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Energy efficiency is not often a precipitating factor in the decision to replace aging windows. However, once replacement is unavoidable because of problems of occupant comfort, maintenance demands, and aesthetics, a window project offers the opportunity to improve energy efficiency and reduce operating costs. Whether owners pay these expenses themselves or pass them along to tenants, energy savings can be a compelling consideration when replacing windows and designing new ones. When embarking on a project, it pays to understand window classes, applicable codes, and types of glass.
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) window types are standardized according to performance grades, distinguished by design pressures:
• R class, 15 psf, typically used in one- and two-family dwellings.
• LC class, 25 psf, usually low- and mid-rise residential buildings.
• CW class, 30 psf, low- and mid-rise buildings with higher loading requirements and heavier use.
• AW class, 40 psf, used in high-rise and mid-rise buildings to meet increased loading requirements and limits on deflection.
Window class selection is dependent upon the 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 condition of the existing substrate and determine whether it has been damaged or has decayed over time. A window’s structural ASTM International integrity is only as good as its attachment to the substrate, and if the substrate itself is unsound, the window could become unstable.
Building codes frequently stipulate requirements for air and water infiltration testing of new window assemblies. Even where the code does not mandate testing, it’s a good idea to review test results from the manufacturer and to conduct laboratory and field performance tests. AAMA and provide guidelines for test methods that should be followed as the industry standard.
The type of glass for a given application may be mandated by code. The three most common types of commercially available glazing are:
• Annealed glass, raw glass that has not been heat-treated, may be limited by code due to its susceptibility to thermal shock and mechanical stress and its tendency to break into large, sharp pieces.
• Heat-strengthened glass, which undergoes controlled heating and cooling to improve strength and fracture resistance, is roughly twice as strong as annealed glass but still breaks into large, dangerous shards.
• Fully tempered glass, which is chemically or thermally treated to improve strength and shatter resistance, breaks into tiny pieces that are 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, the International Energy Conservation Code (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.
Knowing Window Classes, Codes, Glass Types for Window Replacement Projects