QUANTIFYING WINDOW ASSEMBLY PERFORMANCE

Window assemblies should be assessed for structural integrity, air and water infiltration, and potential condensation issues, as well as for energy efficiency. Often, much of this information is available from the manufacturer. However, before relying on published material, it is important to confirm: the data provided pertains to the exact assembly under consideration; and the test specimen incorporated the same glass to be installed in the project. Sometimes, conditions are project-specific and cannot be anticipated in testing performed by the manufacturer. For instance, potential condensation issues that might result from the installation of a replacement window in an existing opening may need to be evaluated through thermal modeling performed by a building enclosure specialist, using software programs such as THERM. Developed by Lawrence Berkley National Laboratory (LBNL), THERM allows design professionals to model two-dimensional heat-transfer effects in building components and evaluate an assembly’s energy efficiency. Although limited in their ability to assess complex real-world conditions, such as thermal massing, THERM and other computer models help anticipate problems with thermal bridging, condensation, moisture damage, and structural integrity. If the variables are too numerous, or there is a need to quantify performance within extremely specific parameters, physical testing of a window assembly—and ideally of a sample of the wall into which it will be installed—can be performed in lieu of computer modeling. For energy performance, tests are typically performed at a testing facility using a hot box, which is an apparatus that aims to replicate conditions typical of what is seen in the field. ASTM C1363-11, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot-box Apparatus, is the recognized reference standard for such tests. During installation, windows should be tested for water penetration, as per ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform or Cyclic Static Air Pressure Difference. By establishing a pressure differential across the building envelope, this test method encourages water from a calibrated spray grid at the exterior to migrate into the building. Window assemblies and the surrounding substrate can then be evaluated for watertightness.

Glazing strategies for energy-efficient windows
The overarching goal of window design is to optimize visible light transmittance, along with exposure to natural light and exterior views for building users. To this end, glazing strategies are frequently employed to:

• maximize energy efficiency;
• take full advantage of natural light and exterior views;
• reduce glare;
• decrease U-factor and SHGC; and
• limit the need for artificial lighting.

Use of window shades and other opaque blinds or screens is not optimal. This is because not only do such window treatments obscure daylight and limit views, but they also tend to be incorrectly operated by building occupants. Shades automatically programmed with sun sensors are a viable option, but they are often costly, consume at least some energy, and—as an active rather than a passive system—require periodic maintenance.

Instead, the industry has advanced a number of technologies that improve efficiency while preserving the natural light and vistas afforded by large areas of glass. By balancing desired levels of visible light with heat gain control, the design team can recommend window assemblies that meet energy-efficiency standards and improve occupant comfort.

Dual glazing
Perhaps the most prevalent glazing strategy for energy efficiency, dual glazing consists of two panes of glass assembled into one integral unit by use of spacers and a perimeter seal. The space between panes is often filled with an inert gas (usually argon) to form an insulating glazing unit (IGU). Dual glazing is often used in conjunction with other strategies, such as tinting, low-emissivity (low-e) coatings, or fritting.

Triple glazing
Similar to dual-glazed IGUs, but with three panes of glass instead of two, triple glazing has not been widely used in the United States due to cost. However, progressively stringent energy codes have increased the prevalence of triple glazing in recent years, which should have the added effect of bringing down manufacturing costs.

Low-e coatings
Low-emissivity coatings are factory-applied treatments to reduce the ultraviolet (UV) and infrared (IR) light that passes through glass, limiting heat gain while preserving VLT. (As with anything installed on the surface of glass, low-emissivity coatings will have some effect on VLT; however, the effect is arguably less than with other methods of reducing SHGC such as tinting or fritting. Hard coatings, which are typically used on single-pane glazing on an exposed surface, have a greater overall effect than soft coats, but again, less than other methods. The surface to which a soft coat is applied has a greater effect on performance than VLT.)