e-News #93: Improving Title 24 Compliance - CBECC-Com

June 9, 2014
Please rate this resource: 

Download: EDR_eNews_093.pdf
(1.3 MB PDF file)

CBECC-Com: Improving Title 24 Compliance

The California Energy Commission (CEC) was tasked with providing a public domain computer program that allows architects, engineers, and energy consultants to demonstrate compliance with California’s Building Energy Efficiency Standards (Title 24, Standards) that go into effect on July 1, 2014. CBECC-Com, or California Building Energy Code Compliance for Commercial Buildings, is the open-source energy modeling program funded by the CEC to meet that goal. The first certified version of the software was released in September 2013, with ongoing development and new features added with every release since then. The current certified version available for public use is v2.0 and is available for free online and a residential version of the software also is available.

At its core, CBECC is a ruleset-based software to support performance-based compliance analysis. In California, this involves building a simulation model of the proposed building and comparing its annual energy consumption to a budget that represents the energy consumption of a “standard design” – or code-minimum – simulation model. To create the standard design model, the software uses a set of mandatory and prescriptive requirements set by the Title 24 Standards. The rules for compliance with the Title 24 are outlined in the Nonresidential Alternate Calculation Method Reference Manual (NACM), on which the approved CBECC- Com ruleset is based.

Development of the CBECC-Com software for the CEC has been a collaborative effort between many groups. The National Renewable Energy Laboratory (NREL) provided support and updates to its OpenStudio software to facilitate integration with CBECC-Com and provided support for required features. Lawrence Berkeley National Laboratory (LBNL) and Pacific Northwest National Laboratory (PNNL) also provided support and guidance in modeling advanced HVAC systems. PNNL also developed the building model prototypes that are used in the CBECC-Com reference method tests.

The CBECC-Com software provides options for modelling compliance for new construction, additions and alterations, and partial compliance. The software architecture of CBECC also allows for inclusion of additional rulesets for specific analysis. For example, a ruleset based on ASHRAE 90.1 could be developed for analyzing building performance against the ASHRAE Energy Standard. This flexibility is possible because the rules processing portion of the software is independent of the simulation engine, which is the US Department of Energy’s EnergyPlus simulation engine.

CBECC includes a Graphic User Interface (GUI) to define the compliance elements of a building. However the GUI is an independent module of the software, allowing third-party software to interface with CBECC via the software’s Application Program Interface (API). The software also contains a reporting module that generates annual energy budgets and PERF-1 reports from the modeling simulations created.

Nonresidential ACM Reference Manual

For buildings seeking compliance under the performance approach, the building for compliance submittal is modeled with an annual energy simulation and compared against a reference building (called the standard design) that is in conformance with the prescriptive requirements of the Standards. The performance approach gives designers greater flexibility in meeting Standards by allowing them to 'trade off' higher efficiency levels of some building components or systems with lower efficiency levels of other components, provided that the components meet all mandatory requirements. The rules which govern the performance method are documented in the Nonresidential ACM Reference Manual (NACM); this manual also contains guidelines for software developers seeking to obtain certification for their programs for use in Title 24 compliance.

The Alternate Calculation Method (ACM) also has undergone substantial revisions for the recent code update. Historically, DOE-2 has been used as the simulation engine for compliance, and the rules in past versions of the ACM have been tailored to fit the capabilities and limitations of the DOE-2 simulation engine. CBECC-Com fulfilled a need to make both the software and the rules that govern compliance with Title 24 more transparent and less dependent on the energy simulation engine used. New technologies and HVAC systems such as chilled beams and variable refrigerant flow systems are increasing in popularity, and other simulation tools have evolved to incorporate new modeling features. In the current NACM there is no dependence on engineering algorithms and other rules specific only to DOE-2. Compliance software programs need only show that they can pass a series of ruleset implementation tests to demonstrate that the proposed design and standard design buildings are properly created and sensitivity tests to show the accuracy of the simulation against the CEC reference method.

EnergyPlus Simulation Engine


ACM – Alternate Calculation Method

API – Application Programming Interface

CBECC – California Building Energy Code Compliance

CEC – California Energy Commission

CBECC-Com – CBEEC Commercial Compliance Software

GUI – Graphic User Interface

NACM – Nonresidential Alternate Calculation Method Reference Manual

SDD – Standards Data Dictionary

TDV – Time Dependent Valuation

UI – User Interface

Performance compliance modelling in California for Title 24, 2008, was based on the DOE-2.1E simulation engine,which is no longer supported by U.S. Department of Energy (DOE). As a result, no significant updates on modelling features or accuracy have occurred recently. As per the California Long Term Energy Efficiency Strategic Plan, all new commercial construction will be Zero Net Energy buildings by the year 2030. To meet this aggressive goal, it was critical that the CBECC simulation engine be capable of informing and supporting decision making by the code agency. It also is important that the simulation engine used for compliance modeling be able to support building features and emerging technologies capable of reducing energy consumption in California buildings. To this effect, the CEC adopted EnergyPlus v8.0 for developing and maintaining the Standards, beginning with the 2013 code cycle.

