There is no simple rating or absolute guarantee of the durability of a window. You may want to study the design and workmanship of the window and rely on recommendations from others who have used similar products. The advice of experienced architects and builders can be helpful. As with other products, warranties can be an indicator of the reliability of the window and its manufacturer. Durability may vary with location; for example, some materials are degraded by salt near the ocean.
These aspects of window durability deserve special attention:
• frame and sashes
• insulating glass seals
• local requirements for structural integrity
Frame and Sashes
Although design and workmanship may be the most important factors determining the long-term durability of sashes and frame, the materials used also play an important role. Some basics about the durability aspects of different frame materials are listed below:
Aluminum: Strong and durable, but readily conducts heat. Condensation and even frost can form. Thermal breaks reduce conduction and improve condensation resistance. The durability of thermal breaks varies by type and quality.
Wood: Easy to repair and maintain. Intrinsically susceptible to rot, but many new wood windows are protected by a durable exterior finish or cladding. Cladding should prevent moisture from forming underneath.
Composite: New wood/polymer composites are very stable. Better moisture and decay resistance than conventional wood.
Vinyl: Good moisture resistance and low maintenance. Tends to expand or contract as temperature changes, but recent designs have improved dimensional stability and resistance to UV radiation and temperature extremes.
Fiberglass: Strong, durable and dimensionally stable.
Insulating Glass Seals
The durability of insulating glass (IG) units is critical to window energy performance. Seal failure leads to the loss of insulating gas and can result in condensation from moisture penetrating between the panes, which can degrade low-E coatings.
From July 2010 on, to minimize the likelihood of seal failure and gas loss, insulating glass in ENERGY STAR windows must be certified through recognized programs. IG certification requires that sample IG units undergo accelerated weathering over extended time periods to determine whether the IG unit can withstand extreme temperature swings, high humidity and UV radiation without failure. IG certification also requires that the manufacturer implements in-house quality assurance.
Field studies have shown that insulating glass failure rates differ greatly depending on the quality of seal systems and manufacturers’ quality assurance. Well-fabricated IG units can retain gas at a loss rate of less than 1 percent per year, resulting in only minimal loss of insulating performance.
Most manufacturers offer a warranty against insulating glass failure which varies from a limited period to the lifetime of the window. A range from 10 to 20 years is common.
Ideally, weatherstripping must survive thousands of operational cycles and years of exposure to sun, weather and water. However, cheaper windows often save on cost by using poorer-quality weatherstripping. To avoid air infiltration and early replacement of the weatherstripping, higher-quality strips are worth the investment.
After some years, any kind of weatherstripping should be inspected in case it warrants to be replaced. Organic felts age fairly quickly, and all felts absorb moisture, reducing their effectiveness. Brush or wiper type weatherstripping eventually gets matted down like a carpet that has had too much traffic. Metal strips are easily dented or bent. Plastics and rubber can become brittle or sticky. Nevertheless, some recent synthetic weatherstripping can be expected to last longer.
Local Requirements for Structural Integrity
Windows must meet numerous standards and building regulations. In special cases, such as in hurricane-prone areas, building codes may require glazing of a certain strength and type to resist breakage during storm conditions. Laminated glass is often used in these cases.
The local building code should be reviewed to determine requirements for wind load resistance and water penetration resistance. Organizations such as the American Architectural Manufacturers Association (AAMA) and the Window and Door Manufacturers Association (WDMA) provide information on these standards, as do local building departments.
Windows can be tested for their structural strength, resistance to air and water penetration, ease of operation, and resistance to forced entry. If these tests are conducted according to the North American Fenestration Standard (NAFS), they result in a performance grade (PG) rating representative of the design pressure (DP) and other performance aspects. Performance grade and design pressure are primarily used for compliance with local code requirements. However, looking for high PG or DP ratings may also help you as a consumer find windows that are particularly good at holding up to the elements in locations with strong wind and/or rain conditions. AAMA and WDMA administer programs through which manufacturers can certify their PG and DP ratings based on tests by independent laboratories.
Another crucial factor for structural integrity is proper installation to avoid air and water leakage with potential damage to both window and wall.