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Understanding the NFRC Window Label

The NFRC label provides performance data for thermal transmittance, solar heat gain coefficient, visible light transmittance and air-leakage.

NFRC window label

The National Fenestration Rating Council is an independent nonprofit organization that provides independent ratings and certification of fenestration performance based on ANSI-accredited standards. This allows for comparisons to be made between products and provides consumers the ability to make a more informed decision when selecting fenestration products.

The NFRC label, issued for factory-built fenestration, or the label certificate, issued for site-built fenestration, provide performance data for thermal transmittance (U-factor), solar heat gain coefficient (SHGC), visible light transmittance (VT) and air-leakage (AL).


The U-factor, reported in units of BTU/°, is a measure of the thermal transmittance of the window assembly, or, how well insulating the window is. The lower the U-factor, the better insulating the window as less heat transfer from hot to cold will occur. It is calculated using the method described in NFRC 100-2020 Procedure for Determining Fenestration Product U-factors.

The U-factor shown on the label is an area-weighted average of the center of glass, edge of glass and frame U-factors. Because the frame and edge of glass typically have higher conductivity than the center of glass--especially aluminum framing--systems with a higher frame-to-glass ratio will likely have a higher conductivity, and higher U-factor. Reducing U-factor is achieved by leveraging high-performance glass while reducing conduction, convection and radiation heat loss through the frame. Holistic fenestration design will often produce the best performance possible.

Solar Heat Gain Coefficient (SHGC)

The SHGC is a rating from 0 to 1 that displays how much heat gain will occur through the insulating glass (IG). The lower the value, the less heat will pass through into the interior. SHGC requirements can vary depending on the region and climate where a fenestration product is installed, with hotter climates, such as Texas, requiring a lower SHGC to reduce heat transfer, while colder climates may require higher SHGC to reduce the amount of heating needed during colder months. Low-emissivity coatings and tints directly impact the SHGC values in most fenestration products. However, the manufacturer will often have to weigh different low-e coatings and tints with occupancy requirements for daylighting and visible light wanted by the end user.

Visible Transmittance (VT)

Visible transmittance is a rating from 0 to 1 that displays how much light will be seen through the glass. The higher the rating, the more light will come through the IG into the interior. This can be affected by the low-e coating or tint selected by the manufacturer. Most consumers opt for the most amount of light transmittance. However, SHGC requirements may affect the amount of light transmittance of an IG.

Air Leakage

Air leakage reflects how much draft you may experience with a fenestration product. The lower the value, the more airtight a product will be, reducing the power consumption needed to keep a space air conditioned or heated.

In short, the NFRC label helps you compare between energy-efficient products by giving you independent ratings in several energy performance categories.


Stephen Aki Technoform

Stephen Aki

Stephen Aki works for Technoform North America assisting window and door designers to develop sustainable, energy-efficient, high-performance systems. He has more than 10 years of experience in the fenestration industry and is a member of the National Fenestration Rating Council and the Fenestration and Glazing Industry Alliance. He can be reached at or 971/832-0502.