CALCULATING FENESTRATION SYSTEM U-FACTOR, SHGC, AND VT .
2020 Building Performance Analysis Conference andSimBuild co-organized by ASHRAE and IBPSA-USACALCULATING FENESTRATION SYSTEM U-FACTOR, SHGC, ANDVT USING PARTIALLY AUTOMATED WORKFLOWSSarah RentfroSimpson Gumpertz & Heger Inc., Washington, DCABSTRACTThis paper explores current workflows andmethodologies used by building enclosure professionalsto calculate and report the effective U-Factor, Solar HeatGain Coefficient (SHGC) and Visible Transmittance(VT) of fenestration assemblies, henceforth referred toas fenestration performance metrics, in various stages ofdesign. Topics covered include a review of manual andpartially automated calculation and simulationworkflows, as well as methods for communicatingperformance targets and goals with the project team,including a web-based data visualization tool developedby the author’s firm.This paper relies on shared modeling experienceinformed by building enclosure design and consulting asthe basis of the discussion. While multiple means forcalculating and reporting fenestration performancemetrics exist, this paper focuses on the primaryexperience of the author and the author’s colleagues.INTRODUCTIONBuilding enclosure design is an iterative process, shapedby the architect’s aesthetic vision and the project team’sevolving understanding of each system’s impact on theperformance requirements of the building. One of thoserequirements is energy efficiency. Whole buildingenergy efficiency requirements continue to become morestringent, and code officials are becoming more vigilantwith enforcement of energy code compliance (Rentfro etal., 2018). Additionally, whole building energyperformance plays a significant role in performancebased voluntary certifications such as LEED andPassive House.The desire for grand views, open office plans anddaylighting have contributed to an increased use offenestration in modern building design, including use ofcustom glazing assemblies and unitized facade systems.Accurate understanding of heat transfer throughfenestration systems is critical, as heat loss/gain throughthe fenestration often exceeds that in opaque walls anddominates the performance of the building enclosure(Der Ananian et al., 2007). For this reason, minormodifications and improvements to thermal and solarperformance of fenestration systems can have asignificant impact on a building’s overall energyperformance. An accurate understanding of the thermaland solar performance of proposed fenestration systemscan contribute to efficient use of daylighting, reducedlighting loads and more appropriately sized HVACsystems.Typical methods for selecting high performancefenestration systems include adhering to publishedperformance criteria, using simple decision-supporttools, and performing customized simulations (He et al.,2017). Workflows for calculating fenestration thermaland solar performance have evolved to include customsimulations, computer automation tools and parametricanalysis, especially in a project’s early design phases.These tools automate the workflow, improve efficiency,reduce human error, and augment the development of anearly and open dialogue with the project team.That said, how fenestration performance metrics arecommunicated with the project team is of equalimportance. The design process is dynamic. Rather thanprovide a static representation of the performanceanalysis to clients, the author’s firm sought to develop anontechnical, interactive tool that project team memberscould use in real time to evaluate the thermal and solarperformance impacts of various fenestration designdecisions.This paper explores the development of calculationworkflows for U-Factor, SHGC, and VT based on theindustry shift toward automation and parametricmodeling. Additionally, this paper presents aninteractive web-based platform used to communicateresults with clients during both early and developeddesign stages.431 2020 ASHRAE (www.ashrae.org) and IBPSA-USA (www.ibpsa.us).For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted withoutASHRAE or IBPSA-USA's prior written permission.
