Low-E Storm Windows

Low Emissivity Windows, more commonly known as Low-E Windows, have a clear coating of metal or metallic oxide on the surface of the glass to reduce the heat loss or gain. The layer on the window is almost completely transparent and reduces the emissivity of the glass, which increases the resistance to heat transfer in both single- and multi-pane windows. Another benefit of Low-E windows is that they block out potentially damaging UV light which can fade fabrics. Generally speaking, Low-E coatings keep heat inside the house in cold climates and heat outside the house in warm climates.

Two types of Low-E coatings exist, soft and hard. Soft coats are more effective at reflecting heat, are better insulators of heat, and are normally found between panes of glass for protection. Hard coats are applied directly to the surface of the glass, and, due to their harder nature, are much more resistant to damage such as scratching. Because of this increased resilience, hard coats are typically used for single-pane storm windows, or other single-pane applications.

There are three common levels of solar gain for Low-E windows—low, medium, and high. High solar gain windows allow almost as much solar energy to pass through them as clear glass, but they do a better job of keeping the heat inside the house. These high solar gain windows are normally used in geographic areas with mild summers and harsh, colder than normal winters. Low solar gain windows, on the other hand, let as little solar energy as possible into the house. This type of window is normally used in regions that have hotter than normal summers and mild winters, such as southern California. The medium solar gain windows allow only a moderate amount of heat to enter during the summer, while still allowing enough solar energy to pass through them during the winter to help heat the home.

Storm windows are typically mounted on the inside or outside of existing single-pane windows to improve their thermal efficiencies. It is estimated that 43 percent of all residential windows are single-pane glass. Storm windows reduce conduction of heat across a window by creating a "dead-air" space between the existing window and the storm window. In addition, storm windows help reduce air infiltration, which is common in older windows. Storm windows come in many different materials and many different styles which can range from rigid glass mounted outside the home to plastic sheets mounted inside a home in an attempt to lower the effect of wind chill on the existing window.

The benefits of traditional clear glass storm windows combined with the technology of Low-E windows creates the potential to achieve nearly equivalent window thermal performance as new double-pane windows at a much lower cost. For example, new windows may cost between $100 and $500 plus installation; a Low-E storm window is in the $60 to $110 price range and is more easily installed.

	Low-E storm windows help stop heat from escaping a house during the winter. By reducing the emissivity of a window and partially blocking out the non-visible spectrum of solar energy that the window is exposed to, the Low-E coating is able to reduce heat flow through the glass up to 30 to 50 percent. The storm window creates a "dead zone" of air between it and the existing window, thereby helping to hinder heat transfer through the window to the outdoors. Storm windows also create a more airtight seal between the window and the house, which also helps prevent the loss of heat through infiltration. These benefits all combine to create a Low-E storm window that is capable of reducing the heating cost of a home by 10 to 20 percent.

Initial Cost  

Low-E storm windows tend to cost approximately 10 to15 percent more than standard storm windows.

Operational Cost  

Low-E storm windows reduce the heating load placed on an HVAC system reducing energy bills.

There are no code requirements for Low-E storm windows.

Existing Homes in Chicago, Chicago, IL, April 2007
Determined the energy savings of installing clear and Low-E storm windows in a cold climate.

Manufacturers’ installation instructions are similar to those of other storm windows.

Typical window manufacturer warranties apply.

Low-E storm windows generally cost between 10 and 15 percent more than traditional clear glass storm windows. However, Low-E storm windows can lower heating and cooling cost by up to 20 percent over single pane windows because they reduce the heat flow through the glass. Also, they are a cheaper alternative to replacing existing single-pane windows with more efficient double-pane windows. Low-E storm windows can also help protect hard wood floors, fabrics, and carpet from fading because they reduce the amount of damaging UV rays that enter a home.