Windows in Green Building

Choosing a Green Window

Casement Windows

A window with a hinged sash that swings like a door, often with fixed panels, too. When windows crank out, casement windows can be used in green building to catch and funnel prevailing breezes into home. Prevailing breezes in the Dallas-Fort Worth area are from the south and southeast.

Clerestory Windows

A clerestory, pronounced clear story, is a high wall with a band of narrow windows along the very top. The clerestory wall often rises above adjoining roofs. In green home building, clerestory windows add natural light to potentially dark rooms or areas, can act as loft windows, and, when they open, can provide a thermal chimney.

Insulated Glazing

Insulated Glazing Unit or Insulating Glass Unit (commonly referred to as IGU) is a set of two or more layers of glass spaced apart and hermetically sealed to form a single glazed unit with an air space between each layer. The most commonly found IGUs are double glazed, i.e. made with two layers of glass. Triple-glazed units are not necessary in the south.

Another name often used in North America is Sealed Insulating Glass (abbreviated SIG).

More about insulated glass in glossary

Low-E Windows

The purpose of low-e windows in the south is to admit less solar heat. Sun must strike the windows for solar heat to be admitted. If sun never strikes windows or only strikes the windows during the winter months, low-e windows are a waste of money. Or worse, low-e windows will stop heat from entering the windows during the winter and destroy a favorite "warm spot" in the sun.

Low-e coatings were originally manufactured to save energy in cold climates. They kept heat in without keeping heat out. Heat needs to be kept out of homes in Dallas-Fort Worth most of the year. When manufacturers introduced windows with coatings which admit up to 40 percent less solar heat, low-e windows became practical for the south.

Note the energy ratings on the Energy Star or National Fenestration Rating Council (NFRC) label. To meet the standards for Dallas-Fort Worth, windows must have a SHGC rating of .40 or lower and a U-factor of .75 or lower. The NFRC label also contains the VLT rating which indicates the percent of visible light able to pass through the glazing. Look for a VLT rating of .60 to .80 (60% to 80%) to obtain adequate daylight from your window.

Even though you can see through low-e coatings, they're not totally invisible and may appear as a slight tint on the glass.

More about Low-E windows.