“My apartment stinks of garlic and I haven’t cooked in days!”

“Why can I smell the garbage chute in my bedroom?”

“If I have to smell Jimmy’s cigarettes one more time…!”

These are among the most frequent odour complaints building managers hear from residents plagued by unwanted scents creeping into their units.

Along with the benefits of a condominium lifestyle — urban living, fun amenity spaces, a diverse community of neighbours — come the pitfalls of a living space intimately shared with dozens of strangers — loud noises, boisterous kids and pets, or unwanted smells. For many, the odour problem is the most egregious. When foreign, and in some cases offensive, odours waft their way into one’s home, it can feel like a particularly intimate invasion. The negative emotional response can be significant.

Odour migration between interior spaces requires an open path of air travel (a door, a duct, a hole in the wall, etc.) and a pressure difference between the two spaces. Although building systems are designed to prevent unwanted air movement, there are often leaky areas that lead to unintended air and odour transfer.

Here are seven common causes of unintentional air movement through condo buildings:

1. Low-pressure corridors

In most residential buildings, fresh outside air is supplied to the corridors via a central make-up air unit, pressurizing the corridors. This allows fresh corridor air to flow into suites through gaps around doors and through the doors themselves, when opened. If the make-up air unit is off, malfunctioning, or undersized, the corridors are not sufficiently pressurized. This can unintentionally allow air to move in the opposite direction (suite to corridor) and then find its way into adjacent suites or common areas.

2. Unbalanced corridor air supply

The fresh air supply from the make-up air unit flows to the corridors in a central duct. Dampers at each branch are adjusted at construction to ensure the correct air volume is delivered to every floor. Over time, the system gets out of balance — dampers can shift; in some cases, residents will manually close the dampers off to stop “the cold air flowing into my suite from this vent.” If a floor’s fresh air flow is diminished due to balancing issues, suite odours may migrate into corridors, then into neighbour’s suites.

3. Wind-driven air movement

Exterior wind pressures are typically higher than those generated by the building mechanical systems. When windows or balcony doors are poorly sealed, or if there is an improper construction in the exterior wall assembly, exterior wind pressures can drive air inwards. The outdoor air may be carrying the smells of the garbage bins, cigarette smoke from the neighbour’s balcony, or kitchen/bathroom/laundry exhaust from surrounding suites.

4. Improper or missing smoke seals

While fires are difficult to prevent, there are strict construction requirements aimed at limiting the spread of flames and smoke. This applies especially to walls and floors between units. Sealant is required to keep smoke contained. A secondary benefit is that smoke seals also help to contain suite-to-suite odours. Mistakes can and do get made, however, and smoke seals may be missing or incomplete, resulting in odour paths. Ducts and conduits within fire-rated floor and wall assemblies can be especially challenging.

Another common odour transfer area due to poor smoke sealing is the often tricky interface at the vertical joint between an interior fire-rated wall and an exterior wall. Concrete walls are relatively simple to seal, but when wall construction consists of steel studs or window walls, it is increasingly difficult to ensure an airtight seal.

5. Elevator shaft air movement

As elevator cars shuttle up and down their concrete shafts, their piston-like action pushes and pulls large volumes of air along with them. This can cause unintended floor-to-floor air flows. For example, air from the garbage room may be sucked into a nearby elevator shaft and distributed to other floors. As elevator door seals wear, unintended air leakage will become more pronounced.

6. Intake and exhaust air short circuiting

Part of maintaining good indoor air quality is exhausting noxious air from kitchens, bathrooms and garbage rooms through the walls or roof to the outside. If the building exhaust air outlets are near windows, doors, or the fresh air intakes to make-up air units, there is risk of short circuiting: odour-laden exhausted air is drawn back into the building and re-circulated as fresh air.

7. Missing backdraft dampers

Suite kitchens and bathrooms are typically exhausted by individual exhaust fans that discharge through vents in the exterior walls. Backdraft dampers are installed in the exhaust ductwork to prevent reverse airflow and ensure the air is traveling in a single direction. If the dampers malfunction, exterior air can be drawn into the building through the exhaust ducts.

Making a diagnosis

While there are many possible causes of odour migration, they are all related to unintended air flows. That said, locating the errant airflow and correcting odour problems is notoriously difficult to do. Add in the fact that they are often transient — “Well, it only happens on Wednesdays at 3 a.m.” — and that people have significantly varying olfactory acuities — “Can’t you smell that? It’s awful!” — and the problem can get downright immovable.

Even with a strong understanding of building systems, solutions are unlikely to be obvious and it may take several measures to determine the origin of the issue. Here are a few common diagnostic steps:

Interview the complainant to put brackets around the problem. In other words, ask: What is the smell? Where is it strongest? When it occurs, who else notices? How long has it been a problem?

Review the condition and performance of the mechanical system, paying particular attention to the makeup air unit in the corridor. Measure the mechanical system airflows both subjectively and quantitatively. Also review the architectural drawings to understand system vulnerabilities.

Use smoke pencils or theatrical fog to illuminate airflow paths. Make selective openings in the drywall to check concealed smoke seals in vulnerable locations. Do blower door testing, which involves installing a large fan on the door of a suite to pressurize or depressurize the space. The goal is to induce air to flow across the breach, making it easier to identify via smoke pencil.

With skill, time and maybe some luck, the problem should be correctable, or at least reducible to a tolerable level. Some odour issues are either intractable, slight or infrequent, in which case it may not be worth the effort and cost to eliminate them completely. Rarely can air-tight perfection be guaranteed. Besides, the benefits still outweigh the risks in living within shouting, and smelling, distance of one’s neighbours.

Dylan Haber is a project manager in the property condition assessments group at WSP Canada Inc. A mechanical engineer by training, his experience includes performance audits and reserve fund studies for condominiums as well as property condition assessments for various building types, including residential, industrial, commercial and retail. He can be reached at dylan.haber@wspgroup.com.