Windows and Home Energy Performance (Part 3)

One important part of choosing energy efficient windows is knowing how well the window does at keeping out the radiant heat of the summer sun.  This may seem confusing because, after all, don’t we want windows to let the light in?

The answer of course, is yes, but there is a difference between light and heat.  A great window can help fill your home with desirable light and at the same time minimize the amount of radiant heat passing through to inside your home.

In the St. Louis area, most homes absorb a large amount of heat radiated by the sun during summer.  In the winter, our homes lose some radiant heat to the outdoors, but in relative terms, not very much.  Keeping radiant heat out in the summer is much more important.  In fact, great windows can do a great job of keeping radiant heat outside during summer and inside during winter.

An interesting aspect of this is that some radiant heat passes through the glass while other heat is first absorbed by the glass and then re-radiated indoors.  Because of this, window manufacturers can ingeniously combine two or even three panes of glass with low-emissivity (low-e) coatings to reduce the heat gain from both the radiation and the re-radiation.

How do we compare windows based on the amount of radiant heat they keep out during summer?  We use something called the solar heat gain coefficient (SHGC).  This measure has been devised as a simple way to sum up how well a window performs in this area.

Solar heat gain coefficients fall on a scale from zero to 1 and represent the ratio of solar heat passing through the glass to solar heat falling on the glass.  The lower the number the better.  When it comes to SHGCs, to be called energy-efficient, a window in the St. Louis area today needs to be at a minimum of .40 or better (lower).

For windows on the south side of the home or in any location where their exposure to the sun creates problems with comfort or energy use, consider going as low as .20.

In choosing replacement windows for your home, other measures such as the u-factor and air infiltration rates matter, but when it comes to the solar heat gain coefficient, get as far below .40 as your can.  For more info on these other measures, read on ….