Basic window design
A truly energy-efficient window starts with good design. As defined by the American Architectural Manufacturers Association (AAMA) in the North American Fenestration Standard (NAFS) (NAFS can also be referred to as American Architectural Manufacturers Association/Window and Door Manufacturers Association/Canadian Standards Association [AAMA/WDMA/CSA] 101/I.S.2/A440-08.), window types are standardized according to performance grades, distinguished by design pressures:
- R class (15 psf [i.e. 720 Pa]), which is typically used in one- and two-family dwellings;
- LC class (25 psf [i.e. 1200 Pa]), which is usually for low- and mid-rise residential buildings;
- CW class (30 psf [i.e. 1440 Pa]), which is for low- and mid-rise buildings with higher loading requirements and heavier use; and
- AW class (40 psf [i.e. 1920 Pa]), which is used in high- and mid-rise buildings to meet increased loading requirements and limits on deflection.
Window class selection depends on application and expected use, with higher performance grades capable of withstanding greater operating force, deflection, and structural loading.
Knowing the applicable building code is critical to window specification. Requirements for structural stability typically cover frame, glass, anchorage, and substrate attachment. An architect or engineer should identify the existing substrate’s condition and determine whether it has decayed or been damaged over time. A window’s structural integrity is only as good as its attachment to the substrate—if the substrate itself is unsound, the window could become unstable.
Building codes also frequently stipulate requirements for air and water infiltration testing of new window assemblies. Even where the code does not mandate testing, it is a good idea to review test results from the manufacturer and conduct laboratory and field performance tests. AAMA and ASTM International provide guidelines for test methods that should be followed as the industry standard.
The code may mandate glass type for a given application. The three most common types of commercially available glazing are:
- annealed—raw glass that has not been heat-treated, which may be limited by code due to its susceptibility to thermal shock and mechanical stress, as well as its tendency to break into large, sharp pieces;
- heat-strengthened—glass that undergoes controlled heating and cooling to improve strength and fracture resistance, which is roughly twice as strong as annealed glass but still breaks into large, dangerous shards; and
- fully tempered—glass chemically or thermally treated to improve strength and shatter resistance, which breaks into tiny pieces less likely to cause injury.
Aside from structural and safety considerations, window options may be limited by energy code requirements, which are becoming increasingly stringent even for existing buildings. As of this writing, IECC is in use or adopted in 47 states, the District of Columbia, the U.S. Virgin Islands, New York City, and Puerto Rico. With each successive edition of the model code, performance criteria will likely continue to become more rigorous.