With homeowners wanting builders to reduce their energy costs, one simple way is with low-e glass.

Windows are no longer comprised of simple panes of glass. They have evolved into complex, engineered systems. Innovations such as low-E (low-Emissivity) glass coatings provide greater energy efficiency and increased occupant comfort while reducing the potential for condensation problems.

Low-E coatings can significantly reduce windows' heat gain in the summer, and heat loss in the winter. They have been introduced not only to decrease these heat losses and gains, but to also help control thermal radiation both to and from the window surface, another major source of occupant discomfort.

How They Work

Over the years, window manufacturers have made significant improvements to their products' performance and energy efficiency. For example, double-pane glass was one of the first window innovations to provide an insulating air layer to reduce heat loss. Subsequently, window manufacturers incorporated sealed, double-pane glass designs into individual window frames, with an air cavity separating the glass pieces.

Further innovations included the use of special gasses to fill the air gap, which provide higher insulating values and better energy performance. However, while these windows reduce thermal conduction heat loss, they do not have an effect on heat radiated through the glass. Heat still permeates the window during the summer, while heat radiates out during the winter. When the temperature is zero F outside, the interior surface temperature of a low-E window will be around 7 degrees warmer than a clear window (52 F vs. 45 F), making for a more comfortable and energy-efficient house. To help control this radiated energy, engineers designed low-E glass coatings.

Low-E coatings consist of microscopically thin metal or metal-oxide layers that are either deposited on, or imbedded in, the glazing surfaces to reduce radiated heat. The coating limits the ability of the glass to emit heat through its surface.

Applications

Low-E coatings are placed on the inside surfaces of the cavity between the windowpanes, with optimal positioning dependent on the climate in which the window is used. Many variations exist for particular applications. For example, in northern climates, where heating costs are high and little or no air conditioning is required, the coating is placed on the interior glass surface. This allows solar radiation to pass through the exterior pane, contributing to heat gain during the winter, but interior heat is reflected back towards the living area. Where cooling costs are the dominant factor, the low-E surface is placed on the exterior pane to reflect solar radiation away from the window.

In some cases, the use of low-E glass can allow a reduction in the size of a home's cooling system, or allow for more glass area without increasing cooling loads. In other words, homes may be designed with larger windows without increasing heating and cooling costs or sacrificing occupant comfort.

Low-E coatings, which are very common today, may add about $15 to the cost of the window. However, they reduce the amount of energy lost through the window by approximately 30 percent.

The NAHB Research Center, on behalf of the national Partnership for Advancing Technology in Housing (PATH) program, is currently working with the U.S. Department of Housing and Urban Development (HUD), ATOFINA chemical company, and the U.S. Department of Energy (DOE) to identify homes that can use low-E coated storm windows over existing single-pane windows. The project's focus is a comparative analysis of the energy performance and cost-effectiveness of the retrofitted low-E storm windows with conventional storm windows.

For more information on low-E window coatings and the PATH low-E storm window project, visit the ToolBase Services program at www.toolbase.org.

Bob Fuller is a research engineer at the NAHB Research Center.