Beyond structural performance, there is another significant factor associated with these tests for performance, namely water management. All window systems are designed to resist water penetration, and the higher the Performance Grade, the more wind-driven rain they should be able to resist. But no system is perfect and some water seepage is certainly possible, meaning planning for it is very important. Vertical sheets of glass will obviously cause rainwater to run straight down their faces.

The design issue becomes how to manage the water once it hits a window framing member, particularly a horizontal member. Sealants and gaskets plus the shape of the horizontal member are usually the first line of defense.

However, depending on the type of window system selected, a secondary system is included to capture and drain away any water that does intrude. Choosing a system that does not allow for this contingency properly, or choosing a system that is exposed to more than it is designed to handle can spell serious problems if water does back up and then leak into the building. At a minimum, it can create an unsightly appearance but worse, it can cause deterioration and failure of other materials. Under the right conditions, it can also lead to problems with mold, mildew and other indoor environmental quality issues. All of this can be avoided by selecting the right system to match the installation and environmental requirements of the building.

Also within AAMA, the Aluminum Material Council exists to focus on standards related specifically to window systems fabricated from aluminum. Since it is estimated that as much as 65% of non-residential buildings use some type of aluminum window system, this is also an important resource for guidance on window design and specification preparation. Recently, hurricane and blast criterion have come to the forefront of fenestration requirements, and the council, in concert with AAMA, has provided particular standards and information in these areas as well.

The National Fenestration Rating Council (NFRC)

NFRC describes itself as "a non-profit organization that administers the only uniform, independent rating and labeling system for the energy performance of windows, doors, skylights and attachment products." Their goal is to provide fair, accurate and reliable energy performance ratings so that architects, code officials, contractors and others can compare different products and make informed product choices. Manufacturers, government agencies, designers and others have recognized the NFRC as the primary resource for energy efficiency ratings that look holistically at a complete window system product. They accomplish this through their signature Component Modeling Approach (CMAST) product certification program which generates whole product energy ratings by testing the components of a particular product or system. Specifically, the three separate components that are tested include the window framing, the glazing and the spacer between multiple layers of insulated glazing.

Incorporating a window system into a building needs to take into account the rest of the design. Here, the windows and entry are protected with a sheltering overhang that reduces the amount of wind and rain that the system is exposed to, helping to enhance its performance. Photo courtesy of Manko Window Systems, Inc.

 

Each manufacturer can provide NFRC testing on established product sizes and configurations to provide information needed to determine how well a product will perform regarding building thermal performance and condensation resistance. By using the information contained on the NFRC test reports, a reliable comparison can be made between one product and another. All energy performance values represent the rating of a window as a whole system taken together and list at a minimum the following values:

U-Factor: The rate of heat loss is indicated in terms of the U-factor (U-value) of a total window assembly. U-factor ratings generally fall between 0.20 and 1.20. (The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.)

Solar Heat Gain Coefficient (SHGC): The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and the amount subsequently released inward. SHGC is expressed as a number between 0 and 1. (The lower a window's solar heat gain coefficient, the less solar heat it transmits.)

Visible Transmittance: The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. (The higher the VT number, the higher the amount of light that is transmitted.)