Arasteh, D.

Finlayson, E.

Huang, J.

Huizenga, C.

Mitchell, R.

Rubin, M.

UNT Digital Library

LBNL-42151TA-378Proceedings of the ACEEE ‘98 Summer Study on Energy Efficiency in Buildings, August 23-28, 1998, Pacific Grove, CAThis work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of BuildingTechnology, State and Community Programs, Office of Building Systems of the U.S. Department of Energy underContract No. DE-AC03-76SF00098.    State-of-the-Art Software forWindow Energy-Efficiency Rating and LabelingDariush Arasteh, Elizabeth Finlayson, Joe Huang, Charlie Huizenga,*Robin Mitchell, and Mike RubinBuilding Technologies DepartmentEnvironmental Energy Technologies DivisionErnest Orlando Lawrence Berkeley National LaboratoryUniversity of California1 Cyclotron RoadBerkeley, CACenter for Environmental Design Research*University of CaliforniaBerkeley, CAJuly 19981State-of-the-Art Software forWindow Energy-Efficiency Rating and LabelingDariush Arasteh, Elizabeth Finlayson, Joe Huang, Charlie Huizenga,*Robin Mitchell, and Mike RubinBuilding Technologies DepartmentEnvironmental Energy Technologies DivisionErnest Orlando Lawrence Berkeley National LaboratoryUniversity of California1 Cyclotron RoadBerkeley, CACenter for Environmental Design Research*University of CaliforniaBerkeley, CAAbstractMeasuring the thermal performance of windows in typical residential buildings is an expensiveproposition.  Not only is laboratory testing expensive, but each window manufacturer typically offershundreds of individual products, each of which has different thermal performance properties.  With overa thousand window manufacturers nationally, a testing-based rating system would be prohibitivelyexpensive to the industry and to consumers.Beginning in the early 1990s, simulation software began to be used as part of a national programfor rating window U-values.  The rating program has since been expanded to include Solar Hear GainCoefficients and is now being extended to annual energy performance.This paper describes four software packages available to the public from Lawrence BerkeleyNational Laboratory (LBNL).  These software packages are used to evaluate window thermalperformance: RESFEN (for evaluating annual energy costs), WINDOW (for calculating a product’sthermal performance properties), THERM (a preprocessor for WINDOW that determines two-dimensional heat-transfer effects), and Optics (a preprocessor for WINDOW's glass database).Software not only offers a less expensive means than testing to evaluate window performance, itcan also be used during the design process to help manufacturers produce windows that will meet targetspecifications.  In addition, software can show small improvements in window performance that mightnot be detected in actual testing because of large uncertainties in test procedures.IntroductionWindows typically account for 3 Quads of annual energy use or approximately one-fourth of theenergy used by the U.S. building sector.  Installing energy-efficient windows could drastically reducethis energy demand.  However, an accurate rating and widely used labeling system is needed to helpconsumers and designers select efficient windows and optimize them for specific applications.Rating and labeling systems for the thermal performance of windows and other fenestrationproducts have recently gained widespread acceptance in the United States, Canada, Australia, and New2Zealand.  Similar programs are being considered in the United Kingdom, Russia, Korea, and elsewhere.The success of such programs results in large part from the availability of software that can consistentlyand accurately evaluate the thermal and optical performance of windows and other fenestration productsat a fraction of the cost of testing them.Software is essential to support a window performance rating system because:• testing is expensive and lengthy.• manufacturers have thousands of different products, many that are different from each other insubtle ways (e.g., the tint of glass used) that can significantly affect energy performance.• manufacturers can most effectively produce efficient windows that meet particular performancespecifications if window performance data can be generated during the design process beforeproducts are actually available for testing.• some performance properties (like annual energy ratings) are virtually impossible to test for butcan be simulated.