Low-E windows are designed to keep the heat outside during the summer and the heat inside during the winter. This is done by a coating of an energy transfer reduction material on the window. The coating does not block natural light from passing through the glass. Up to 95 percent of light still makes it through the window. Low-E windows are branded under many different names, including:

• Sun-Coat
• SunBlocker
• SunGate

Not all Low-E windows perform the same. Some are designed to better keep heat out and some are designed to better keep heat inside.  The sticker provides information on the U-factor and and solar heat gain coefficient. The U-factor measures the flow of heat. The SHGC is a measurement of radiation created by the sun that is flowing through the window. 

The other factor of Low-E windows is the level of natural light that makes it through the glass. The visible light transmittance number is a measurement of light. Low-E windows have good visible light transmittance levels, but the thickness of the Low-E coating can reduce clarity. Find the appropriate balance for your situation.