Tech Talk

Specifying Energy Analysis (gbXML) Settings

gbXML, or the Green Building XML (extensible markup language) schema was developed in the late 1990’s to enable interoperability between building design models and engineering analysis tools. Its use by major CAD vendors and several major engineering simulation vendors, helps to streamline time consuming building takeoffs.

gbXML, or the Green Building XML (extensible markup language) schema was developed in the late 1990’s to enable interoperability between building design models and engineering analysis tools. Its use by major CAD vendors and several major engineering simulation vendors, helps to streamline time consuming building takeoffs. gbXML helps to remove some cost associated with the design of energy efficient buildings, all in the name of helping architects, engineers, and energy modelers design more energy efficient buildings.  A person can define the gbXML information used by third-party energy analysis software. gbXML settings specify the parameter values used by third-party software applications when calculating energy use.

export gbxml screenshot

The following parameters must be specified prior to exporting the building model to a gbXML file for use with an energy analysis application or performing heating and cooling loads analysis:

  • Building Type – specifies the type of building according to the gbXML schema (similar to ASHRAE).
  • Location – identifies the city, and longitude and latitude for the building
    gbxml location map
  • Building Service – specifies the heating and cooling systems for the
  • Building Construction – specifies the type of construction, which determines the materials and insulation (U-values) for the
  • Building Infiltration Class: Specifies an estimate of outdoor air that enters the building through leaks in the building
    NOTE  Infiltration is not exported to gbXML.

    Infiltration can be specified as:

      • Loose – 0.076 cfm/sqft for tightly constructed walls.
      • Medium – 0.038 cfm/sqft for tightly constructed walls.
      • Tight – 0.019 cfm/sqft for tightly constructed walls.
      • None – infiltration is excluded from the calculation of loads.
        gbxml report
      • Report Type: Specifies the level of information provided in the heating and cooling loads report. You can specify Simple, Standard, or Detailed for Report Type.
        gbxml project summary
    • Ground Plane – specifies the level that serves as the ground level reference for the building. Surfaces below this level are considered to be underground. The default level is zero.
      NOTE Ground plane does not affect Heating and Cooling Loads calculations. It is used with gbXML export.
    • Project Phase – specifies the stage of construction (Existing, New Construction).
    • Sliver Space Tolerance – specifies the tolerance for areas that will be considered sliver spaces.
    • Export Default Values – determines whether certain default values will be exported. When checked, the default values for People and Electrical Loads, Occupancy, Lighting, and Power Schedules, and building/space type Construction Types are exported together with all user specified values. When cleared, only user-specified values are exported. Only used with Export gbXML.
    • Export Complexity – specifies the level of detail provided for openings, and whether shading surface information is exported. Shading surfaces are surfaces that are not adjacent to any space, and include surfaces that create a solar obstruction. Only used with Export gbXML. Simple complexity is used for heating and cooling loads analysis.
      • Simple – curtain walls and curtain systems are exported as a single opening (without individual panels). Simple is more appropriate for energy analysis.
      • Simple with shading surfaces – same as simple, but with shading surface information exported.
      • Complex – curtain walls and curtain systems are exported as multiple openings, panel by panel.
      • Complex with shading surfaces – same as complex, but with shading surface information exported. Shading surfaces are not associated with any room/space (roof overhang, free-standing wall).
      • Complex with mullions and shading surfaces – same as complex, but with mullion and shading surface information exported.
        Mullions in curtain walls are exported as shading surfaces. A simple analytical shading surface is produced from mullions, based on the centerline, thickness, and offset.

You can optionally specify information about construction elements, such as windows and doors, and interior and exterior wall, floors slab, roof, and ceiling, construction.
To specify gbXML settings:

  1. Click Manage tab » Settings panel » info icon(Project Information).
  2. In the Instance Properties dialog, click Edit for the Energy Settings parameter.
  3. In the Energy Settings dialog, do the following:
    • For Building Type, select one of the predefined values.
    • For Location, specify the project location.
    • For Project Phase, select Existing or New Construction.
    • For Sliver Space, specify a tolerance value for sliver spaces. All areas that are within the sliver space tolerance are considered sliver spaces.
      NOTE Other sliver space conditions must also be satisfied for an area to be considered a sliver space.
    • Enter optional construction parameter values.
  4. Click OK twice.

About the Author: Bill Johnson

Bill Johnson has spent more than 25 years’ applying MEP & AEC design solutions for large commercial companies, actively developing Autodesk® Revit® implementation strategies, techniques, and procedures for architectural and MEP companies. He is an Autodesk MEP Implementation Certified Expert and is a trusted advisor for many of today’s leading architectural, engineering and construction decision makers. Additionally, Bill has had the advantage of working as Lead Designer for several multimillion-dollar communication sites included structural, electrical, HVAC, conduit, cable plans and equipment layouts – making me an excellent problem solver, with intricate knowledge of BIM workflows and solutions.

Contact Bill Johnson

I have spent more than 25 years’ applying MEP & AEC design solutions for large commercial companies, actively developing Autodesk® Revit® implementation strategies, techniques, and procedures for architectural and MEP companies. My passion for, and deep knowledge of, the industry has lead me to work for multiple, well-respected companies including Autodesk, TEECOM Design Group, GTE/GTEL, Greg LeDoux and Associates, Scottish Power in England, Ideate, Inc. and now AEI. As an Autodesk MEP Implementation Certified Expert, I am proud to act as a trusted advisor for many of today’s leading architectural, engineering and construction decision makers. Additionally, I have the advantage of having been the Lead Designer for several multimillion dollar communication sites which have included structural, electrical, HVAC, conduit, cable plans and equipment layouts – making me an excellent problem solver, with intricate knowledge of BIM workflows and solutions.

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