• simulations produce consistent results (large uncertainties in test procedures mean that smallimprovements may not be detectable by testing; these improvements would show up insimulation results, however).Software tools are used as part of rating systems in the same way that laboratory test facilitieswould be used to examine actual products: independent simulation laboratories, accredited by anindependent agency, perform simulations and report the results.Properties for Evaluating Window PerformanceHistorically, window thermal performance has been reported in terms of:• U-factor: the amount of heat loss (or gain) through a window; the inverse of R-value• Solar Heat Gain Coefficient (SHGC): the fraction of incident solar radiation that comes throughthe window• Visible Transmittance: the fraction of visible light that is transmitted by the window• Air Infiltration: the air leakage through the window.These properties need to be evaluated under standard environmental conditions (indoor andoutdoor temperatures, wind speeds, etc.) in order for performance ratings to be comparable amongproducts.  These are the properties of most interest to engineers, architects, and other professionals andare also used in energy codes because they are, to first-order, static properties that belong to the windowitself rather than the building in which it is installed.  Software can model window U-factor, SHGC, andVisible Transmittance in typical installations (Air Infiltration is virtually impossible to model).In addition to the above factors, thermal performance ratings would be most useful to consumersif they indicated the energy and cost impact of a particular window in a typical application.  Such anassessment can be based on knowledge of the window’s properties (U-factor, Solar Heat Gain, VisibleTransmittance, and Air Infiltration.)  Software simulations have long been used in similar ways toevaluate building energy impacts; however, modeling has typically focused on whole buildingperformance rather than the impacts of specific components such as individual windows.3Software Tools Developed at LBNL for Rating SystemsTo meet the needs of window rating systems in the U.S. and other countries, LBNL hasdeveloped a series of publicly available PC programs for determining window thermal and opticalperformance.WINDOW (Arasteh, Finlayson & Huizenga 1994) is the cornerstone of LBNL’s fenestrationsoftware series; it calculates total window U-factors, SHGC, visible transmittance, and other relatedoptical properties.  WINDOW 4.1 uses a heat-transfer analysis method that is consistent with theNational Fenestration Rating Council (NFRC) rating procedure.  WINDOW can be used to analyzeproducts made from any combination of glazing layers, gas layers, frames, spacers, and dividers underany environmental conditions and at any tilt.  As input, WINDOW requires information on the opticalperformance properties of a window’s glazing materials (see discussion of Spectral Database and Opticsprogram below) and the results of a detailed two-dimensional heat-transfer model on the window's frameand edge thermal performance.  This heat-transfer information can be generated using THERM (seebelow).  The software program FRAME (Enermodal Engineering Ltd. 1991), developed in Canada, alsoperforms two-dimensional heat-transfer calculations.The Spectral Database (Rubin 1998) which accompanies WINDOW is a compendium of spectraldata (transmittances, reflectances, emissivities) on all major glass products produced in the U.S. for usein residential and commercial window products.  These data are measured by manufacturers (who haveparticipated in a round-robin collaboration to standardize measurement practices) and then undergo peerreview as well as review by LBNL to check for consistency.  The approximately 1,000 entries in thisdatabase represent single glass layers.  Laminated layers (two pieces of glass “glued” together with alaminate material) are typically not reported in this database of measured values due to the large numberof possible combinations.  Likewise, “glue-on” or “applied” films combined with glass layers are alsotypically not reported in this database due to the large numbers of possible combinations.In order to ensure that laminates and applied films can be incorporated into a window rating andlabeling process, the Optics program has been developed.  Optics (Rubin, von Rottkay & Powles 1988)is designed to allow its users to compute the properties of any laminate or applied film by specifying itscomposition.  The properties of laminated glazing materials or glazings with applied films added to themcan thus be created by Optics for use with the WINDOW program without the need to make a sample andmeasure its optical properties.THERM (Finlayson et al. 1998) provides WINDOW with required two-dimensional heat transferindices on window frame and edge performance.  