Knowing how to read an NFRC label makes window shopping simple for everyone. The NFRC is the National Fenestration Rating Council, a non-profit which specializes in the energy performance of windows, doors, and skylights. The NFRC’s Energy Performance Label has been designed to provide consumers with accurate energy performance ratings for all window and door products. The NFRC’s testing is only conducted by certified labs, and their results are validated by independent agencies.

If you know how to read it, an NFRC label can tell you a lot about the way windows will perform. They can tell you whether they will keep heat contained in the winter, and whether they will fill your home with natural light. An NFRC label provides you with all the information you need to compare windows and purchase the ones best suited to your home’s particular energy needs.

On every NFRC window label, you will find the manufacturer’s name, the window’s name, and the window’s style. Below that information, you should see four labeled rectangular boxes, most of which will contain numbers. These numbers indicate a window’s energy performance ratings in four important categories. Here’s everything you need to know about these categories and how to evaluate windows’ NFRC ratings.

What’s in the Label?

 An NFRC label displays up to four numerical ratings—one for each category of energy performance. These categories include U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, and Air Leakage. U-Factor, SHGC, and VT are all required to be listed on the NFRC label. Manufacturers may choose whether or not to include Air Leakage rating on their window labels, but this quality must be tested regardless. Unlike Air Leakage rating, which is determined through an actual test, U-Factor, SHGC, and VT are all computer simulated. Understanding what these categories measure and the kind of numbers to look for in each of them makes window shopping much easier.

U-Factor measures how well a window resists heat flow. In other words, it gauges how good a window is at keeping heat from escaping through it. U-Factor ratings range from 0.20 to 1.20. When window shopping, look for windows with lower U-Factors because the lower a window’s U-Factor, the better it will be at keeping heat in your home.

Solar Heat Gain Coefficient quantifies a window’s protection against heat from sunlight. This quality becomes especially important in the summer months, when excess heat transfer can lead to a sharp increase in home cooling costs. SHGCs range from 0 to 1. The closer the rating to 0, the less you’ll have to shell out on cooling costs.

Visible Transmittance is a measure of the amount of natural sunlight a window will allow into your home. A window with a high VT rating could save you on artificial lighting costs. High amounts of natural sunlight is also great for your mood and enhancing your home’s beauty. VT is also measured from 0 to 1. However, VT is the one category of energy performance for which you want to look for higher number ratings. A window with a higher VT rating will fill your home with more light.

Air Leakage gauges how much air will be able to “leak” through a window—through cracks in its frame, sash, or other components. Air leakage ratings range from 0.1 to 0.3. As with U-Factor and SHGC, you want to seek out windows with lower ratings when window shopping. Windows with lower air leakage ratings will be better at keeping out drafts.

This rating is especially important to pay attention to because a product with lower air leakage numbers will perform better than a product which allows more air leakage, even if the U-Factor of each product is the same. A window can’t regulate heat transfer effectively if too much air is able to pass through it.

Using Labels to Your Advantage

 Understanding the information on an NFRC label makes window shopping straightforward. Settling upon the right windows for your household is just a matter of comparing energy performance ratings. While browsing windows at different stores, you should take photos of the NFRC labels on windows you like. After you’ve explored your options, go back through the photos and compare the ratings.

Consider your household’s specific energy needs. Which categories are priorities for you as a homeowner? Do you need to cut down on the costs of heating and cooling, or is increasing the amount of natural light coming into your home currently more important? Ask yourself questions like these and use and your knowledge of NFRC ratings to evaluate your options.

Knowing how to read and assess a NFRC label is the first step in making an informed and rewarding window purchase.