A curtainwall system, designed by Greenfield Architects, was used to create a dynamic look for Albertson's Denver, PA distribution center. (Architect: Greenfield Architects, Lancaster, PA)

"The aesthetics of windows give buildings character and style," he says. "They are one of the most important elements in an architect's palette, because light affects the moods of the people inside."

Windows come in a multitude of options to suit the builder's needs and can affect the look of a building, its energy efficiency, and its ventilation.

"Window choice is critical," says Robert J. Dudish, an architectural representative for Adam Wholesalers. "Like the foundation, window systems are difficult to change once they've been installed. If you make the wrong choice, they can be expensive to replace."

Fox says most commercial window frames are made of aluminum because they are lightweight and resistant to corrosion. In buildings where security is important, window frames can be made of steel for added strength. Both systems come with a multitude of finishing options and colors. He says there are three main types of aluminum window systems:

• Unit windows, which are smaller, prefabricated windows in any desired size, and the most widely-manufactured.

• Storefront systems, common in buildings of two to three stories, and appropriately used when a lot of glass is preferred to showcase items inside.

• Curtainwall systems, used in buildings higher than three stories. They are composed of metal, glass, and other surfacing materials or components supported within a metal framework, and are designed to protect from wind and water infiltration at higher elevations.

According to Fox, curtainwall systems are very sophisticated, and keep water out with an intricate system of internal gutters that equalize pressure in the window cavity. Robert Foster, senior marketing manager for Kawneer Corp., says, "Curtainwalls can be individual windows in an office or a three-to-four-story facade, a common feature in high-rise office buildings of six or more stories, or monumental buildings." Fox adds that curtainwalls are available in varying designs, including half-circles and triangles, and come in varying depths.

The architect's use of glass enhances Messiah College's Jordan Center.

Foster says most institutions want "fixed" windows (windows that do not open), but also utilize "single hung" windows, those in which one half slides up or down, and "double-hung" windows, in which the top goes down and the bottom goes up. "Projected windows" (also called "awning" windows) open so that the top projects in or the bottom projects out. A favorite choice in monumental buildings, such as museums and hospitals, is the "pivot" window, which can pivot horizontally or vertically from the center and allow large pieces of glass to be washed easily.

Fox says window systems can be thermally broken, which means the inner and outer frames are separated with a thermoplastic resin that reduces thermal conduction through the frames, and eliminates or greatly reduces condensation. This treatment also reduces the infiltration of sound, which is important in office buildings and hotels.

The type of glass used in window systems is dependent on the window's purpose, says Fox.

• Float glass, the most commonly used glass in low-level commercial buildings, is made by pouring molten glass on molten tin and allowing it to cool slowly. This process creates a flat surface, which presents little distortion.

• Insulated glass, which is manufactured as a unit, is made up of two or three panes of glass, and the air space between the panes can be filled with a gas, such as argon or krypton to further increase the energy efficiency.

• Tempered glass is heat-treated and cooled quickly, which makes the glass four times as strong as float glass. When shattered, it breaks into tiny pieces to prevent serious injuries. It is commonly used as safety glass in doors, sidelights and other areas susceptible to breakage and where required by building codes.

• Laminated glass is several pieces of glass bonded together with plastic resin that makes the glass even stronger than tempered glass. Laminated glass is used in bullet-resistant doors and windows and, because of its density, where needed to deaden sound. Tempered and laminated glasses are ideal in high-rise construction because of their durability and strength to resist high wind pressures.

• Tinted glass, as well as reflective glass, is popular in today's buildings. Reflective glass is produced by coating the glass with a thin layer of metal oxide. A relatively new product on the market is Low-emissivity glass. Low-e glass, which it is commonly called, has a coating or a film applied to it that allows certain wavelengths to either pass through or reflect out. For example, when it's cold, Low-e glass allows solar rays to enter the building during the day, but prevents heat from escaping at night.

For visual appeal, Fox says laminated glass can be designed and produced with a patterned interlayer, such as horizontal lines or squares. This design option provides architectural interest, and can help reduce light transmittance and glare. "When an architect needs to close out mechanical spaces or hide the interstitial space of an all-glass building," he says, "a good choice is spandrel glass, which is heat strengthened or tempered glass with an opaque coating applied on the back."

"People spend up to 12 hours a day working inside your building," says Ned d'Entremont, a commercial sales representative for the Gunston Corp. "The right choice in windows can go a long way in affecting their attitudes."