How to Interpret and Utilize Window Performance Labels

By Randy Williams

20 Clubs

Ever wonder what all those numbers listed on a window label mean? In this article, guest contributor Randy Williams explains how to make use of performance data, particularly thermal performance data, when picking out windows for your project.

Back when I was starting out as a general contractor, I relied on the salesperson at my lumberyard to determine the window performance level needed. After all, a window is a window, right? Well, as it turns out, no.

There are a lot of window selections that should be made based on the location of the project and specific goals of the home. For instance, if you live in a cold climate, you may want to choose a window with more solar heat gain and a lower U-Factor. If you live in a hot climate, less solar gain may be needed.

All this information, and more, will be listed on the certification label that’s placed on the window before it leaves the factory. That label is unique to the window configuration and includes performance information specific to its type (casement, double-hung, picture, etc.), materials, glass coatings, and other features.

I’ll explain how to put that data to good use but first, a little background.

Where does the data on the label come from?

Andersen® windows and patio doors are National Fenestration Rating Council (NFRC) certified. This independent, non-profit industry organization establishes objective window, door, and skylight energy performance ratings to help homeowners, builders, architects, and others making purchasing decisions.

Windows and doors that are NFRC certified are tested by an independent, third-party testing lab. The information gathered during testing includes the following:

  • U-Factor
  • Solar Heat Gain Coefficient (SHGC)
  • Visible transmittance
  • Air infiltration


U-Factor values indicate the rate or speed of energy (heat) loss through the window. Considering this measure helps you understand the insulation value of a window.

You might be familiar with R-Value, which measures heat resistance. With R-Value, the higher the number, the better the insulation properties. With U-Factor values, on the other hand, lower numbers are better.

Depending on location, code may specify maximum window U-Factor values to match the local climate zone. For example, code requirements in a cold climate might range from 0.30 and in a hot climate might range from 0.50.

Solar Heat Gain Coefficient (SHGC)

SHGC measures how much solar heat gain is allowed through a window. This metric has a listed value between 0 and 1 with lower numbers allowing less solar heat gain through.

Much like U-Factor, energy codes may specify what is allowed based on climate location. For example, 0.25 may be required in hot climates, 0.40 in more moderate climates, and there’s not likely to be a requirement for SHGC in cold climates (zones 6, 7, and 8).

Visible transmittance

Visible transmittance measures how much visible light enters through a window. It’s listed as a range between 0 and 1. One is the maximum amount of visible light and 0 would be a window that you’d have a hard time seeing through.

Usually, more panes of glass and different glass coatings that help improve window performance will also reduce the amount of daylight moving through the window, which results in a lower visible transmittance number.

Air infiltration

Air infiltration measures air leaking through the window assembly itself, not air leaking around the window due to any air sealing deficiencies that might occur during installation. Air that leaks through the different parts and pieces of a window will affect comfort and the operating cost of a home.

The number range is from 0.1 to 0.3, with 0.3 being the maximum air leakage allowed.

What else is listed on the label?

Other information that will be listed on a window label includes: the manufacturer’s name, window model, materials used, number of panes, glass coatings, and any gas fill used in between the panes.

If a window meets the requirements of any energy certification programs, such as ENERGY STAR®, that may also be listed. Visit the ENERGY STAR website for information about certification requirements in your climate zone.

Choosing a window based on how it will perform in your climate will yield better results for your project, and performance metrics will help you make the right window choice.