New window technologies have increased energy benefits and comfort, and have provided more practical options for consumers. This selection guide will help homeowners, architects, and builders take advantage of the expanding window market. The guide contains three sections: an explanation of energy-related window characteristics, a discussion of window energy performance ratings, and a convenient checklist for window selection. Selecting the best windows for a specific home invariably requires tradeoffs between different energy performance features, and with other non-energy issues. An understanding of some basic energy concepts is essential to choosing appropriate windows and skylights.

Three major types of energy flow occur through windows:

• Non-solar heat losses and gains in the form of conduction, convection, and radiation.
• Solar heat gains in the form of radiation.
• Airflow, both intentional (ventilation) and unintentional (infiltration). This airflow can be exponential if windows are poorly installed.

Insulating Value

The non-solar heat flow through a window is a result of the temperature difference between the indoors and outdoors. Windows lose heat to the outside during the heating season and gain heat from the outside during the cooling season, adding to the energy needs in a home. The effects of nonsolar heat flow are generally greater on heating needs than on cooling needs because indoor-outdoor temperature differences are greater during the heating season than during the cooling season in most regions of the United States. For any window product, the greater the temperature difference from inside to out, the greater the rate of heat flow. AU-factor is a measure of the rate of non-solar heat flow through a window or skylight. (An R-value is a measure of the resistance of a window or skylight to heat flow and is the reciprocal of a U-factor.) Lower U-factors (or higher R values), thus indicate reduced heat flow. U-factors allow consumers to compare the insulating properties of different windows and skylights. The insulating value of a single pane window is due mainly to the thin films of still air on the interior and moving air on the exterior glazing surfaces. The glazing itself doesn’t offer much resistance to heat flow.Additional panes markedly reduce the U-factor by creating still air spaces, which increase insulating value.

In addition to conventional double-pane windows, many manufacturers offer windows that incorporate relatively new technologies aimed at decreasing U-factors. These technologies include low-emittance (low-E) coatings and gas fills.

A low-E coating is a microscopically thin, virtually invisible, metal or metallic oxide coating deposited on a glazing surface. The coating may be applied to one or more of the glazing surfaces facing an air space in a multiple-pane window, or to a thin plastic film inserted between panes. The coating limits radiative heat flow between panes by reflecting heat back into the home during cold weather and back to the outdoors during warm weather. This effect increases the insulating value of the window. Most window manufacturers now offer windows and skylights with low-E coatings. The spaces between windowpanes can be filled with gases that insulate better than air. Argon, krypton, sulfur hexafluoride, and carbon dioxide are among the gases used for this purpose. Gas fills add only a few dollars to the prices of most windows and skylights. They are most effective when used in conjunction with low-E coatings. For these reasons, some manufacturers have made gas fills standard in their low-E windows and skylights.

The insulating value of an entire window can be very different from that of the glazing alone. The whole-window U-factor includes the effects of the glazing, the frame, and, if present, the insulating glass spacer. (The spacer is the component in a window that separates glazing panes. It often reduces the insulating value at the glazing edges.) Since a single-pane window with a metal frame has about the same overall U-factor as a single glass pane alone, frame and glazing edge effects were not of great concern before multiple-pane, low-E, and gas-filled windows and skylights were widely used. With the recent expansion of thermally improved glazing options offered by manufacturers, frame and spacer properties now can have a more pronounced influence on the U-factors of windows and skylights. As a result, frame and spacer options have also multiplied as manufacturers offer improved designs.

Window frames can be made of aluminum, steel, wood, vinyl, fiberglass, or composites of these materials. Wood, fiberglass, and vinyl frames are better insulators than metal. Some aluminum frames are designed with internal thermal breaks, non-metal components that reduce heat flow through the frame. These thermally broken aluminum frames can resist heat flow considerably better than aluminum frames without thermal breaks. Composite frames may use two or more materials (e.g. aluminum-clad wood, vinyl-clad wood) to optimize their design and performance, and typically have insulating values intermediate between those of the materials comprising them. Frame geometry, as well as material type, also strongly influences thermal performance properties. Spacers can be made of aluminum, steel, fiberglass, foam, or combinations of these materials. Spacer thermal perfor- mance is as much a function of geometry as of composition. For example, some well-designed metal spacers insulate almost as well as foam.

Preventing Condensation

Air can hold varying amounts of water vapor or moisture. The warmer the air is, the more moisture it can hold. The amount of moisture in the air, expressed as a percentage of the maximum amount the air could hold at a given temperature, is called its relative humidity. For health and comfort, indoor air should contain some moisture. The relative humidity should generally be between 30% and 40% at normal room temperature. The relative humidity of air can be increased by adding more moisture or by reducing the temperature.

When the relative humidity reaches 100%, the air can hold no more moisture, and water begins to condense from it. The temperature at which this condensation occurs is called the dew point temperature of the air. When moist air comes in contact with a cold surface in a home, it may be cooled to its dew point temperature, resulting in condensation on the surface.

When shopping for windows, look for the National Fenestration Rating Council (NFRC) label as your guide to buying energy-efficient windows. The NFRC is a non-profit public/private collaboration that provides contractors and homeowners with standardized, unbiased methods of comparing various brands and types of windows.

To the right is an example of an NFRC label. All the parts of the label are described, but the U-factor and Solar Heat Gain Coefficient, which rate the efficiency of the entire window (glass and frame), are the most important in helping choose the best window for your purposes. All labels have U-factors; Solar Heat Gain Coefficients and Visible Light Transmittance are now being added. Air leakage ratings and annual heating and cooling factors will be added in the near future.

info courtesy of nfrc.org, www.eere.energy.gov