Resolving Odour Transfer Issues in Condominiums Using Logical Sequencing

Since recreational cannabis was legalized throughout Canada this fall, engineers are increasingly being asked how to resolve unintended odour transfer in condominiums. Leading up to October 17th, 2018, there seemed to be much concern and confusion with Boards of Directors scrambling to institute rules or by-laws in order to limit or manage the use of cannabis in their communities. Ultimately that concern may have been similar to Y2K, given that people were already smoking in their units before cannabis legalization, and have continued to do so thereafter.

Although the smell of cannabis is the current concern, countless odours move unwanted through a building. Building staff and professionals alike can learn to understand the principles of odour movement and how to best investigate them. This article does not cover the cause of every odour transfer issue, but it does lay out a logical sequence of steps to properly diagnose what might be facilitating odour movement within your condominium.


Step 1: Gather Information

Much of the investigation relies on information available before the investigation begins in earnest: owner reports, building drawings, and documents.  At the onset of the investigation the following questions should be reviewed:

  • Are there building drawings available (Architectural and HVAC)?
  • Are the odours experienced inside the units or in the corridors?
  • Where in the unit or corridors are the odours noted?
  • Can a specific odor originating unit be identified?
  • Will the occupants of the reporting and odour originating units cooperate with the investigation?
  • Do the occupants of the odour originating unit create odours in only one section of their unit (e.g., do they smoke only in the living room?)
  • What time of day are the odour transfers noted?
  • What time of year are the odour transfers noted?
  • What are the patterns of behaviour in the reporting and originating units?
  • Do the occupants leave their windows open?
  • Do the occupants turn on the exhaust fans, and when?
  • Have secondary weather-stripping or door sweeps been installed on their doors?
  • Are the odours noted only when high winds are present?
  • Are the odours noted only when adjustments are made to the building mechanical systems (Seasonal turn over)?
  • Are the units served by a central exhaust system?
  • What sizes are the individual exhaust systems and where are they located (kitchen, bathroom)?
  • Have the exhaust systems been cleaned or serviced recently?
  • Does the make-up air unit run continuously?
  • When was the make-up air system last serviced?
  • When was the make-up air system last balanced?
  • Is there a history of condensation inside the building?

The responses to these questions permit investigators to plan their course of action. If investigators do not begin with these questions, they are setting themselves up for failure.

Step 2: Overall Building Airflow

Odours consist of molecules that float and are carried by air movement. Outdoors, air (and the odours it carries) is moved by the wind – the neighborhood BBQ smells great downwind. In buildings or occupied spaces, however, air moves from high pressure areas to low pressure areas.

The movement of air in high-rise buildings is affected by man-made mechanical systems, building envelope openings, and seasonal effects, including:

  • Mechanical systems:
    • Make-up air units
    • Distribution louvers and ducting
    • Unit exhaust fans (bathroom and kitchen)
  • Building envelope openings:
    • Windows/Doors
    • Envelope deficiencies
  • Seasonal effects:
    • Winter – buildings are heated and the warm air rises, creating a HIGH pressure at the top half of the building and a LOW pressure in the bottom half of the building
    • Summer – Buildings are cooled and the cold air sinks, creating a HIGH pressure in the bottom half of the building and a LOW pressure in the top half of the building
  • Warmer months – windows more likely to be open

In common high-rise ventilation, fresh air is provided to the units by way of the rooftop make-up air unit pumping air to the corridors, which then feed the units through a passive vent near the door or in openings around the door. This make-up air creates a high-pressure zone in the corridor preventing smells from the lower pressure units from entering the corridors.

This pattern of high and low pressure can break down if the building-wide ventilation systems are compromised or operating poorly.  Problems could include:

  • The make-up air unit is turn off or cycled down too far, removing positive pressure from the corridors
  • The make-up air unit is old, undersized, or not serviced, limiting the amount of air provided to the corridors
  • The system is unbalanced, distributing air flow improperly to different floors

If building wide systems are off-kilter, no unit specific approach will be able to fully address an odour transfer issue.

Step 3:  Unit Specific Airflow

After confirming that air is getting into the corridors properly, it is necessary to ensure it has unobstructed access to the unit. Typically, air is brought into the unit through openings around the doors, or passive wall vents adjacent to the doors. Air is then exhausted through fans in the kitchen and the bathrooms:

These unit specific components can fail, become obstructed, or be used in ways that compromise the air flow and design pressurization thus allowing air and odours to move in unplanned directions.

  • Door sweeps and jambs may have been blocked with weather stripping
  • Passive air vents may be dirty or blocked
  • Exhaust fans may be dirty or blocked (see if a piece of paper will stick to the vent when it is turned on)
  • Exhaust fans may be old and inefficient
  • Exhaust fans may be misused (if the occupants of the unit smelling the smoke turn on their exhaust fan, they will decrease the relative pressure in their unit, increasing the intensity of the smell)

In townhouse units, and specialty high-rises, units may get their fresh air, not from a common Make-Up Air unit, but from a dedicated Heat or Energy Recovery Ventilator (HRV/ERV).  As in the case of the make-up air unit, if the HRV/ERV is not functioning properly, the unit’s pressurization will not meet design levels, probably creating unexpected air pathways and odour transfers.


Step 4: Fog Testing to Determine Compartmentalization Failures

After possible building-wide and unit-specific air flow problems have been eliminated or resolved, investigators should then begin to look for defects or deficiencies in the separations between units. Units are typically compartmentalized. When a penetration, such as a pipe, pierces this compartmentalization, we expect it to be properly sealed. When compartmentalization is compromised, the possibility of odour transfer increases.

Many possible pathways and causes of odour transfer exist. Rather than aimlessly making holes in walls before the source of the problem is accurately identified, investigators should identify problem areas using a fog test. Investigators perform a fog test to recreate the odour transfer path (visible fog will behave like invisible odours and move with the air flow). Blower doors may be used to depressurize specific areas to promote fog transfer.

By depressurizing the areas where the smells are reported and creating a detectible fog in the suspected source area, investigators can identify trouble spots by visually tracking the fog transfer. The most common trouble areas are:

  • Gaps around electrical plugs in shared walls
  • Gaps around pipe penetrations beneath sinks and bathtubs
  • Complex air/vapour barrier interfaces in shared walls


Step 5:  Exploratory Openings

Only after fog testing has confirmed suspected areas of failed compartmentalization should holes in drywall be made to locate and correct deficiencies. To yield the best result, investigators should make exploratory openings that are:

  • Centralized at areas of concern or confirmed by fog test
  • Not located solely  at reported odour sites

Sufficiently large to identify any deficiency Limited in number

It is important to remember that some openings may not yield obvious results. At the exploratory openings step, it may be determined that a single or multiple compartmentalization failures are causing odour transfer. Once the defect has been identified, sealing solutions are usually straightforward:



Resolving odour transfer is an iterative process. At every step in the investigative process, odour reports should be reviewed and reconfirmed, because the initial steps may resolve the problem sufficiently, rendering subsequent steps and costs unnecessary.

Following a methodical step-by-step process ensures the following:

  • Costly interior finishing work can be avoided if ventilation proves to be the problem.
  • Lower overall costs, because the investigation stops once the source of the odour transfer problem has been identified.
  • The least expensive (low hanging fruit) solutions to the problem are identified early in the investigation.
  • A decreased chance of failing to resolve the issues because of an unmethodical investigation.

In short, a methodical investigative process empowers building staff and professionals to deal with the perennial problem of odour transfer. Now go find out who cut the cheese!