Windows And Thermal Performance

by Editor on May 17, 2013

Replace old windows

When finding replacement windows, homeowners need to take into account the thermal rating of the window.

Windows are one of the contributing factors to the loss of energy efficiency in the home.  Air leaks from around the sides of the window, causing the temperature to fluctuate within the home.  In addition, windows allow sunlight to enter the home, which can lead to a natural increase in temperature.  In order to address these problems, windows come with thermal performance ratings.  The ratings inform homeowners about the energy efficiency of the windows, most homeowners do not know what the ratings actually mean.  The three primary measures of thermal resistance in windows are the U Factor, Solar Heat Gain Coefficient (SHGC) and the Visible Transmittance (VT).

U Factor

The U Factor of a window is the measure of heat flow through the window and into the home.  When the U Factor is low, the window does not allow much heat to transfer through the glass, and the window is considered energy efficient.  Heat can flow through a window in three different ways.  First, direct heat, or conduction, which is heat that moves directly through the glass.  Convection occurs when air comes in contact with glass that is warmer or cooler than the air around it.  Heat loss by convection can be combated with the use of specific gases, such as Argon and Krypton, that are more resistant to temperature changes.  Finally, windows allow heat flow through thermal radiation.  When the windows are either significantly warmer or cooler than the bodies in the room, the window allows heat exchange via thermal radiation.  When all of these issues are taken into account, the U Factor is calculated.  When combined with the R Factor of the other insulation in the home, windows with a low U Factor can give a home optimal energy efficiency.


The Solar Heat Gain Coefficient (SHGC) is a tricky number when it comes to evaluating a window’s thermal performance.  SHGC is the measure of solar heat that is transferred to the interior of the home through the window.  The reason that the SHGC is a tricky number is that some homes and some climates need a low SHGC and some need a much higher number.  In warm climates, a low SHGC is normal.  In places like Texas and Arizona, homeowners do not want any more heat in the home than is absolutely necessary, especially during the summer.  On the other hand, in places like Pennsylvania and Maine, a high SHGC is beneficial.  This helps to lower the heating costs of the home by utilizing the sun’s natural warmth instead of fuel.  Whatever the situation may be, homeowners can discuss their SHGC preference with their window installers so that they have the best window solution for their home needs.


Visible Transmittance is the last measure of thermal performance for windows.  Heat is not the only factor in determining a window’s thermal performance; ultraviolet and infrared light are taken into account as well.  Windows with a high VT will allow almost all light to come through the window, while a low VT will block more of the light.  The windows then act as a barrier, protecting the homeowners and their families from harmful radiation.

Homeowners should take into account the thermal performance ratings of their windows before the windows are installed.  By getting windows with the right thermal performance numbers, homeowners will have windows that provide an adequate barrier between the conditioned air inside the home and the air outside.

About the Author

Hello my name is Samanta Holland, and I am writing this article to inform others about windows.  I just went through a large decision on what windows to buy and I thought since I did, this would be the best time to write about it.  I used a company called replacement windows Lancaster PA and they were great for me and made the process much easier. It won’t be the same for every person though, you need to find a company right for you and a style. I hope this helps someone looking, thanks for reading this article.

photo by: Song_sing

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