Understanding Energy Efficient Window Terminology

Understanding Energy Efficient Window Terminology When you start looking for new vinyl replacement windows for your home, you might think it will be a pretty simple task. After all, windows don’t seem like terribly complicated items. But if you’re specifically looking for energy efficient windows, some of the vocabulary associated with them can be a little overwhelming. We want to help take some of the guesswork out of buying your new energy efficient windows! Let us explain some of the terms that are very commonly used when talking about energy efficient windows.

Low-E/Low-Emissivity Glass

As the sun shines in through your window, some of the heat from the sun will naturally come in along with it. While getting a little extra heat without having to turn up the thermostat can be nice during the winter, it can also heat your home up on hot, sunny summer days. Low-E or low-emissivity glass windows have a very thin, barely visible metallic coating on them that works to reflect heat back to its source. During the summer, this would be toward the sun and during the winter, this would be the inside of your home. This way, the heat in your home stays in during the winter and your home stays cooler during the summer.

When heat from the sun can transfer into your home, the amount of heat that transfers through the window is called solar heat gain. If you’re looking for the most energy efficient windows, you’ll want ones with a low solar heat gain coefficient.


A window’s U-factor refers to the amount of heat that is conducted through a window. Unlike solar heat gain, a U-factor does not specifically deal with heat from the sun. The U-factor is the rate that heat is transferred from the inside of a home to the outside, so this would also apply to heat from your furnace. The lower a window’s U-factor, the more energy efficient it is.


Since it’s very important that energy efficient windows don’t conduct heat very well, you’ll want to look for windows with a high R-value. A window’s R-value refers to how well it conducts heat and you’ll want windows with a high R-value.

Insulated Glass

When you have double or triple pane windows, there will be a small gap between each pane of glass. With insulated glass windows, this gap is filled with argon, an inert gas that insulates better than regular air, making the window more energy efficient.

Warm Edge Spacer

Have you ever touched your windows on a cold winter day and noticed that they felt colder along the edges ? Or maybe you’ve looked at them and noticed condensation on the glass along the edge. In some types of windows, panes of glass are separated by aluminum, which is a very conductive material. Warm edge spacers, on the other hand, are made of less conductive materials and prevent heat from being transferred along the outer edges of windows.