Multifamily Energy Modeling: How It Works and Why It Matters

Energy modeling is the foundation of energy code compliance for multifamily buildings in Massachusetts. It is how you prove your building meets code, how you optimize your design for cost, and how you qualify for incentive programs.

If you are building 4+ units, you need an energy model. Here is how it works.

What Energy Modeling Is

Energy modeling is a computer simulation of how your building will perform thermally. The model takes your building's geometry, orientation, wall assemblies, windows, HVAC systems, domestic hot water systems, lighting, and appliances and calculates how much energy the building will use.

The model compares your building's projected performance against a reference building that represents code minimum. If your building performs better than the reference, it passes. The degree to which it outperforms the reference determines your HERS score and your incentive eligibility.

This is not a rough estimate. A properly built energy model accounts for every unit type, every orientation, every floor level, and every system interaction. On a large multifamily project, the model might include 20 to 40 distinct unit configurations.

Why Multifamily Projects Need It

For single-family homes, there is a prescriptive path: meet certain insulation values, install certain equipment, and you comply without modeling. For multifamily buildings with 4+ units in Massachusetts, the prescriptive path is either unavailable or impractical. Energy modeling is the standard compliance method.

Beyond compliance, modeling is the only way to make informed decisions about your building systems. Without a model, you are guessing at whether a particular HVAC system or window specification will meet code. With a model, you know.

We have seen projects where the architect specified triple-pane windows throughout the building because they assumed it was necessary for code. The energy model showed that standard double-pane windows with a slightly better HVAC system would meet code at $8 per square foot less. That is the kind of decision modeling enables.

The Software and Tools

For multifamily projects in Massachusetts, we use OpenStudio-ERI as the primary modeling engine for feasibility studies and compliance path analysis. For final QA submission and HERS certification, we use Ekotrope, which is a RESNET-accredited platform designed specifically for residential energy modeling.

For commercial areas of mixed-use buildings or projects that fall under ASHRAE 90.1, we use COMcheck for prescriptive compliance and can run full ASHRAE models when the project requires it. Many multifamily projects have both residential units (CMR 22) and commercial common areas that need separate compliance documentation.

The software matters less than the modeler's experience with your building type. Multifamily energy modeling has nuances that generic training does not cover: corridor pressurization, shared wall interactions, elevator and common area loads, central vs. individual mechanical systems, and sampling group definitions.

How Modeling Feeds Into the Compliance Path Decision

The energy model is not just a pass/fail tool. It is a design optimization tool.

When we run a feasibility study, we create multiple energy models of the same building with different system configurations. One model might use ductless mini-split heat pumps with a standard envelope. Another might use ducted central systems with an upgraded envelope. A third might use a centralized VRF system.

Each model produces a HERS score and a compliance result. We then pair those results with construction cost data to find the combination that meets code at the lowest total cost.

This is where the savings come from. On a recent project, we modeled a building with ductless heat pumps and found that all units passed code with a standard envelope. The developer asked what would happen if they switched to ducted central heat pumps because their GC preferred the installation. We remodeled with ducted systems, and half the units in the building failed code, even after adding triple-pane windows.

The model showed exactly why: ducted heat pumps have significantly lower rated heating efficiency than ductless systems (HSPF2 of roughly 8.6 for ducted vs. 10.5+ for ductless, a gap of about 20%). Both systems lose efficiency in cold weather, but ductless starts from a higher baseline. That efficiency gap translates directly into HERS points, and no reasonable envelope upgrade can close it. The HVAC decision determined whether the envelope strategy would work.

Without the model, the developer would have made the switch during construction and discovered the problem at final testing.

ECC's Approach

We model every project we work on, starting with the feasibility study and continuing through construction. Our process:

Feasibility modeling: We create models for 3 compliance path options using your schematic plans. This gives you the data to make system decisions before anything is locked.

Design development modeling: As your plans evolve, we update the model to reflect actual specifications and confirm compliance.

Permit documentation: We prepare the energy model documentation required for your building permit submission.

Construction support: We verify that installed systems and materials match the model assumptions. If something changes during construction, we update the model and confirm the project still complies.

Final certification: After testing, we finalize the model with as-built data and issue the HERS certificates required for your Certificate of Occupancy.