Tips for your Home
New window technologies have aided to increase energy benefits and comfort in the home, while providing practical options and alternatives for consumers. This selection guide will help homeowners, architects and builders, in particular, 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 right window for a specific home invariably requires trade-offs between different energy performance features, and must factor in other non-energy issues. An understanding of some basic energy concepts is therefore 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)
Non-solar heat flow through a window is the result of a 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 non-solar 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.
As a general rule for all window products, the greater the temperature difference from inside to out, the greater the rate of heat flow there will be. A U-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-emitting (low-e) coatings and gas fills. As an example, 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. It can also be applied to a thin plastic film inserted between panes. The coating limits heat flow between panes by reflecting heat back into the home during cold weather and back to the outdoors when the weather is warm. In turn, this has the effect of increasing the insulating value of the window. Most window manufacturers, as such, now offer windows and skylights with low-e coatings.
The spaces between window panes 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 performance is as much a function of geometry as of composition. For example, some well-designed metal spacers insulate almost as well as foam. Due to their orientation and their greater projected surface areas, domed and other shaped tilted and horizontal skylights have significantly higher U-factors than do vertical windows of similar materials and opening sizes.