Trying to understand what is the best choice for improving the efficiency of glass in your home or office can be confusing. Hopefully this brief overview will help simplify what to look for.*
The measure of the rate of non-solar heat loss or gain through a window system The lower the U- Factor the better.
The inverse of the U-Factor. The higher the better.
Solar Heat Gain Coefficient (SHGC):
The fraction of solar radiation both transmitted and absorbed through a window and released inward. Lower is better.
Visible Transmittance (VT):
A fraction of the visible light spectrum transmitted through a window. Higher is better if you enjoy bright rooms.
Finally, and most important if you have an extensive amount of windows and desire natural light and energy efficiency.
Light-To-Solar Gain (LSG):
The ratio between SHGC and VT which is the best gauge of the relative efficiency of glass in transmitting daylight while blocking heat gain. The higher the number, the more light transmitted without adding excessive amounts of heat.
Hope this helps!
* Sources: (U-Value, R-Value and SHGC) Bill Lingnell, technical director, Insulating Glass Manufacturers Alliance. (VT and LSG) Department of energy.