THERM’s two-dimensional heat transfer analysis isbased on the finite-element method that allows the user to model complicated frame details and evenentire window cross-sections, if warranted.  The latest version of the program, THERM 2.0, includes aradiation view-factor model that improves the program’s accuracy for modeling products with self-viewing surfaces, such as greenhouse windows and skylights (see Figure 1).  THERM is a MicrosoftWindows™-based program that includes a graphic interface developed to allow for quick and accuratedrawing of window cross-sections.4Figure 1.  THERM 2.0 includes a radiation view-factor model thatimproves the program’s accuracy for modeling products with self-viewing surfaces such as greenhouse windows and skylights.RESFEN (Huang et al. 1997) can help consumers and builders pick the most energy-efficient andcost-effective window for a given application.  It calculates heating and cooling energy use andassociated costs as well as peak heating and cooling demand for specific window products.  Users definea problem by specifying the house type (single- or two-story), geographic location, orientation,electricity, and gas costs, and building configuration details (e.g., wall type, floor type, HVACsystems).  Window options are defined by specifying size, shading, and thermal properties: U-factor,SHGC, and air leakage rate.  RESFEN calculates energy use and cost; different window options can becompared.  RESFEN 3.0 is a major improvement over previous versions because it performs hourlycalculations using a customized version of the DOE 2.1E energy analysis simulation program.Issues and ConclusionsNot all products can be simulated using the four software programs described above.  Testprocedures are still necessary for the approximately 1% of products that cannot be modeled, as well asfor advanced products not yet incorporated in the simulation programs.  Spot testing is also needed tovalidate simulation tools, showing that real product performance is consistent with simulation results.The use of software tools, such as those developed by LBNL, can save the cost of significantamounts of testing and provide the large numbers of data needed to support window thermal performancerating systems.  Data obtained from software simulations are reliably consistent and reveal small changesin window efficiency that might be obscured by uncertainties in actual testing of products.Manufacturers can use simulation programs in the design process, to help achieve specified performance4targets.  The integration of these four tools, WINDOW, THERM, Optics and the Spectral Database, andRESFEN allows the user to quickly and efficiently see the energy and cost impacts of componentchanges.For more information on LBNL’s fenestration modeling software and/or to obtain copies, seehttp://windows.lbl.gov or contact the author(s).AcknowledgmentsThis work was supported by the Assistant Secretary for Energy Efficiency and RenewableEnergy, Office of Building Technology, State and Community Programs, Office of Building Systems ofthe U.S. Department of Energy under Contract No. DE-AC03-76SF00098.ReferencesArasteh, D.K., E.U. Finlayson, and C. Huizenga.  1994.  “WINDOW 4.1: A PC Program forAnalyzing Window Thermal Performance in Accordance with Standard NFRC Procedures.”  LawrenceBerkeley National Laboratory Report LBL-35298.  Berkeley, CA.Enermodal Engineering Ltd.  1991.  “FRAMETM -- A Computer Program to Evaluate the ThermalPerformance of Window Frame Systems, Version 3.0.” Waterloo, Ontario, Canada.Finlayson, E.U., D.K. Arasteh, C. Huizenga, M.D. Rubin, and M.S. Reilly.  1993.“WINDOW 4.0: Documentation of Calculation Procedures.” Lawrence Berkeley National LaboratoryReport LBL-33943.  Berkeley, CA.Finlayson, E.U., R. Mitchell, D. Arasteh, C. Huizenga, and D. Curcija.  1998.  “THERM 2.0:A PC Program for Analyzing the Two-Dimensional Heat Transfer Through Building Products.”Lawrence Berkeley National Laboratory Report LBL-37371, Rev. 2.  Berkeley, CA.Huang, J., R. Sullivan, D. Arasteh, and R. Mitchell.  1997.  “RESFEN 3.0: A PC Program forCalculating the Heating and Cooling Energy Use of Windows in Residential Buildings.”  LawrenceBerkeley National Laboratory Report LBNL-40682, Berkeley, CA.Rubin, M. 1998.  “WINDOW 4.1 Optical Data Library #5 Including NFRC Certified Data.”Lawrence Berkeley National Laboratory Report LBL-35298 Addendum #5.  Berkeley, CA.Rubin, M., K. von Rottkay, and R. Powles.  1988.  “Window Optics”, Solar Energy, 62.

State-of-the-art software for window energy-efficiency rating and labeling

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State-of-the-art software for window energy-efficiency rating and labeling

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