Have you ever lamented the fact that the rooms in your home with the most windows are almost uninhabitable in the summer? That is due to solar heat gain.

Solar heat gain refers to the process by which solar radiation is admitted into your home as heat. This radiation, generated by the sun’s rays, enters your home through your windows. It can be transmitted directly through your windows’ glazing or absorbed into their glazing and frames. Either way, this radiation gets released as heat inside your home.

Some types of windows will allow a lot of solar heat into your home, while others will only allow a small amount. Understanding how solar heat gain works will allow you to select windows that will keep your home’s heating and cooling needs in check while maximizing your household’s energy efficiency.

Solar Heat Gain Coefficient

To assist you in selecting the most energy efficient windows for your home, the National Fenestration Rating Council (NFRC) lists every certified window’s solar heat gain coefficient (SHGC) on its window label.

Expressed as a number between 0 and 1, a window’s SHGC is the fraction of solar radiation allowed to pass through it. For instance, a window with a SHGC rating of 0.35 will allow 35 percent of available solar heat to pass through it and into your home. The higher the rating, the more solar heat the window transmits.

Multiple aspects of a window’s assembly, including its glazing, frame, and spacers, are factored into its SHGC rating.  

SHGC and Energy Efficiency

The goal of an energy efficient window is to reduce the amount of times your home’s climate controllers—your heating and air conditioning—need to cycle at any given point during the year.

Solar heat gain can be a blessing or a curse for energy efficiency, depending upon factors like your area’s climate and the time of the year.

In the winter, solar heat gain can help maintain a desired temperature within your home, relieving your heater and reducing energy costs. During the summer, though, excess solar heat gain can cause your home to overheat, increasing AC usage and cooling costs.

Choosing the Right SHGC

So should you choose windows with a high SHGC for the cold winter months or a low SHGC for the hot summer ones?  

The answer is not always clear. After all, there aren’t any glass packages available that provide the best possible solar heat gain results in both seasons.

To determine which windows will be most energy efficient for your home, take climate and temperature control into consideration. Think about your household’s temperature control needs. During an ordinary calendar year, do you expend more energy and expenses on heating or cooling?

If heating is your home’s primary need, seek out windows with a higher SHGC. If cooling is more critical to your home’s energy efficiency, look for windows with a lower SHGC. In most cases, you’ll have to compromise a bit to achieve the best possible balance for your home.

By taking into consideration your home’s climate and temperature control needs, you should be able to use SHGC ratings to find windows that offer your home a happy medium for energy efficiency—adequate heating assistance in the winter without excess heating in the summer.