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Tips on Quantifying Window Assembly Performance


Window assembly performance should be assessed for structural integrity, air and water infiltration, and potential condensation issues, as well as for energy efficiency. Often, much of this quantifying information is available from the manufacturer. Before relying on published material, however, confirm that the data provided pertains to the exact assembly under consideration, and that the test specimen incorporated the same glass that would 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, using software programs such as THERM. Developed by Lawrence Berkley National Laboratory, THERM allows design professionals to model two-dimensional heat-transfer effects in building components and evaluate an assembly’s energy efficiency. Although limited in its ability to assess complex real-world conditions, such as thermal massing, THERM and other computer models can 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, an apparatus that aims to replicate conditions typical of what is seen in the field. The ASTM International Standard 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 Standard 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.

This Quick Read comes from Craig A. Hargrove, AIA, LEED AP, senior vice president and director of architecture with Hoffmann Architects, Inc., which specializes in building exteriors. He can be reached at c.hargrove@hoffarch.com. Read more from him about knowing when window replacement is inevitable.

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