0.32 is typically ideal.

What is the SHGC? (Solar Heat Gain Coefficient)

The Solar Heat Gain Coefficient (SHGC) represents how much heat from the sun passes through a window. People in colder regions may prefer a higher SHGC, around 0.35 to 0.60, to allow more solar heat gain. Those in mixed climate zones generally aim for an SHGC of 0.30 or lower to minimize additional heat. The higher the SHGC, the more heat is added from sunlight; the lower the SHGC, the less heat is added. This is where “Low-E” glass coatings are beneficial.

Air Leakage

Air leakage measures how much air escapes through a window. Regardless of the zone, it is desirable to have this number as low as possible. The window will have an air leakage rating, but proper installation is also crucial for minimizing air leakage.

What Qualifies as an Energy Star Rated Window?

To qualify as Energy Star rated, a window must meet the following criteria:

• U-Factor: 0.30 or less
• SHGC: 0.40 or less
• Air Leakage: 0.3 cubic feet or less

Windows that meet these criteria are certified through rigorous testing. Many Energy Star windows are triple-pane, use a heavier gas than argon (which is typical in double-pane windows), and may have insulated vinyl cavities. However, the most important factor in window performance is proper installation.