Terms to Know When Choosing Windows – U-Factor, SHGC & VLT

There is more to know about windows than casement, double-hung, Low-E, double or triple pane. While Low-E and the number of panes can give a basic idea on the performance characteristics of a window assembly, it is also good to know a couple other key terms.

U-Factor – This is equivalent of R-value for insulation. The insulating performance of a window is expressed as the inverse of the assembly’s R-value. A low U-Factor indicates a better insulating window.

SHGC – Solar Heat Gain Coefficient is a measure of how well a window blocks out heat from the sun. This is indicated by a number that ranges from 0 to 1. A lower number indicates less solar heat coming through. In a warm climate it is desirable to have a high SHGC. The opposite is true often true in cold climates where we want to maximize the amount of solar heat in order to minimize the use of a heating system.

VLT – Visible Light Transmittance, sometimes referred to as Visible Transmittance (VT) is the amount of visible light that passes through a window. This is expressed as a number from 0 to 1.  A higher number indicates that more light passes through the glazing. As can be expected, this must be weighed against the SHGC since more light through a window also tends to mean more heat.

Theses three factors come into play when designing a structure. The windows that face east and west will often have different values than those that face south while the north windows will have properties that are different still. For example, it may be desirable for north facing windows to have a very low U-Factor and a high VLT, the SHGC is not important since those windows will only see direct sunlight in the winter months, when the extra heat may be desirable. The nuances of window performance are just one of the myriad of small, yet important, bits of knowledge that an architect possesses.