GLAZING

The Washington State Energy Code defines glazing as all areas, including the frames, in the shell of the conditioned space that let in natural light including windows, clerestories, skylights, sliding or swinging doors and glass block windows. There are various criteria for selecting window glazing products. Whatever your priority may be when selecting glazing, it is wise to consider the energy-related consequences and costs of the glazing product you are about to purchase.

DETERMINING GLAZING EFFICIENCIES

Glazing efficiency is measured by U-values (rate of heat loss) or thermal conductance of a material. The higher the conductance (U-value), the greater the flow; in this case, greater heat loss. In the State of Washington, the energy code for new electrically heated homes (Zone 1) require glazing to have a prescriptive U-value of 0.40 (Option IV) or lower and are dependent on glazing % of floor area. Residences heated by other fuels require a prescriptive U-value of 0.65 (Option IV) or lower and are also dependent on glazing % and HVAC equipment and efficiency.

RATING GLAZING COMPONENTS

The testing requirements of the National Fenestration Rating Council (NFRC) have established a single rating system that enables utilities, builders, and consumers to compare product performances. Performance certification ensures that:

• Rated products reaching the marketplace are built in the same manner

• The appropriate product ratings and labels are put on the correct product (all glazing components meeting these testing standards will have an NFRC label).

• The manufacturer maintains an in-house quality assurance program to support accurate and consistent energy performance ratings of its products.

Multiple layers of glass or plastic films improve thermal resistance and reduce the heat loss attributed to convection between window layers. Other factors that can greatly enhance the performance of the glazing are low emissivity (low-e) coatings and gases which are installed between the panes of glass. Low-emissivity (low-e) coatings have a transmittance of visible light that resembles that of clear glass, but have a low emmittance, which reduces radiant heat transfer between two glazing layers. Heat radiated to the glass from inside the building is radiated back inside. While air is a relatively good insulator, there are gases (such as argon, krypton, and carbon dioxide) with lower thermal conductance. Using one of these non-toxic gases in an insulating window reduces heat transfer between the glazing layers.

WHAT'S NEW IN THE GLAZING WORLD ?

Superwindows (U-values of 0.2- 0.25) were first introduced and marketed by wood manufacturers and are now available in vinyl frames. These windows utilize gas fills and low-e coatings. Superwindows offer up to four times the insulating value of aluminum double pane windows. They also provide builders with design flexibility in meeting energy codes and conservation programs such as Super Good Cents.

WINDOW FRAMES MAKE A DIFFERENCE

Aluminum frames are less expensive than other types of frames, but don't insulate as well. Wood frames provide good insulation, but cost more than aluminum and require regular maintenance. Vinyl or aluminum clad wood frames don't require maintenance, but may cost more than wood. However, vinyl frames provide excellent insulation, require no maintenance and are reasonably priced.

WHAT ELSE CAN BE DONE ?

There are two basic ways that you can weatherize your windows:

• Add storm windows

Advantages: lower cost; retains look of windows; can be installed inside or outside; Disadvantages: must be removed for cleaning; condensation between units

• Replace existing windows with new windows.

Advantages: insulated glass and improved window frames; better weather-stripping; new screens
Disadvantages: higher cost than storm windows; may be difficult to install; may change look of home.