Software: The Key to Effective Ratings

The success of window ratings programs, from the United Kingdom to Korea, depends on consistent, accurate, and inexpensive software. Real-life measurement of the thermal performance of windows in typical residential buildings is expensive. The lab testing costs a lot, and the costs are multiplied-window manufacturers often offer hundreds of individual products, each of which has different thermal performance properties. With more than 1,000 window manufacturers in the United States alone, a testing-based rating system would be prohibitively expensive to the industry and to consumers.

Computer testing cuts costs. Software can also be used to fine-tune windows during the design process. Finally, software testing has fewer uncertainties than physical testing, so it can show small improvements that might not otherwise be detected.

Lawrence Berkeley National Laboratory (LBNL) has developed a series of publicly available PC programs for determining the thermal and optical performance of windows. RESFEN evaluates annual energy costs; WINDOW 4.1 calculates a product's thermal performance properties; THERM is a preprocessor for WINDOW that determines two-dimensional heat transfer effects; and Optics is a preprocessor for WINDOW's glass database.

WINDOW is the cornerstone of LBNL's fenestration software series. It calculates total window U-factors, SHGC, visible transmittance, and other related properties. WINDOW 4.1 uses a heat transfer analysis method that is consistent with the NFRC rating procedure. WINDOW can be used to analyze products made from any combination of glazing layers, gas layers, frames, spacers, and dividers under any environmental conditions and at any tilt. As input, WINDOW requires information on the optical performance properties of a window's glazing materials and the results of a detailed two-dimensional heat transfer model on the window's frame and edge thermal performance. This heat transfer information can be generated using THERM. The software program FRAME, developed in Canada, performs similar heat transfer calculations.

A spectral database accompanies WINDOW. It is a compendium of transmittances, reflectances, and emissivities on all major glass products manufactured in the United States for use in residential and commercial windows. These data are measured by manufacturers who have standardized their measurement practices. The numbers are then peer reviewed and reviewed by LBNL to check for consistency. The 1,000 or so entries in this database represent single glass layers. Since there are so many possible combinations of glazings, the database generally does not include laminates (two pieces of glass fixed together) or applied films combined with glass layers.

For laminates and applied films, LBNL developed Optics. Users specify the composition of a laminate or glass-film combination. The program computes the properties for use with WINDOW, so it is not necessary to make a sample and measure its optical properties.

THERM provides WINDOW with required two-dimensional heat transfer indices on window frame and edge performance. THERM's two-dimensional heat transfer analysis is based on the finite-element method, which allows the user to model complicated frame details and even entire window cross sections, if warranted. The latest version of the program, THERM 2.0, includes a radiation view factor model that improves the program's accuracy for modeling products such as greenhouse windows and skylights. THERM is a Windows-based program that includes a graphic interface developed to allow for quick and accurate drawing of window cross sections.

RESFEN can help consumers and builders pick the most energy-efficient and cost-effective window for a given application. Users define a problem by specifying type of house (one- or two-story), geographic location, orientation, electricity and gas costs, and building construction details. They then specify the size, shading, and thermal properties of the windows. RESFEN calculates the energy use and cost for that window selection, so users can compare different window options. RESFEN 3.0 is a major improvement over previous versions because it performs hourly calculations using a customized version of the DOE 2.1E energy analysis simulation program.

Not all products can be simulated using the four software programs described above. Test procedures are still necessary for the approximately 1% of products that cannot be modeled, due to their geometric complexity, as well as for advanced products not yet incorporated into the simulation programs. Spot testing is also needed to validate simulation tools, showing that real product performance is consistent with simulation results.

For more information on LBNL's fenestration modeling software, or for copies, go to the LBNL Web site: http://windows.lbl.gov.

Dariush Arasteh, Robin Mitchell, Elizabeth Finlayson, and Mike Rubin are researchers at Lawrence Berkeley National Laboratory in Berkeley, California. Charlie Huizenga is a research specialist at the University of California at Berkeley.

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