FENESTRATION PERFORMANCEMETRICSThe following performance metrics define the thermaland solar impacts of a fenestration system on buildingenclosure performance and overall energy use.U-FactorAlso known as thermal transmittance, the U-factor is therate of heat transfer across a component due toconduction, convection, and long-wave infraredradiation (NFRC, 2017a). In North America, this valueis calculated in accordance with NFRC 100 – Procedurefor Determining Fenestration Product U-Factors fortransparent and translucent enclosure systems, andASHRAE Standard 90.1 – Energy Standard forBuildings Except Low-Rise Residential Buildings foropaque enclosure systems. The U-factor is used toquantify the thermal performance of a system forcomparison purposes. For example, manufacturers cancompare against each other’s systems or designers cancompare against prescriptive code requirements. Thesevalues are also used as an input for a whole buildingenergy model. The U-factor may also be used as part ofa broader analysis to understand the effects the proposedfenestration system may have on occupant thermalcomfort.Solar Heat Gain Coefficient (SHGC)The SHGC is defined as the ratio of solar heat gainentering through the fenestration product (as a functionof directly transmitted solar heat and absorbed solarradiation that is reradiated, conducted, or convected intothe space) to the solar radiation striking the fenestrationproduct (NFRC, 2017b). The SHGC is a number thatranges from zero to one. In North America, this value iscalculated in accordance with NFRC 200 – Procedurefor Determining Fenestration Product Solar Heat GainCoefficient and Visible Transmittance at NormalIncidence. Similar to the U-factor, the SHGC is used toquantify performance of a given system and to determinecompliance with the applicable energy codes, as well asto determine the impacts a given system might have onoccupant thermal comfort.Visible Transmittance (VT)The VT is defined as the ratio of visible light entering thespace through the fenestration product to the visible lightstriking the surface of the fenestration product (NFRC,2017b). This value is also a number that ranges from zeroto one and is reported as a percentage. In North America,it is calculated in accordance with NFRC 200 and is usedto determine compliance with the applicable energycodes, and in daylighting and occupant comfort analyses.For opaque components (e.g., frames, dividers, opaqueinfill panels), the VT is zero (NFRC, 2017b).Effective Performance MetricsAs part of the permitting process, project teams are oftenrequired to provide documentation to the AuthorityHaving Jurisdiction (AHJ) to show that the proposedperformance metrics meet the requirements of theapplicable energy code. Some design professionalsincorrectly report the glazing manufacturer’s publishedthermal performance values (e.g., center-of-glass Ufactor) for the entire fenestration system; however,these glazing values do not account for additional heatflow that occurs through the highly conductive framecomponents and spacer bar at the edge of the glazing(Der Ananian et al., 2007).Energy codes are clear in their definitions and requirecalculation of an “effective” or “overall” value thataccounts for the fenestration system as a whole: glazing,spacer bar, and frames (IECC, 2018). Effective valuesare determined using an area-weighted average of theperformance of the fenestration system’s componentparts (e.g., center of glass, edge of glass, frame, dividers,etc.) over the area of the whole fenestration unit asshown in the following equation for the effective Ufactor (NFRC, 2017a):[Eq.1]𝑈𝑈𝑒𝑒𝑒𝑒𝑒𝑒. [Σ(𝑈𝑈𝑓𝑓𝐴𝐴𝑓𝑓) Σ(𝑈𝑈𝑒𝑒𝐴𝐴𝑒𝑒) �𝐶𝐶𝐶𝐶) OMPUTER SIMULATION ANDAUTOMATION TOOLSIn North America, the National Fenestration RatingCouncil (NFRC) has defined the methodology to test aswell as calculate the aforementioned fenestrationperformance metrics. The NFRC has approved a fewcomputer simulation tools for performing thesecalculations (NFRC, 2017a; NFRC, 2017b). Buildingenclosure professionals and design teams rely on thesetools, and are more recently turning to scripting tools toautomate these calculations. This paper references thecomputer software tools outlined in Table 1 below.Note that there are other computer software tools usedfor similar purposes, and the list in Table 1 is notintended to be comprehensive. Further, buildingenclosure professionals also use three-dimensional finiteelement analysis programs, such as HEAT3 andANSYS, to perform thermal simulations of discretecomponents or systems with complex geometry;however, this type of analysis is not covered in thispaper. Similarly, Grasshopper and Rhino are not the onlytools used for this kind of parametric design. Someothers include Dynamo (Autodesk Revit) ija, 2018); however, these tools were notspecifically used to develop the basis of this paper.432 2020 ASHRAE (www.ashrae.org) and IBPSA-USA (www.ibpsa.us).For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted withoutASHRAE or IBPSA-USA's prior written permission.
