Why does moving heat pump water heaters into the corridor improve the HERS score?

A heat pump water heater cools the air it sits in. Inside a unit that cooling lands on conditioned space the HVAC is paying to heat, and the model counts it against the unit. In the common corridor the load lands outside the rated units, so each unit sheds it and the model credits the improvement. On the 36-unit project we modeled, that was up to 10 percent.

How much envelope can corridor water heater placement save?

On the 36-unit project, the headroom from corridor placement let the team drop triple-pane windows to standard double-pane, remove continuous exterior insulation entirely, and cut full underslab insulation to a minimal slab perimeter detail. The dollar value scales with the unit count and the premium that was in the envelope budget.

Does the corridor hack work on every multifamily building?

No. The corridor needs the space and ventilation to host the equipment, the envelope has to be carrying premium items worth stripping, the mechanical layout has to support the relocation, and the compliance path has to credit where the load lands. When those do not line up, the headroom shrinks or the construction cost eats the savings.

Is relocating water heaters to the corridor allowed under the Massachusetts Stretch Code?

The Stretch Code is a performance path, so what matters is that the building hits its HERS target with the equipment as modeled. On this project the corridor placement was part of a compliant model that cleared code on a stripped envelope. The placement has to be coordinated with the plumbing and mechanical design, not just the energy model.

Why Moving Heat Pump Water Heaters Into the Corridor Stripped the Envelope Budget

On a 36-unit Massachusetts project, putting heat pump water heaters in the corridors gained up to 10% HERS headroom and dropped triple-pane windows and CI.

June 01, 2026

If you are trying to find out how lean your building envelope can get before it fails the Massachusetts Stretch Code, the answer usually starts with the expensive items. Triple-pane windows. Continuous exterior insulation. Full underslab insulation. On a recent 36-unit project, we cleared all three off the budget without failing compliance. The move that paid for it was relocating the heat pump water heaters into the common corridors.

The Real Project

The team came in with a clear question. How lean can the envelope get before the building fails code. They assumed the answer required the premium envelope. Triple-pane windows, continuous exterior insulation, and full underslab insulation were all in the budget as mandatory items.

We modeled the building with the heat pump water heaters relocated from inside the units to the common corridors. That single change lifted the units by up to 10 percent in the model. That headroom was the entire budget for the envelope downgrade.

With the corridor placement in the model, the building cleared code on a stripped envelope. Standard double-pane windows in place of triple-pane. Zero continuous insulation. Minimal slab perimeter insulation in place of a full underslab layer.

On a 36-unit building, the difference between the premium envelope and the stripped envelope is real money. None of it shows up until the model runs with the water heaters in the corridor.

Why Corridor Placement Moves the Model

A heat pump water heater makes hot water by pulling heat out of the surrounding air. That is why it is efficient, and it is also why its location matters in the energy model. Wherever the unit sits, it cools that air and adds a load that the space has to make up.

Inside a dwelling unit, that cooling effect lands on conditioned space. In the heating season the water heater is pulling heat out of a room the HVAC system is paying to keep warm, and the model counts that interaction against the unit. The efficiency of the appliance is real, but part of it is eaten by the space it sits in.

In the common corridor, the picture changes. The corridor is a shared space with a different conditioning profile, and the cooling effect from the water heaters lands there rather than inside the rated units. The dwelling units shed the load they were carrying when the equipment sat inside them, and the model credits each unit with the improvement. On this building that was up to 10 percent.

That 10 percent is headroom. The HERS target did not move, so every point the corridor placement bought was a point the envelope no longer had to deliver. That is what let the triple-pane windows, the continuous insulation, and the full underslab layer come off the budget.

When the Corridor Hack Does Not Pay Off

Corridor placement is not a free win on every building. Several things have to line up.

Corridor space and ventilation have to support the equipment. The common corridor needs the room and the air volume to host the water heaters and carry the cooling effect. A tight corridor without the air to give up does not deliver the headroom.

The envelope has to have something worth trading. The hack pays off when the budget is carrying a premium envelope that can be stripped back. On a building already specified to a lean envelope, there is nothing to downgrade and the headroom has no budget to release.

The mechanical layout has to support the relocation. Pulling the water heaters out of the units changes the plumbing runs and the mechanical coordination. When the layout fights that move, the construction cost can eat the envelope savings.

The code path has to credit the placement. The improvement shows up because the model credits where the load lands. On a different compliance path or a different rating the credit can be smaller, and the headroom shrinks with it.

The Decision Framework

The takeaway on this project is that a mechanical decision can buy an envelope budget. The water heater location looked like a plumbing and coordination question. In the model it was an envelope question worth standard windows, zero continuous insulation, and a minimal slab detail on a 36-unit building.

The conventional approach treats the envelope and the mechanical systems as separate budgets that get value-engineered on their own. They are not separate in the model. A point bought on the mechanical side is a point the envelope does not have to spend, and the cheapest path to the HERS target is usually the one that moves points between the two.

Every building trades differently. The corridor that has the air to host the equipment on one project is too tight on the next, and the envelope that has premium items to strip on one building is already lean on another. The only way to know whether the corridor placement pays off on a specific building is to model it with the equipment in both locations before the envelope budget is set.