Just because a window is new and has two panes of glass, doesn’t mean it’s necessarily an energy efficient replacement window. In order for a window to be ENERGY STAR® rated, it must meet certain rating criteria. The National Fenestration Rating Council (NFRC) is an independent organization that rates windows so homeowners can compare windows to one another.

Most energy efficient windows have common characteristics including:

  • Superior design and construction
  • Quality materials
  • Low-E double or triple pane glass

Of course, proper installation is key to any window’s energy efficiency.

Design & Construction

Keep in mind that different materials have different construction techniques that are considered superior. However, because vinyl windows are the most common replacement window material, we’re going to talk about that. Vinyl replacement windows should be design with an insulated frame for both the window and sashes. Ideal construction includes fusion welded seams and lifts incorporated into the window itself. Lifts that are screwed in post construction can lead to weakening and a less energy efficient replacement window over time.


Again, speaking of vinyl windows only, you want virgin vinyl, not recycled. The problem with recycled vinyl windows is that the vinyl isn’t as strong. Over time, the window will lose its shape. When it’s no longer the shape it was designed and constructed to be, air leaks begin, leading to a less energy efficient window. Foam insulation is a better option for frame fills as that helps prevent heat transference.


In order for any window to be labeled an energy efficient replacement window, it must have Low-E glass. However, this is a bit of a misnomer. The glass itself isn’t different than the window glass you have today. What is different is that each pane of glass has a coating that reduces heat loss in the winter and heat gain in the summer. For a more detailed description, click here. Of course, you also want a double or triple pane window. Less important is what is between those panes of glass. Argon and Krypton are inert gases that provide greater insulation than air, however, the cost for those is not always worth the savings.

Trust Peak with the Installation

We only sell energy efficient replacement windows and you can trust that our team will properly install them. That means you’ll reap the maximum benefit from the windows you were promised. Call us today at (703) 468-4769 for your free estimate for new windows for your home.