Multi-Family HERS Ratings: Air Sealing Between Units
Multi-family HERS ratings introduce challenges that rarely appear in single-family homes. While the modeling process may look similar on paper, the field verification and blower door testing process can quickly expose weaknesses in how units are separated from one another.
In Massachusetts, each dwelling unit in a multi-family building receives its own HERS rating. That means each unit must be tested individually with a blower door to measure air leakage.
When units are not properly compartmentalized, air can move freely between them. During a blower door test, this leakage may appear as air entering from adjacent units instead of from the exterior envelope. The result can be:
Blower door results that are higher than expected
Difficulty meeting Stretch Code or Specialized Code airtightness targets
Confusing test conditions where airflow paths are difficult to identify
From a building science perspective, the goal is simple: each unit should behave like its own airtight box.
Achieving that outcome, however, requires careful planning and execution during construction.
The Core Challenge: Air Leakage Between Units
In multi-family construction, the thermal boundary and air boundary do not always align cleanly. Shared walls, floors, and ceilings introduce dozens of opportunities for unintended airflow.
Common problem areas include:
Rim joists and band joists where framing from adjacent units meets
Plumbing and electrical penetrations through shared assemblies
Top and bottom plates of unit partition walls
Vertical utility chases connecting stacked units
Dropped ceilings or soffits that extend across unit boundaries
When these areas are not sealed carefully, they create pathways that allow air to move between apartments instead of being contained within the tested unit.
From a testing perspective, this can create a frustrating situation: the unit may appear “leaky” even though the exterior enclosure is well built.
Solution 1: Treat Each Unit as a Separate Air Compartment
The most effective strategy for multi-family energy performance is intentional compartmentalization.
Compartmentalization means designing and building each dwelling unit so that air leakage is minimized across shared walls, floors, and ceilings, not just the exterior envelope.
This approach improves:
Blower door test results
Energy efficiency
Indoor air quality
Odor and sound control between units
In practice, it means treating the unit boundary as an air barrier, similar to the way exterior walls are treated in a single-family home.
When compartmentalization is done well, the blower door test becomes much more predictable.
Solution 2: Air Seal the Rim Joist and Band Joist Between Units
One of the most overlooked leakage areas in multi-family buildings is the rim joist or band joist between adjacent units.
Because framing from multiple units often meets in this area, it can create hidden air pathways that connect apartments horizontally.
Best practice is to:
Install solid blocking at the unit boundary
Air seal the assembly with spray foam or sealant
Insulate the cavity after sealing
When left untreated, this location can allow significant airflow between units, particularly during blower door testing.
Blocking and sealing the rim joist area is one of the highest-impact improvements builders can make.
Solution 3: Use the Airtight Drywall Approach at Unit Partition Walls
Another effective strategy is the airtight drywall approach (ADA).
This technique focuses on sealing the drywall to the framing at critical locations so that air cannot move through wall assemblies.
In multi-family buildings, ADA is especially valuable along shared partition walls between units.
Typical details include:
Gaskets or sealant at bottom plates
Gaskets or sealant at top plates
Sealing around door frames located in partition walls
These details are relatively simple to implement but can significantly reduce air leakage between units.
They also have the advantage of being visible and inspectable during construction, making them easier to verify compared to hidden air sealing measures.
Solution 4: Seal All Mechanical, Plumbing, and Electrical Penetrations
Penetrations through floors, ceilings, and walls are unavoidable in modern buildings. However, when they occur at unit boundaries, they must be sealed carefully.
Typical problem areas include:
Electrical wiring through partition walls
Plumbing stacks passing between floors
Refrigerant lines and mechanical piping
Ventilation ducts or bath fan penetrations
Each of these openings can create a direct air pathway between apartments.
Best practice is to seal these penetrations with:
Caulk
Fire-rated sealant where required
Spray foam for larger gaps
Even small openings can contribute to measurable air leakage during blower door testing.
Solution 5: Pay Close Attention to Vertical Chases and Framing Transitions
Vertical chases are another frequent source of leakage in multi-family buildings.
Because these chases often run continuously through several floors, they can connect multiple units if they are not sealed at each level.
Common examples include:
Plumbing chases
Ventilation shafts
Mechanical risers
To maintain proper compartmentalization, these areas should be:
Blocked at each floor level
Air sealed at the unit boundary
Without this blocking, the chase can behave like a vertical air highway between apartments.
Why This Matters for HERS Ratings
For builders working in Massachusetts, these details are not just theoretical—they can directly influence HERS ratings and code compliance.
When air leakage occurs between units:
Blower door tests may show higher ACH50 values
Projects may struggle to meet Stretch Code airtightness targets
Diagnosing the leakage path becomes more complicated
In contrast, buildings that focus on strong compartmentalization typically see:
More predictable blower door results
Better unit-level HERS scores
Improved comfort and air quality for occupants
Ultimately, the goal is simple: each unit should perform as if it were its own airtight home within the larger building.
Achieving that level of performance comes down to careful attention to air sealing details during construction.