EnergyPlus is a publicly-funded software supported by the DOE, Building Technologies Program. EnergyPlus not only incorporates modeling features of DOE-2, but also includes new modelling capabilities that were absent in DOE-2. EnergyPlus simulates loads, systems, and plant generation simultaneously, which is a different approach from DOE-2 where loads and systems were simulated separately. It is expected that adoption of EnergyPlus as the simulation engine for CBECC and Title 24 compliance will enable a broader representation of energy efficient technologies in compliance modelling since EnergyPlus undergoes continuous development for modeling algorithms to incorporate low-energy technologies.

Ruleset Based Software for Compliance Modeling

Although California has a long history of performance-based compliance modelling, the building industry has not been able to benefit from this approach as much as it could. This primarily is because energy models used for code compliance lag behind technological advances in the energy efficiency sector. Also, energy models built to inform design or predict energy savings from energy efficiency measures seldom are integrated with energy models built for compliance. It’s anticipated that adoption of EnergyPlus as the simulation engine for compliance will result in a more integrated and standardized approach to compliance modelling.

A downfall of prior energy models built for Title 24 compliance is that they often need to be modified for use with other programs. This is because the prescriptive and mandatory requirements vary between programs and between building codes, and there are limited software options available for automated standard design generation. Software programs that support the standard design generation often have a proprietary source code, meaning that compliance models generated in one software program may differ from models generated in another software program.This makes it difficult for code agencies and rating authorities to verify and modify the rulesets used in the software. To compound the situation, some energy codes allow for modelers to manually define the baseline or standard design model, but in these cases the quality of the model depends on the modeler’s interpretation of code. As a result, modeling results change between software tools and results also differ when the same building is modelled by different energy modelers. A significant amount of time and effort goes into model generation, documentation, and review of results due to these non-standardized practices. The adoption of open source ruleset-based software will help address some of these issues.

The standardized interpretation of the rules by a code body, such as the CEC for Title 24 compliance, will ensure uniformity in automated standard design models. The addition of more rulesets opens up the possibility for using a single user model for various compliance types. Existing rulesets can be modified to create new rulesets like an ASHRAE 90.1-based ruleset for LEED, rulesets for incentive programs, or rulesets for local code jurisdictions. Addition of rulesets will enable a one-stop modelling solution for multiple modelling purposes. Since third-party software can utilize the compliance analysis features of CBECC via an API, more software tools can include this capability in their product offerings. This can provide users more options when choosing a modeling software.


Software Architecture

CBECC Architecture 01
Figure 1. CBECC-Com software architecture diagrammed.
Image: Architectural Energy Corp.

Figure 1 illustrates the architecture of the CBECC-Com software. The software is modular in design to allow for flexibility and additional functionality modifications in the future.

The Graphic User Interface (GUI) allows for creation of an analysis model in a file format called “SDD XML”. SDD stands for “Standards Data Dictionary”, a collection of data terms and properties that are directly correlated to the Energy Standards, such as Title 24. SDD was developed to allow designers and energy analysts to specify compliance models using data that is relevant to compliance. The inputs provide enough detail to characterize the energy efficiency level of building components, while eliminating the need for entry of building model inputs that are not readily accessible to the designer.

A design goal of CBECC-Com is to encourage third-party software developers to create custom GUIs for compliance that can communicate with the core rules processor of CBECC. The architecture diagram shows that these third-party interfaces can be used instead of the CBECC GUI, provided that they are able to generate a valid SDD XML input file.

Rulesets is the module that defines how the input model should be analyzed according to Title 24 and the NACM. A detailed summary of this module is provided in following sections, but in essence it describes what Title 24 defaults should be used in the analysis, and what the Standard Design (code minimum) building properties are that set the energy budget for compliance. It also ensures that the building design meets the mandatory code requirements. Because the ruleset is modular, it could be replaced with alternate rulesets representing different energy codes, standards, or incentive programs.

The Compliance Engine reads the ruleset and automatically generates the final Proposed Design and Standard Design input files – in SDD XML format – to be used to determine compliance. These two models are communicated to OpenStudio using its Application Programming Interface (API). OpenStudio has the functionality to translate SDD XML to a native EnergyPlus input file. The Proposed Design and Standard Design models are translated to EnergyPlus IDF and then simulated to calculate annual energy consumption and TDV energy.

CBECC Tree 02
Figure 2. A screen capture of the CBECC-Com software architecture.