Table 1: Tools for Partially Automating Fenestration Thermal and Solar Performance CalculationsProgram (Developer)DescriptionThermal and Solar Performance Simulation ToolsTHERM (Lawrence Berkeley Two-dimensional (2D), static heat transfer analysis program used to calculateNational Laboratory, LBNL)component U-factors of fenestration assemblies (NFRC, 2017c).One-dimensional (1D), static heat transfer analysis program used to calculatecenter-of-glazing thermal and solar properties for glazing buildups.WINDOW (LBNL)WINDOW can also be used to calculate effective U-factor, SHGC and VT forsimple fenestration assemblies based on inputs from THERM (NFRC, 2017c).Used to calculate optical properties of glazing layers to be input intoOptics (LBNL)WINDOW (NFRC, 2017c).Automation ToolsRhinoceros(Robert McNeel & Associates)Grasshopper(Robert McNeel & Associates)Honeybee (Ladybug Tools)Excel (Microsoft Office)Referred to as Rhino. Computer aided drafting program (2D and 3D) that canbe used to generate the geometry of fenestration assemblies.Visual programming environment and plug-in to Rhino that is used toautomate parametric calculations of 2D and 3D geometry.Plug-in for Grasshopper that is used to facilitate interoperability betweenGrasshopper and energy simulation programs, including THERM.Programmable spreadsheet tool, which can be used for performing handscripted and macro-enabled calculations.EARLY DESIGN WORKFLOWSIntegrated project delivery and building informationmodeling (BIM) have shown that early collaborationbetween members of the design team as well as betweenthe design and construction teams has a valuable impacton the overall performance and efficiency of theimplementation of the project (Construction UsersRound Table, 2004). Furthermore, some of the mostimportant design decisions that have significant impactson building performance are made at the conceptualstage of a project, such as building massing, orientation,volume, shading, daylight strategies, etc. (Aksamija,2018).Building enclosure professionals do not often usethermal and solar performance simulation tools to informdecisions in early stages of design (e.g., schematicdesign) because information about the enclosure systemsis limited and not refined enough for thermal modelingto be helpful. For instance, the project team may have asense of general rough opening dimensions and mullionsightlines but not have chosen a specific product orfenestration assembly. In this early stage of design,rather than modeling based on assumptions, a designermay use published performance data from fenestrationmanufacturers or industry resources such as ASHRAEHandbook of Fundamentals (Chapter 15, Tables 1 and 4)to provide rough estimates for fenestration performancemetrics.Manual Workflow (Early Design)For the purpose of this paper, the term “manualworkflow” is used to describe calculation procedures inwhich humans must transfer information and databetween individual steps of the process. In early design,the manual workflow procedure is relatively simple, andconsists of the following steps also shown in Figure 1below:1. Reference published literature (e.g., manufacturer’spublisheddata, ASHRAEHandbookofFundamentals) for fenestration performance metrics(U-factor, SHGC, and VT).2.If needed, perform numerical calculations to obtaineffective performance values.Published LiteratureGlazingVLT/SHGCComponentU-FactorsManual EffectiveSHGCEffectiveVLTFigure 1 – Manual Workflow in Early Design433 2020 ASHRAE (www.ashrae.org) and IBPSA-USA (www.ibpsa.us).For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted withoutASHRAE or IBPSA-USA's prior written permission.
Partially Automated Workflow (Early Design)Automating CalculationsArea-weighted calculations can very quickly becometime intensive and are prone to human error. The processoften uses spreadsheet tools such as Microsoft Excel tocalculate effective performance metrics based onmanually input data including fenestration unit sizes andindividual component performance metrics. Theauthor’s firm and others (see Reference Procedure forSimulating Spandrel U-Factors published by theFenestration Association of British Columbia) havedeveloped macro-enabled worksheets that perform thecalculations required to obtain effective thermal andsolar performance metrics. This is one means to partiallyautomate the workflow and introduce efficiency;however, it can still be prone to user error associatedwith manual data input into the worksheet. If not caughtearly on, transfer errors can contribute to inaccuracies indownstream tasks that rely on building enclosureperformance inputs, such as a whole building energymodel or envelope trade-off analysis.Note that while Excel is readily available andprepackaged with in-program scripting capabilities(using Visual Basic for Applications, VBA), buildingenclosure professionals may choose to use differentprogrammable calculators. The author’s firm, forexample, has created a tool using the C# programminglanguage to be compatible with an interactive web-basedplatform for project team members to use (referenceResults Reporting section below).Automation with Parametric Design and ComputerAided Drafting ToolsIn the author’s experience, parametric design andcomputer aided drafting tools, such as Grasshopper andRhino, are well suited for automation and can improveefficiency while analyzing a wider range of designoptions, generally with improved quality control (QC).This process involves assigning key parameters tovarious features of the fenestration system, as shown forexample in Figure 2 below, that the team anticipates maychange as the design develops and are often studiedparametrically.Parametric design studies for building enclosure andfenestration systems most often includes varying thegeometric and material properties. For example,designers may want to understand the impact ofenlarging a vision glazing panel compared to changingthat panel to a shadow box assembly. By integrating thecapabilities of parametric design and buildingperformance simulations, multiple design variables canbe tested rapidly (Aksamija, 2018). The parametricdesign approach is most effective when parameters areprioritized early on by the project team.Zone A: Spandrel1) Vision Glass2) Frit3) Shadow BoxSpandrelHeightIntermediate mullion(only if spandrel isa shadow box)Zone B.1: Vision1) Vision GlassPanelHeightZone B.2: Visionor Frit1) Vision Glass2) FritB-1WidthB-2WidthFigure 2 – Example Fenestration System ParametersThe key change in the workflow is the use of a scriptedsequence, developed in Grasshopper for example, thatcontrols the workflow with limited user interference. Inthis case, the resulting workflow, as shown in Figure 3,includes the following steps:1.Develop a directory (e.g., plain text file or Excelfile) of relevant component performance metrics(e.g., manufacturer’s published center-of-glass Ufactor and frame/edge U-factors from ASHRAEHandbook of Fundamentals).2.Generate geometry in Rhino. In early stages ofdesign, the geometry built in Rhino typicallyconsists of an elevation with general dimensions(e.g., unit width and height, mullion width, framingmember lengths, etc.).3.Use Grasshopper to apply parameters to Rhinogeometry, which can then be manipulated such .4.Develop a script in Grasshopper to perform theanalysis calculations. The script may includesourcing component performance metrics , and exporting results.5.Manipulate parameters and run simulation(s).434 2020 ASHRAE (www.ashrae.org) and IBPSA-USA (www.ibpsa.us).For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted withoutASHRAE or IBPSA-USA's prior written permission.
