How to Measure the Performance of Energy Efficient Windows

We recently reviewed the various types of environmentally friendly windows available, and now we’re here to delve further into that topic by looking at how to measure the performance of these windows.  There are five basic principles that influence a window’s energy efficiency, as well as two leading certifications that rate products based on these criteria.


U-factor (also known as U-value) is the overall rate of heat transfer/loss of a window. When rated, the makeup of the entire window—glazing, frame, and spacers—is taken into account.  Ratings typically fall between 0.15 and 1.20, with a lower number indicating a product that is better at reducing heat transfer and insulating.  Double-pane windows can often have U-factors of 0.30, and some high-performance triple-pane windows can have rates as low as 0.15.  

Solar Heat Gain Coefficient (SHGC) 

The SHGC (also commonly known as solar gain) is the increase in temperature due to solar radiation transmittance through a window, both that which is directly transmitted and which is absorbed and then released internally.  To put it more simply, it’s how much heat from the sun is blocked.  SHGC measurements range from 0 to 1, with 0 being the lowest possible solar gain.  In hot climates, a low solar gain is very important, while in cold areas, people may want a higher gain to lessen their heating bills.  

Visible Transmittance (VT) 

The VT refers to the amount of light within the visible part of the spectrum that can pass through a window.  VT rating includes the impact of the window frame, as well as the glazing.  The measurement values range from 0 to 1. With a higher VT number, more light is transmitted, which is preferred to maximize natural daylight.   

Air Leakage (AL) 

Air leakage looks at how much air passes into a building through the window.  The measurements range from 0.1 to 0.3.  Have a drafty window?  That’s a higher AL.  

Condensation Resistance (CR)

A window’s CR rating expresses how well it can resist the form of interior condensation, measured with a number between 1 and 100.  The higher the value, the better a window is at keeping out condensation. 

We know this is a lot of information to take in, which is why it’s so helpful that two organizations exist to monitor and consolidate all of this data.

                                      ENERGY STAR

                                      ENERGY STAR

A collaboration between the Department of Energy (DOE) and the Environmental Protection Agency (EPA) established in 1992 under the authority of the Clean Air Act, ENERGY STAR is a designation for products that meet a certain energy performance criteria.  Four geographical zones—Northern, North/Central, South/Central, and Southern—have required properties in the categories of U-factor, solar heat gain coefficient, visible transmittance, and air leakage.  Look for the ENERGY STAR seal on windows to see if they qualify. 

National Fenestration Rating Council (NFRC)

An example of an NFRC label 

An example of an NFRC label 

 The NFRC is a non-profit organization “that administers the only uniform, independent rating and labeling system for the energy performance of windows, doors, skylights, and attachment products.”  The Council is comprised of researchers, government agencies, manufacturers, suppliers, and code officials.  Formed in response to the energy crisis in the 1970s, NFRC develops and administers energy-related rating and certification programs.  For windows, these ratings include U-factor, Solar Heat Gain Coefficient, Visible Transmittance, Air Leakage, and Condensation.  Look for the NFRC label on products. 

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