HERS Ratings for Multifamily Buildings: Everything You Need to Know

If you are building a multifamily project in Massachusetts, you almost certainly need a HERS rating. But multifamily HERS ratings work differently than what most people expect from the single-family world, and the differences matter for your budget and timeline.

What a HERS Rating Actually Is

HERS stands for Home Energy Rating System. It is a standardized method for measuring how energy-efficient a building is. A HERS score of 100 represents a code-minimum reference home. Lower scores mean better performance. A score of 0 would be a net-zero building.

For multifamily projects, the HERS rating is not optional. It is how you prove to the building department that your project meets the energy code. The rating is based on an energy model of your building combined with field inspections and testing during construction.

How Multifamily Differs from Single-Family

Single-family HERS ratings are straightforward. One building, one model, one rating.

Multifamily is a different animal. A 200-unit building might have 15 to 40 distinct unit types based on size, orientation, floor level, and exposure. Each unit type needs its own energy model. The number of models required depends on the building geometry, and getting the grouping wrong means either wasted modeling time or failed inspections.

The sampling protocol also differs. Not every unit gets individually tested. RESNET sampling guidelines allow testing a representative sample, but the sample must be chosen correctly. If a sampled unit fails, additional units get pulled into testing, which delays your CO.

This is where experience matters. A HERS rater who primarily does single-family work may not understand multifamily sampling, unit grouping, or the interaction between shared walls, corridors, and common areas.

The HERS Rating Process for Multifamily

Pre-construction: Your HERS rater reviews plans and creates the energy model. This is where the target HERS score gets established and compliance path gets confirmed.

Rough-in inspection: The rater inspects insulation installation, air barrier continuity, and mechanical rough-in. This happens before drywall. If issues are caught here, they are cheap to fix. After drywall, they are expensive.

Final testing: Blower door testing measures air leakage. Duct leakage testing (if applicable) measures HVAC distribution losses. The results must meet the thresholds in the energy model.

Certificate: Once testing passes, the HERS rater issues certificates for each unit. These certificates are required for your Certificate of Occupancy.

What Score Should You Target?

The target HERS score depends on your compliance path and your municipality's code tier. Under Stretch Code, most multifamily projects need to achieve a HERS score of 45 or lower. Under Specialized Code, the targets are more aggressive.

The key insight is that hitting a lower score is not always more expensive. Sometimes a different HVAC system or envelope assembly actually costs less and performs better. That is what a feasibility study reveals.

We have seen projects where switching from one mechanical system to another dropped the HERS score by 8 points while also reducing construction costs. The score and the budget are not always in conflict.

HERS Ratings and Incentive Programs

Your HERS rating directly affects your eligibility for incentive programs like Mass Save. Better-performing buildings qualify for larger incentives, and the HERS certificates are part of the documentation required to claim those incentives.

On a recent 296-unit project, the HERS ratings showed that several buildings were already meeting Energy Star MFNC thresholds even though code did not require certification. By submitting the paperwork for voluntary certification, the developer captured an additional $468,000 in incentives at minimal incremental cost.

The HERS rating is not just a compliance document. It is a tool for finding money.