Published LiteratureGeometryin nsEffectiveSHGCGenerate frame geometry in THERM, manuallyassign boundary conditions, and run THERMsimulation(s).3.For simple fenestration configurations, importTHERM simulations back into WINDOW tocalculate effective performance values (referenceforthcoming Automating Calculations section). Formore complicated fenestration configurations,manually export component U-factors for frame,edge of glazing and center of glazing from THERM,and perform numerical calculations to obtaineffective performance values.ComponentU-FactorsAutomated DataExchangeEffectiveU-Factor2.EffectiveVLTFigure 3 – Automated Workflow in Early DesignBy parametrizing the geometric inputs of the model,Grasshopper can automatically update the area-weightedanalysis based on the user’s preference. In early design,it is also possible to use Grasshopper to evaluate a seriesof non-geometric modifications to the fenestrationsystem (e.g., glazing build-up, thermal properties of theframe, introduction of an opaque element such as ashadow box, etc.) as long as the user provides therelevant component performance metrics. In someinstances, if the proposed system is similar to previouslymodeled systems, then it may be possible to use pastexperience or to use previously calculated componentperformance metrics. However, this should be done withcare, as many factors, including adjacent perimeterconditions and dimensional ratios, can affect fenestrationperformance metrics. These factors can be exploredfurther with the use of computer simulation tools, whichgenerally occurs duringlaterdesignstages(developed ortTHERMManual DataTransferComponent(GlazingU-Factors VLT/SHGC)Manual EffectiveSHGCEffectiveVLTFigure 4 – Manual Workflow in Developed tTHERMDEVELOPED DESIGN WORKFLOWSIn later stages of design (e.g., Design Development andConstruction Document phases), the project team has amore defined vision of the proposed enclosureassemblies, including specific fenestration and glazingsystems. In these phases, building enclosureprofessionals often rely on computer simulation toolssuch as THERM and WINDOW to calculate refined,project-specific component performance metrics.Manual Workflow (Developed design)The manual workflow during developed design, asshown in Figures 4 and 5, includes the following steps:1. Model the glazing assembly in WINDOW, usingOPTICS as required, and manually import it intoTHERM.ComponentU-FactorsManual eVLTFigure 5 – Man
Topics covered include a review of manual and partially automated calculation and simulation workflows, as well as methods for communicating . Referred to as Rhino. Computer aided drafting program (2D and 3D) that can . (2D and 3D) that can be used to generate the geometry of fenestration assemblies. Grasshopper
AAMA 507-15, Standard Practice for Determining the Thermal Performance Characteristics of Fenestration Systems Installed in Commercial Buildings ANSI/NFRC 100-2014, Procedure for Determining Fenestration Product U-Factors ANSI/NFRC 200-2014, Procedure for Determining Fenestration Product Solar Heat Gain
E. National Fenestration Rating Council (NFRC) / American National Standards Institute (ANSI): 1. ANSI/NFRC 100-2014 Procedure for Determining Fenestration Product U-Factors 2. ANSI/NFRC 200-2014 Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence 3.
including the U-factor times the area and C-factor or F-factor times the perimeter. The SHGC requirements for fenestration in Table C402.4 are also met. The total UA proposed and baseline calculations are documented. REScheck or COMcheck is deemed to provide UA calculation documentation. Mandatory 703.1.1.2 Prescriptive R-values and fenestration
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Subp. 21. Projection factor or PF. “Projection factor ” or PF“ ” means the ratio of the horizontal depth of the external shading projection divided by the sum of the height of the fenestration and the distance from the top of the fenestration to the bottom of the furthest point of the exterior shading projection, in
Fundamental Factor Models Statistical Factor Models: Factor Analysis Principal Components Analysis Statistical Factor Models: Principal Factor Method. Fundamental Factor Models. The common-factor variables ff. t. gare determined using fundamental, asset-speci c attributes such as. Sector/
C402.4.3 Maximum U-factor and SHGC. The maximum U-factor and solar heat gain coefficient (SHGC) for fenestration shall be as specified in Table C402.4. The window projection factor shall be determined in accordance with Equation 4-5. PF A/B (Equation 4-5) where: PF Projecfion factor (decimal).
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