Once the results are generated, they are returned to the Compliance Engine which compares the results of the Proposed and Standard design simulations. If the Proposed Design results in less Time Dependent Valuation (TDV) energy consumption than the Standard design, then the building is found to comply with Title 24 and the software generates a message stating that the building will “pass”.

If the building passes compliance, then the final step is to generate the Title 24 compliance forms. The forms are generated by an online report generator funded by the CEC which reads through the CBECC input and output data files and automatically formats them into the Title 24 PERF-1 form.

Key Components of CBECC-Com

Graphical User Interface (GUI)

CBECC Envelope 03
Figure 3. Envelope tab, example of spaces and surfaces within a Building.
Image: CBECC-Com User Manual

As detailed an architecture as Figure 1 may imply, CBECC-Com has a simple, straightforward interface. The user is provided with simple screens to plug in information necessary to build the data model. Modeling files from other tools in .xml format also can be opened within the CBECC-Com GUI.

For example, the OpenStudio plugin for Sketchup allows users to export three-dimensional building geometry via the SDD export functionality in the software.

There are two main tabs in the GUI: the Envelope tab and the Mechanical tab. As the names suggest, all non-mechanical attributes of the model are built in the Envelope tab and mechanical attributes in the Mechanical tab. The GUI allows users to choose the compliance type based on the scope of their project. Options include New Construction, Additions and Alterations, and Partial Compliance.

CBECC Envelope 04
Figure 4. Envelope tab, showing construction assemblies from the
material library.
Image: CBECC-Com User Manual

CBECC-Com comes with a library of modeling attributes. For Title 24 compliance, users are required to use modelling inputs prescribed by the NACM for certain cases. For example, only CEC-approved materials can be used to represent the construction assembly layers in the model. The materials library in CBECC-Com contains all CEC-approved materials. Materials can be brought into the model by importing them from the library via the interface. If there is no perfect match for the building material used in the project, the modeler should select the material closest in performance from the material library. The library objects in CBECC-Com also are called in when a prescribed input is required by the ruleset. For California Title 24 compliance, schedules and various internal loads are prescribed by the NACM. CBECCCom allows users to leave such prescribed modeling inputs blank, thereby saving modeling time. During the compliance analysis, the software assigns appropriate values to such fields by pulling them from the library.

CBECC Mech 05
Figure 5. Mechanical tab, showing examples of a Fluid System in tree view.
Image: CBECC-Com User Manual


Organization of the Envelope Tab

The GUI follows a parent-child organization structure. Project is at the highest level of the organizational tree with child components arranged under it. In the Envelope tab, as seen in Figure 2, the child components listed under Project are Building data, Schedules, Construction Assemblies, Materials, Door and Fenestration assemblies, Space Function defaults and luminaires.

Under the Building tree are listed the spaces in the building and the surfaces making up the spaces, as seen in Figure 3. The user can build a set of construction assemblies from the material library for reuse in building envelope components, as shown in Figure 4.


Organization of the Mechanical Tab

CBECC Mech 06
Figure 6. Mechanical tab, showing example for an Air System in tree view.
Image: CBECC-Com User Manual

HVAC systems can be constructed by combining individual HVAC components. The component approach allows for greater flexibility in building a wide range of HVAC system types. Air side components are available under the Building Data and water side components are available under the Project data. Figures 5 and 6 show greater detail on how mechanical systems appear in the

Once the input model is created, the analysis can begin.


The “Perform Analysis” button in the GUI initiates the simulation of the model. Depending on the choice of analysis type set in the model, the data model created in the GUI gets validated and simulated. If there are errors in the data model due to incomplete information or undersized HVAC systems, the simulation aborts and the user is presented with the list of errors. If the model is validated and the simulation is successful, a Results summary window pops up in the GUI. The results summary provides the end use breakdown for site and TDV energy and informs the user if the models “pass” or “fails” compliance. In addition to the summary, detailed modelling reports produced during simulation can be accessed. Various EnergyPlus simulation outputs are available in .xml, .csv, html and pdf formats.


Figure 7. A screen capture of NACM language converted to software rules, specifically for roofs.

The NACM contains building descriptors required to build a compliance model. For each building descriptor, the NACM provides rules for transforming a user model into a proposed and standard design model. The NonRes Ruleset in CBECC-Com consists of rule blocks for each building descriptor in the Standards Data Dictionary. Each rule block is a logical conversion of the ACM text into a syntax that can be processed by CBECC-Com. Figure 7 shows an example of NACM language converted to software rules for user model, Proposed Design, and Standard Design models.

Report Generator

The report generator collects data from the data model and analysis results and automatically populates the Title 24 performance reports. As described earlier, the summary of results can be viewed in the UI after a successful simulation. In addition to the simulation results, a PERF-1 report is generated for compliance documentation. The report generated is a separate module; therefore, reports could be added or customized
to support other energy codes or programs.


