For Designers

Durability

The durability of a window or skylight directly affects its energy performance. If, for example, insulating glass seals or low-emissivity coatings fail, a product's thermal performance will be seriously compromised. Durability denotes a product's ability to perform as designed over a period of time. Important elements of a fenestration product's durability include the ability to resist condensation and air and water infiltration while maintaining thermal and structural performance as well as aesthetic features.

Impact of Durability Issues on Energy Performance
A durable window system will meet its energy performance criteria - U-factor, solar heat gain coefficient, and air infiltration parameters - over time. Failure of components such as sealants and gaskets will degrade thermal performance and may lead to loss of insulating gas in units with gas infill, in addition to permitting air and water to penetrate the glazing system and the building. The NFRC is in the process of developing long-term durability test procedures aimed at determining the effects of individual physical weathering and durability impacts on specific product U-factors, solar heat gain coefficients, air-infiltration rates, and condensation resistance.

The durability of insulating glass units is critical to maintaining the energy performance of window and door assemblies. Minimum performance levels have been established in the United States and Canada requiring that insulating glass units conform to ASTM E 2190 or CAN/CGSB 12.8. Proof of compliance may be by a test report from an independent accredited laboratory or by certification of the IGU by IGMA, IGCC, ALI or another certifying agency acceptable to the specifier. These certification programs require compliance with the performance standards and the establishment of a reliable quality control program at the manufacturing plant. The IGMA technical manual, TM-4000-02 "Insulating Glass Manufacturing Quality Procedure Manual," and IGU certification represents a significant increase in quality assurance requirements for the manufacture of insulating glass.

The IGMA quality assurance provisions, or equivalent, should be the minimum level of quality assurance acceptable for IGU production. However, there are a number of quality assurance provisions that are referenced in ISO EN 1279 - which should be considered by the manufacturer. Standards related to durability and durability testing are listed in the references. Most of these standards relate to the durability of individual components (such as IGU sealants) or impacts from specific conditions (such as seismic and wind).

There is no simple rating or absolute guarantee of the durability of a window. Just as in making any other important choices, the designer or homeowner must study the design and workmanship of the window, and then rely on the recommendations from others who have used similar products and the manufacturer's reputation. The advice of experienced architects and builders can be helpful to the homeowner. 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.

Maintenance
Older windows with wood exposed on the exterior require periodic painting and replacement of the glazing compound that holds the glass in place. Newer windows made with vinyl or metal frames or cladding over the wood frame require very little exterior maintenance. If the exterior frame material is wood, then the cost and effort of periodic painting should be considered in selecting a window. However, many new wood windows have a very durable exterior finish, applied in the factory, which is far better than traditional paint finishes.

Structural Capability and Water Resistance
Windows available on the market must meet numerous standards and building regulations. In special cases, such as a building in a hurricane-prone area, 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.

Preventing water transmission and minimizing unwanted air movement are prime requirements of windows and other fenestration products. Manufactured window units are designed to prevent water intrusion and if it does occur, divert it to the exterior. Water that appears to be the result of leaking windows and doors is often a result of penetration at another location.

The local building code should be reviewed to determine requirements for wind load resistance and water penetration resistance. Organizations such as AAMA and WDMA provide these standards, as do local building departments.