Windows and Home Energy Performance (Part 2)

If you have good reasons to replace the windows in your home, then it makes sense to take advantage of the opportunity to replace them with windows that are energy efficient.  The question is, how do you know whether a window is energy efficient?

A good place to start is with the u-factor.  The u-factor is a measure of how well a window does at stopping the transfer of heat by conduction.

To understand this, think of insulation.  Your home probably has insulation in the attic, walls and other places.  For a given type, the thicker the insulation, the better it stops conduction.  This ability is measured in something called R-value.  For a given insulation type, thicker insulation has a higher R-value and thinner insulation has a lower R-value.

The u factor is the inverse of the R-value.  In other words u = 1 / R.  So windows with a lower u-factor do a better job of stopping conduction, windows with a higher u-factor don’t do as well.  U-factors range from 0 to 1.

As an example, consider a newly constructed home in the St. Louis area.  The attic insulation should have an R-value of at least R-49.  The insulation in the walls cavities should have an R-value of at least R-13.  The windows should have a u-factor of .35 or less.

Windows don’t have R-values, but if they did, a window with a u-factor of .35 would have an R-value of R-2.9.  This is because u = 1 / r.  Since R-2.9 is small compared to R-13 or R-49, it is obvious that windows don’t do nearly as good a job at stopping the transfer of heat by conduction as insulation does.

We put up with the fact that windows do a poor job of stopping heat transfer because windows have other benefits.  Windows allow natural light to enter our homes, they provide us with a view to outdoors, and they stop most infrared radiation from entering our homes.

The u-factor of a window is determined by several things.  How well does the glass conduct heat?  How many panes of glass does the window have?  What type of gas is between the panes?  How well do the sash, frame and spacers conduct heat?

The u-factor does a good job of summing up how well a window stops the transfer of heat by conduction.  When shopping for replacement windows in the St. Louis area, look for windows with a u-factor of .35 or less.  Just remember, the lower number is better.