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.
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.