CBECC Workflow 08
Figure 8. CBECC-Com workflow for nonresidential compliance analysis.
Image: CBECC-Com User Manual

As shown in Figure 8, the basic steps in CBECC-Com include:

  • Creating User Model by entering information in UI.
  • Launch Compliance Analysis- CBECC-Com generates the Proposed and Standard Design Models in SDD XML format.
  • OpenStudio translators convert the SDD XML files into EnergyPlus IDF files
  • The IDF files get simulated in EnergyPlus v8.0. The simulation engine runs the Proposed Design Annual Simulation, Standard Design sizing simulation, and Standard Design Annual Simulation.
  • Results are retrieved and presented in a results summary Screen in the CBECC-Com UI.

Building Geometry in CBECC-Com

There are two options to create building geometry in the user model – the detailed geometry method or the simplified geometry method. The detailed Geometry method allows users to build their building geometry in the Sketch up and convert graphical 3D model into an energy model utilizing OpenStudio plugin. The modelling steps to follow in this tool are: tracing the building floor plan, generating building stories and spaces, and drawing surface attributes like doors, windows, skylights, and wall types. After building the geometry, the model is exported to an SDD XML format via OpenStudio plugin.

For the simplified geometry method, the building geometry is created within CBECC-Com’s GUI. In this method, the user can create attributes of building geometry by entering characteristics that define these objects. No reliance on graphic drawing tools is required for this method. The modeler will take detailed information form the construction drawings to define building geometry components. For example, required inputs for walls are area and azimuth; for roof area, azimuth and tilt are required.

There are instances when the simplified geometry approach may be well-suited for a given project. Users should understand the pros and cons of the both geometry representation approaches before choosing the one most apt for the project. As the name suggests, the simplified geometry approach is simpler than the detailed approach. With the simplified approach, spatial relationship between various surfaces cannot be defined, hence software features like daylighting and credits for daylighting controls and shading from shading devices are not available for Simple Geometry. Projects using the Simplified Geometry must still meet all applicable mandatory and prescriptive daylighting requirements, even though daylighting is not modeled.


The list of features supported by CBECC-Com v2 is available in the Quick Start Guide. An additional version is planned for release in July 2014 that will include several new HVAC modeling features.

CBECC-Com Training Materials (and more information)

Guides and Manuals

The Quick Start Guide and User Manual are available in the “Help” section of the main menu bar of the software’s user interface. The Quick Start Guide is a good starting point for a novice user providing an overview of the software. The document lists capabilities and limitations of the software and enhancements in each release version. It also includes lists of example files that can guide a user in assembling various building and HVAC system types in the software. The sections on various menu bar items can provide a feel for the layout of the interface and how building attributes can be accessed and edited by the user. The User Manual provides detailed information for using CBECC-Com. The document is divided into several sections with each section providing detailed topic-specific information. It is recommended that users review the various sections relevant to the building model to get a better understanding of modelling workflow.

Online Training Videos

A series of short training videos are available online. Each of these videos walks users through various attributes of the software. The videos are available to be viewed online or downloaded for offline viewing via a media player.

CBECC-Com Training Sessions

California utilities have developed a free, two-day training on CBECC-Com. The training is “hands on” with a goal of ensuring that attendees become comfortable developing Title 24 compliance models from start to finish. The CBECC-Com 2013 Title 24 Nonresidential Compliance Software Training will be held at utility energy education centers.

Southern California Edison – Irwindale, CA
June 25-26 (Wednesday–Thursday)

Sacramento Municipal Utility District – Sacramento, CA
July 16 ( Wednesday)

PG&E – San Francisco, CA
June 18-19 (Wednesday–Thursday)

San Diego Gas & Electric – San Diego, CA
June 16-17 (Monday–Tuesday)

About e-News

Don't miss future issues - to sign up for a free email subscription, please visit our newsletter subscription page. Send letters to the editor, suggestions on topics for future issues, or other comments to the e-News editor via our Comments & Feedback form.

e-News is published by Energy Design Resources (www.energydesignresources.com), an online resource center for information on energy efficiency design practices in California.

Savings By Design (www.savingsbydesign.com) offers design assistance and incentives to design teams and building owners in California to encourage high-performance nonresidential building design and construction.

Energy Design Resources and Savings By Design are funded by California utility customers and administered by Pacific Gas and Electric Company, Sacramento Municipal Utility District, San Diego Gas and Electric, Southern California Edison and Southern California Gas Company, under the auspices of the California Public Utilities Commission.


Download: EDR_eNews_093.pdf
(1.3 MB PDF file)

 Earlier Comments

by , at