September 9, 2019

A homeowner or designer who wants to build green has many choices in today’s market. Choosing the right energy-efficient windows for your home can mean sorting through seemingly endless window models to find the right combination of style, color, price and energy performance.

Energy performance labels are invaluable for helping you compare different types, models and energy efficiency ratings of windows to find the ones that will best meet your needs. An energy performance label can tell you at a glance how well the windows perform and in what specific ways, so you can easily make energy-efficiency comparisons across brands and between different window styles and types.

What is the difference between the types of labels found on energy-efficient windows? What do all the numbers mean? If you’re not familiar with energy performance labels, you may find yourself scratching your head as you try to compare windows. This guide will help you make sense of it all so you can make an intelligent buying decisions.

Two Types of Energy Performance Labels for Windows: What’s the Difference?

If you have been shopping for windows, you may have noticed two distinct types of energy performance labels. One is the sky blue and white ENERGY STAR® label. This label is issued by the U.S. government’s ENERGY STAR® program and signifies that the product has met or exceeded certain standards for energy efficiency.


You also may have come across the energy performance label issued by the National Fenestration Rating Council (NFRC). The NFRC is an independent, nonprofit organization that sponsors an energy efficiency and certification program for windows and doors. While the ENERGY STAR® label simply indicates that a window is energy efficient, the NFRC label tells you why and how.


What Do All the Numbers Mean?

If you take a closer look at the NFRC label, you will see a certification seal along with information about the window — such as model number, material and window style. Underneath these, you will find four energy performance rating numbers. While at first it might seem simpler to have a single, all-encompassing number rating for each window; there’s good reason to break window energy performance into several categories.

Let’s take a look at each rating, why it’s important, and what you should be looking for to get the most energy-efficient windows.

  • U-factor — Technically speaking, the U-factor (or U-value) of a window is a measure of the rate at which non-solar heat moves through it. Practically speaking, it is similar to the R-value of insulation in that it tells you how well the window retains heat. Unlike R-value, though, the lower the number the better when it comes to energy efficiency. 
  • Solar heat gain coefficient — While U-value is typically used to measure heat loss, the solar heat gain coefficient measures the amount of radiant solar heat the window will let in. The lower the number, the more efficient the window will be at shading the home. For most buildings, look for a low solar heat gain coefficient to help reduce summertime cooling bills. However, if you plan to use the window for passive solar heating applications, you may prefer a window with a higher rating.
  • Visible Transmittance (VT) — While windows are transparent, they will always block a small percentage of light. This number indicates the amount of visible light the window will transmit. If you’re looking to maximize the natural lighting capabilities of your home, look for a higher VT rating; a lower rating can be helpful if you want to reduce glare.
  •  Air leakage — Just as it sounds, this number indicates how well the window prevents drafts. The lower the rating, the tighter and more energy efficient the window will be.

As you can see, the efficiency of a window ultimately depends, in part, on the intended use of the window and the conditions you want to create in your home. Now that you know what to look for in a window energy efficiency label, you should be able to make an informed decision when it comes to choosing the best windows for your home.

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