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How to Read a Window Label

With the current emphasis on energy efficiency, you want to make sure that any new windows you install in your home perform the way you expect. The National Fenestration Rating Council (NFRC) realize this, which is why manufacturers are required to include an NFRC label with energy performance ratings on all new windows, skylights, and doors. By understanding the numbers and codes on this label, you can make an economical decision when replacing the windows in your home.

Here is an example window label from Marvin:

example window label

Product Description

At the top of every NFRC label, you’ll see:

  • The manufacturer’s name
  • Window frame material, such as vinyl, fiberglass, aluminum, wood, or wood-clad
  • Glazing type, including single, double, or triple pane glass
  • Argon or krypton gas in-fill, if present
  • Low-E coating, if present
  • Window operating style, such as sliding, double-hung, casement, or awning


The first energy performance rating on the window label is the U-Factor, which ranges from 0.02 to 1.20. This is a measure of how well the window prevents heat from passing through it. The lower the U-Factor, the better its insulating value is.

Solar Heat Gain Coefficient

To the right of the U-Factor on a window label is the Solar Heat Gain Coefficient (SHGC), which ranges from 0 to 1. This measures how well the window hinders radiant heat gain caused by exposure to direct sunlight. A lower SHGC rating means less solar heat is transferred to your home for more efficient window performance.

Visible Transmittance

Under “Additional Performance Ratings,” you’ll see Visible Transmittance (VT), a rating that ranges from 0 to 1. This tells you how much visible light passes through the window. The closer the value is to 0, the less light that passes through the window. It’s important to balance a window’s SHGC and VT ratings to block solar heat gain without darkening your view substantially.

Air Leakage

To the right of the VT rating on a window label, you’ll see a number for Air Leakage, an optional rating that most manufacturers choose to include. This tells you how much heat gain and loss occurs through cracks in the window assembly. The closer the number is to 0, the better.

Condensation Resistance

This is another optional rating that many NFRC window labels include. The Condensation Resistance (CR) ranges from 0 to 100 and measures how well the window resists the formation of condensation on the interior surface. This is a desirable quality because condensation can drip down the pane and cause water damage to the windowsill and behind the wall. The closer the CR rating is to 0, the better.


Many NFRC window labels feature additional certifications at the bottom. Here are some that you might see:

  • Window and Door Manufacturers Association (WDMA): The WDMA is a trade association that defines standards of excellence in the window and door industry. A window with the WDMA’s approval is manufactured to meet this organization’s high standards.
  • Hallmark Certified: The WDMA’s Hallmark Certification is a mark of excellence. Architects and contractors accept this certification industry-wide.
  • Energy Star: The Energy Star logo on a window means it goes above and beyond industry standards to provide an exceptionally efficient window for the region in which it’s being sold.

Browse Window Labels at the Quality Window & Door Showroom

The best way to compare windows and their labels is to visit one of our local showrooms in Maryland and Northern Virginia. We have a huge selection of windows on display from some of today’s best manufacturers, including Weather Shield and Infinity from Marvin. Stop by today, or contact Quality Window & Door to request a free window installation estimate.

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