Low E Windows

Low-emissivity glass, also known as low-E glass, uses a microscopically thin and virtually invisible metal or metallic oxide layer incorporated in the glazing surface to control heat transfer through insulated windows, reducing energy loss by 30% to 50%. Since the heating and cooling energy lost through windows accounts for 4% of all U.S. energy consumption every year, the Department of Energy is beginning to push more efficient windows as a standard, with low-E windows at the forefront. Meanwhile, as more low-E windows are put into service, unexpected problems have arisen. Inspectors, who conduct energy audits, as well as energy-conscious consumers, can benefit from knowing more about these energy-efficient windows and the advantages and problems associated with them.
 

The super-thin coating utilized in low-E glass helps to reduce the transfer of heat through the window in order to keep heat from entering the home in hot climates, and to prevent heat from escaping from the home to the exterior in colder climates. The low-E coating is typically used on multi-pane windows with argon gas-fills. Argon is an inert gas which has higher resistance to heat flow than air, and is non-toxic, colorless and odorless. In hotter climates with east- or west-facing windows, the low-E coating is applied to the outer layer of the glazing to help keep the heat out. In colder, heating-dominated climates, it is applied to the inner layer to keep warmth in.

Installing low-E windows can provide significant energy efficiency and savings, especially in hotter climates where windows with a low solar heat-gain coefficient (SHGC) are most effective. The SHGC rating for a window is the quantification of its properties in relation to its ability to transmit heat from solar radiation. A high SHGC means more solar heat is allowed to pass through the window and a low SHGC means less solar heat will pass through. While a window with a higher SHGC can actually be beneficial in some climates where cooling needs are minimal, because it allows for more solar heat gain to warm the interior during winter months, windows with a low SHGC rating is best for hot climates where blocking the sun’s heat is much more important.