Which HERS inputs move the score the most on Massachusetts multifamily projects?

On most all-electric multifamily buildings we model in Massachusetts, domestic hot water, air leakage, and window performance are the three inputs with the highest HERS elasticity. A central heat pump water heater plant is often worth 3 to 5 HERS points compared to an electric resistance baseline. Envelope air sealing targets below 0.25 CFM50 per square foot of envelope consistently outperform mechanical upgrades of similar cost.

How much does it cost to run what-if scenarios before we lock the mechanical spec?

Scenario modeling during schematic design is part of the Feasibility Study we provide at no cost, and it is the cheapest time in the project to make these decisions. Once the mechanical bid package is locked, changes cost roughly ten times as much to implement. The right cadence is one modeling pass at schematic, one at design development, and one at permit.

Do I need a high-efficiency ERV for the Specialized Stretch Code or Passive House?

The Specialized Stretch Code does not require a specific ERV efficiency, it requires the building to hit a HERS Index target, and most projects clear that target without a premium ERV. Passive House does effectively require a high-SRE unit, because the annual heating demand limit is tight enough that ventilation recovery becomes a material input. If you are not on a Passive House path, run the scenario before committing to the premium equipment.

Why a Premium ERV Upgrade Did Nothing for Our HERS Score

A design team asked us to model a higher-efficiency ERV upgrade on their multifamily project, expecting a couple of HERS points. The score did not move. Here is where the HERS elasticity actually lives on all-electric multifamily.

April 22, 2026

A design team asked us to model a higher-efficiency ERV on their multifamily project, expecting the upgrade to buy them a couple of HERS points. We ran it. The score did not move, not even half a point. The budget they were about to spend on the premium unit would have bought them a real HERS change somewhere else.

The Real Test

The project is a mid-rise multifamily building using central ERVs serving unit ventilation at code-minimum CFM. The baseline spec was an ERV with 68 percent sensible recovery efficiency at standard rating conditions. The proposed upgrade was a unit at 82 percent SRE, roughly $40,000 more in equipment and controls across the building.

We rebuilt the model with the upgraded unit, held everything else constant, and re-ran the HERS calculation. The Index rounded to the same whole number as the baseline. In the unrounded output the score moved by less than a tenth of a point.

That is not a modeling error. That is the scoring engine telling you the upgrade is not worth the spend.

The Diminishing Returns Math

HERS assigns energy load categories a share of the total index based on modeled consumption. On a tight multifamily envelope with heat pump heating and cooling, mechanical ventilation energy sits at roughly 3 to 5 percent of total annual energy use. Going from 68 percent to 82 percent SRE improves that slice. The whole slice is small.

The rating already credits the presence of heat recovery once the SRE clears the threshold for recognition. The delta between a decent ERV and a premium ERV is a small improvement on a small number. The HERS math reflects that honestly.

On the same project, swapping DHW from electric resistance to a central heat pump water heater plant moved the HERS Index by 4 full points. Tightening the air leakage target from 0.25 to 0.20 CFM50 per square foot of envelope moved it another 2 points. Those are numbers a design team can spend against.

The Scoring Weight Factor

HERS is a ratio of as-designed energy use to a reference home built to a 2006 IECC envelope and equipment package. Each end-use category contributes to the numerator in proportion to its modeled energy. Ventilation, hot water, heating, cooling, and lighting each have their own weight, and the weights shift with climate zone, fuel choice, and building geometry.

On an all-electric multifamily building in Massachusetts, DHW and envelope air leakage carry disproportionate weight, because DHW is a year-round load and leakage drives both heating and cooling penalties. Ventilation is a smaller lever, because the reference home also has mechanical ventilation, so the delta to the proposed design is narrow.

When an ERV Upgrade Still Moves HERS

Premium ERVs earn their spend on certain project types. We have specified them many times. The question is whether the building geometry and compliance path actually reward the extra efficiency.

Passive House projects. PHIUS and PHI both require high-SRE ERVs to hit the annual heating demand target, and the score moves meaningfully because the ventilation share of total energy is higher in a superinsulated envelope.

Projects close to a code ceiling. When the target is a HERS 45 and the design is landing at 46, a fraction of a point from the ERV can be enough to push you across. It is a compliance tool, not an energy strategy.

Buildings with elevated ventilation rates. A project with mechanical ventilation well above code minimum, for example a senior housing building with enhanced IAQ targets, has a larger ventilation load and a larger response to SRE improvements.

The Decision Framework

Model before you spec. Every HERS input has a different elasticity, and the elasticity depends on the rest of the building. A $40,000 ERV upgrade that does nothing on one project can be worth 3 HERS points on the next. The job of the energy model is to tell you which levers are live on your specific project, so the budget lands on inputs that actually move the score.