Windows are an important component of your home, but it can be confusing to understand all the terminology that comes with them. For any homeowner, it is crucial to have a grasp of window terminology. From glazing to mullions, this handy glossary will help demystify window terms and make sure you are well-informed.


Glazing refers to the glass in your windows. Double-glazed windows have two pieces of glass with a gap between them that creates better insulation. Triple-glazed windows have three pieces of glass with two gaps that provide even better insulation. Other types of glazing include low-e coatings, which reflect heat back into your home, and tinted glass, which reduces glare and heat.


Mullions are the strips of material that divide a window into smaller panes. The purpose of mullions is primarily aesthetic, but they can also improve the structural integrity of the window. You may also hear the term "muntins" used, which refers to the thin strips of material that hold the glass in place within each pane.


The sash is the part of the window that moves up and down or side to side. Single-hung windows have one moveable sash, while double-hung windows have two. Sliding windows have one or two moveable sashes that slide horizontally. Casement windows have one moveable sash that swings outward from one side.

Air leakage

Air leakage refers to the amount of air that can pass through your windows when they are closed. A higher air leakage rating means that your windows are less efficient at keeping out drafts. Look for windows with a low air leakage rating to maximize your energy savings and improve your home's insulation.


The U-factor, also known as the overall heat transfer coefficient, is a crucial metric used to quantify the insulating performance of your windows. By measuring the rate of heat transfer through the window assembly, it provides valuable insights into the efficiency of your windows in reducing heat loss and enhancing comfort within your home. A lower U-factor signifies that your windows excel at minimizing heat transfer, resulting in increased energy efficiency and a more comfortable living space.

Understanding window terminology is part of taking care of your home. Familiarizing yourself with terms like glazing and mullions will empower you to make well-informed choices regarding the selection, upkeep, and repair of your windows. Enhancing your knowledge in this area ensures that you have the necessary tools to make the best decisions for your windows. By paying attention to air leakage and U-factor ratings, you can maximize your home's energy efficiency and comfort. So the next time you need to make decisions about your windows, you'll be well-equipped and informed.