Energy efficiency is one of the most important factors to consider when buying windows, but the different terminology and volume of information can quickly become overwhelming. To help make comparison shopping easier, we’ve compiled an overview of the main performance markers that determine a window’s energy efficiency rating.


In Canada and the United States, ENERGY STAR is the certification system used to identify energy-efficient products such as windows and doors. The ENERGY STAR symbol indicates that a product has met or exceeded certain standards after being tested by an independent laboratory.

Having a single set of standards makes it easier for consumers to compare products. However, those standards occasionally change to keep up with evolving technology or environmental policies, so you should always ensure you’re referring to the latest figures.

It’s also important to note that not all ENERGY STAR products offer the same level of energy efficiency. To understand the differences, take a closer look at the individual performance markers.

U-Factor / U-Value

U-factor, sometimes called U-value, is a measurement of a product’s resistance to heat loss. Windows with a lower U-factor provide better insulation, meaning they don’t let as much heat escape as windows with a higher U-factor. Choosing windows with a low U-factor is particularly beneficial during the colder months, since you won’t have to compensate for heat loss by running your heating system as high or as often.

Solar Heat Gain Coefficient (SHGC)

Solar heat gain coefficient (SHGC) refers to the amount of solar radiation transmitted through a window and into a home or building. SHGC will appear as a number between 0 and 1, with a lower number indicating that the window transmits less solar heat.

The type of glass plays a significant role in determining SHGC. Low-E glass typically reduces solar heat gain with its protective coating, which can help control air conditioning use during the summer. However, windows with a higher SHGC may be desirable depending on the climate, the room’s orientation and the level of surrounding shade. Talk to your contractor or a window dealer about finding the perfect solution for your space.

Visible Transmittance (VT)

Expressed as a percentage, visible transmittance refers to the amount of visible light allowed to pass through a window. A lower number means less light will enter the home or building. This is typically the result with Low-E glass windows, which use a special coating to reduce solar heat gain. But the highest quality Low-E windows can achieve a comfortable balance—letting in lots of natural light while minimizing unwanted heat gain.

Air Leakage

Air leakage is a measurement of the amount of air that passes through cracks in a window. A higher rate of air leakage indicates the window is more susceptible to heat loss and gain, which reduces energy efficiency and interior comfort.

Air leakage can also result from improper installation, especially if it occurs around the frame.


R-value is often mentioned in relation to U-value since both are reflections of the same process. Whereas U-value refers to the rate of heat loss through a window, R-value refers to the window’s ability to retain heat. An energy-efficient window should have a low U-value and high R-value.

Energy Rating (ER)

This figure captures the overall energy-efficiency score of a window. It encompasses U-factor, air leakage and SHGC, giving consumers a more convenient way to compare products. The standards for energy ratings vary according to climate zone. To earn the ENERGY STAR label in Canada, a window must have a minimum energy rating of 25 in zone 1, 29 in zone 2 and 34 in zone 3.