What Is Dwelling Unit Compartmentalization?

Sealing each dwelling unit against its neighbours, the corridors and the shafts — then testing it. The 2025 code lowered the tested leakage value for all multifamily units, with garden-style units excepted.

Sealing between units, not just to outside

In a house, airtightness means sealing the envelope against the weather. In an apartment building the more interesting boundary is internal — the walls, floors and ceilings between one unit and the next, and between units and the corridor.

Compartmentalization treats each dwelling unit as its own enclosure. It is confirmed by a leakage test on the unit rather than on the building, which is what makes it a different exercise from a whole-building air leakage measurement.

Why it matters more than it sounds

Air moving between apartments carries everything in it. Cooking smells, tobacco smoke, pet allergens and noise all travel the same leakage paths. A leaky demising assembly means residents are breathing air conditioned — and used — by their neighbours.

There is a stack effect problem too. A tall building behaves like a chimney, drawing air in low and pushing it out high. Where units are poorly sealed to the shared corridors and shafts, that pressure difference drives continuous air movement between floors regardless of what any ventilation system is doing.

And it is what makes controlled ventilation work at all. A supply or balanced system can only deliver air from where it intends if the unit does not have larger uncontrolled openings elsewhere.

The 2025 change

The tested leakage value was lowered for multifamily units — a tighter target than the previous cycle required.

Garden-style units are excepted from that lowered value. The reason is geometric: a garden-style unit typically has its own exterior entrance and far less shared surface with neighbouring units, so the problem the requirement addresses is much smaller. It also has no stack effect across many floors to fight.

Where the leakage actually is

Rarely the drywall. It is the penetrations — plumbing stacks, electrical boxes, ducts crossing demising walls, the gap around a bathtub, recessed fittings, and the interface where a demising wall meets the floor slab or the ceiling plane.

Every one of those is created by a different trade, and none of them is anybody's designated responsibility unless somebody makes it so. That is the practical difficulty with compartmentalization: it is not a product you specify, it is a sequence of details several trades have to get right.

It has to be tested, so it has to be sequenced

Because compliance is demonstrated by a test on the unit, the sealing has to be complete and the unit finished enough to test — but any failure has to be findable and fixable. A unit that fails after finishes are on is expensive to chase.

Projects that succeed here test a first unit early, find the systematic leaks, and fix the detail for every unit after it. Projects that test everything at the end find the same leak multiplied by the unit count.