ASHRAE 90.1 Appendix G is one of the most important methods in commercial building energy modeling. It is used for LEED energy credits, above-code performance evaluation, utility programs and, in newer editions of ASHRAE 90.1, performance-based code compliance.
At its core, Appendix G asks one question: how much better is the proposed building than a standardized baseline building? That sounds simple, but the details matter. A small baseline modeling error can change the reported savings, LEED points or compliance result.
What is ASHRAE 90.1 Appendix G?
Appendix G is the Performance Rating Method in ASHRAE 90.1. Instead of checking each envelope, lighting and HVAC requirement one by one, the modeler creates two whole-building simulations:
- The proposed building model: the actual design, including its envelope, HVAC, lighting, controls, schedules and systems.
- The baseline building model: a standardized reference building created from ASHRAE 90.1 rules.
The proposed and baseline models are simulated with the same weather file, energy rates, operating assumptions and simulation program. The performance difference is then used to calculate energy-cost or performance improvement.
Baseline model vs proposed model
The proposed model represents what the design team intends to build. It should reflect the real geometry, orientation, envelope assemblies, glazing, lighting power, HVAC system, service water heating, controls, renewables and regulated energy systems.
The baseline model is not simply the same building with worse equipment. It is a rule-based reference building. Appendix G defines how the baseline envelope, HVAC system type, lighting power, fan power, equipment efficiencies and other inputs must be assigned.
| Item | Proposed Building | Baseline Building |
|---|---|---|
| Geometry | Actual design | Same basic geometry and floor area |
| Envelope | Actual U-values, SHGC and assemblies | ASHRAE-defined baseline envelope |
| HVAC | Actual system and controls | System type selected by Appendix G rules |
| Lighting | Actual lighting design and controls | Allowed lighting power density and baseline controls |
| Schedules | Standardized project schedules | Same schedules, unless rules require otherwise |
Why the baseline model is so sensitive
The baseline model often drives the final savings result more than people expect. Common issues include:
- Wrong baseline HVAC system selection
- Incorrect window-to-wall ratio treatment
- Incorrect envelope U-values or SHGC
- Mismatched thermal zoning
- Inconsistent schedules between baseline and proposed models
- Incorrect lighting power density assumptions
- Fan power and pump power errors
- Unmet load hours that distort results
When the baseline is too weak, savings are overstated. When the baseline is too strong, a good design may appear worse than it is. A defensible baseline is therefore essential for LEED reviewers, code officials and incentive programs.
How Appendix G supports LEED
For LEED BD+C projects, energy modeling is often used to demonstrate minimum energy performance and optimize energy performance credits. Appendix G provides the comparison framework: baseline versus proposed.
The proposed design earns credit when it performs better than the ASHRAE-defined baseline. Measures such as improved envelope, lower lighting power, high-efficiency HVAC, heat recovery, better controls, reduced fan energy, daylight-responsive lighting and renewable energy can all contribute to performance improvement.
Appendix G and code compliance
Historically, Appendix G was widely used for rating above-code performance. In later ASHRAE 90.1 editions, it became more directly usable as a compliance path. This matters because it creates a more consistent modeling framework for codes, LEED, utility programs and performance-based design.
The model still has to follow the specific ASHRAE 90.1 edition required by the authority having jurisdiction, LEED rating system or owner requirement. (See our guide to ASHRAE 90.1-2016 vs 90.1-2019 for how the editions differ.)
What project teams should provide
To build a reliable Appendix G model, the energy modeler needs:
- Architectural drawings and area schedules
- Envelope U-values, insulation, glazing and SHGC
- HVAC system type, efficiencies, controls and ventilation rates
- Lighting layouts or lighting power densities
- Occupancy and equipment schedules
- Service hot water system details
- Renewable-energy system details, if any
- Project location and applicable code version
- LEED or compliance pathway requirements
Better inputs mean fewer assumptions, fewer review comments and a stronger final submission.
The takeaway
ASHRAE 90.1 Appendix G is not just a software workflow. It is a structured comparison between a real design and a standardized baseline building. The value of the model depends on how carefully both sides are built, documented and reviewed.
For architects, MEP engineers, developers and LEED consultants, a clean Appendix G model can show where the design performs well, where it loses energy, and how to target improvements before submission.
Why Appendix G needs careful documentation
Appendix G results are only as strong as the documentation behind them. Reviewers need to understand how the baseline was created, how the proposed design was represented, and why each major input is defensible. Good documentation reduces comments and protects the credibility of the savings claim.
Appendix G workflow
- Confirm the applicable ASHRAE 90.1 edition and compliance or rating path.
- Build the proposed model from the real design documents.
- Create the baseline model using Appendix G rules.
- Check unmet load hours, schedules, zoning and end-use results.
- Prepare the model narrative and reviewer-ready documentation.
Common Appendix G mistakes
Typical mistakes include selecting the wrong baseline HVAC system, misapplying window area rules, using inconsistent schedules, overlooking process loads and failing to explain unusual end-use results. These issues can change the final percent improvement and trigger review comments.
What a strong deliverable should include
A useful analysis should not only give a pass/fail result. It should explain the assumptions, show the major inputs, summarize the result clearly and identify what the design team can do next. For technical buyers such as architects, MEP engineers, LEED consultants and owners, the value is in decision support as much as final documentation.
- A short executive summary written for the design team, not only the reviewer.
- Clear input assumptions for geometry, envelope, HVAC, lighting, schedules and loads.
- Key outputs organized by end use, zone, system or compliance metric.
- Identification of the measures that move the result most strongly.
- Reviewer-ready documentation with traceable model inputs and supporting notes.
QA/QC before submission
Most modeling problems are not caused by the simulation engine. They come from mismatched areas, incorrect schedules, missing system assumptions, unrealistic internal loads, or outputs that were never sanity-checked. A professional workflow should include a QA/QC pass before any report is submitted.
Typical checks include comparing modeled floor area with drawings, reviewing peak loads, checking unmet hours, confirming end-use breakdowns, reviewing envelope and system assumptions, and making sure the final narrative matches the submitted design documents. This is where a model becomes defensible.
How project teams should use the results
The best use of analysis is not simply to file a report. The results should guide design choices. If one end use dominates, the team can target that system. If comfort or daylight risks appear in specific zones, the team can revise glazing, shading, controls or HVAC strategy before construction documents are complete.
For owners and developers, this also creates a clearer business conversation: which measures are required for compliance, which improve certification outcomes, which reduce operating cost, and which support long-term decarbonization goals.
FAQ
Is the baseline a typical building? No. It is a standardized reference building defined by ASHRAE 90.1 rules.
Can Appendix G be used for LEED? Yes. It is central to many LEED whole-building energy modeling workflows.
Can two modelers get different results? Yes, if assumptions differ. That is why documentation and QA/QC matter.
Related DOE Modellers articles
Need an Appendix G model for LEED or code compliance?
We prepare ASHRAE 90.1 Appendix G baseline and proposed energy models for LEED, energy-code compliance, optimization and submission documentation. Let's talk about your project.
Get in touchThis article is general guidance, not a substitute for the published standard or advice for a specific project. Always verify requirements with the standard and the authority having jurisdiction.