Building Stack Effect in High-Rise Apartments: Winter Comfort, Pressure and Energy Loss

During an Ontario winter, a high-rise apartment building can behave like a very large chimney. Warm air inside the building is less dense than the cold outdoor air. Pressure differences drive outdoor air into lower levels, move air upward through shafts and other vertical pathways, and push indoor air outward near the top of the building. This seasonal pressure pattern is known as building stack effect.

Stack effect can contribute to cold entrance lobbies, whistling doors, drafts around lower-floor suites, odour movement, uneven ventilation, elevator-door issues and heat loss. At upper levels, warm and moisture-laden indoor air may be driven into or through exterior assemblies, increasing the potential for condensation where surfaces are cold enough.

The effect becomes more pronounced as building height and the difference between indoor and outdoor temperatures increase. However, the final pressure pattern is not created by temperature alone. Wind, envelope leakage, open doors, exhaust systems, corridor air supply, parking-garage ventilation and the arrangement of elevator shafts, stairwells and service chases all influence how air actually moves.

Stack effect cannot be switched off with one control adjustment. It can often be managed through a coordinated combination of ventilation maintenance, air balancing, controls, compartmentalization and building-envelope work. The first step is to understand the whole building as a connected pressure system, just as it is when addressing broader multi-unit residential HVAC performance.
 

What Is Building Stack Effect?

Stack effect is air movement caused by differences in air density. In winter, the air inside a heated building is generally warmer and less dense than the outdoor air. The indoor and outdoor columns of air therefore develop different pressure profiles over the height of the building.

In a simplified high-rise building during cold weather:

• Outdoor air tends to enter below the neutral pressure level.

• Air moves upward through elevator shafts, stairwells, service risers, gaps and other connected paths.

• Indoor air tends to leave the building above the neutral pressure level.

The ASHRAE Handbook discussion of ventilation and infiltration describes the lower portion of a heated building as depressurized relative to outdoors and the upper portion as pressurized when other forces are excluded.

Real buildings are more complicated. Interior partitions, shaft doors, floor slabs, mechanical systems and leakage paths divide the building into connected pressure zones. A tall building may have more than one neutral pressure level, particularly where elevator shafts serve different groups of floors or effective horizontal separations interrupt the vertical air path.
 

The neutral pressure level

The neutral pressure level is the elevation where indoor and outdoor air pressure are approximately equal under the conditions being considered. Below that level, winter stack pressure tends to draw outdoor air inward. Above it, stack pressure tends to push indoor air outward.

The neutral pressure level is not automatically at the midpoint of the building. Its position is influenced by:

• The location and size of exterior leakage openings

• Interior compartmentalization

• Open entrance, loading and service doors

• Elevator and stair shaft construction

• Mechanical supply and exhaust airflow

• Window and balcony-door operation

• Building additions and renovations

Exhausting more air can move the neutral pressure level upward and increase negative pressure at lower floors. Adding supply air can move it downward. Neither response is automatically beneficial throughout the entire building, which is why whole-building measurements matter.
 

Stack effect, wind and mechanical pressure act together

Facility teams often use the phrase "stack effect" to describe every winter draft. In practice, measured pressure is the combined result of several forces:

• Stack pressure: Created by indoor-outdoor temperature and density differences over height

• Wind pressure: Changes with wind speed, direction, facade orientation and surrounding buildings

• Mechanical pressure: Created by make-up air, corridor supply, suite exhaust, central exhaust, garage fans and commercial tenant systems

• Piston effects: Temporary pressure changes caused by elevator movement

The National Research Council Canada's archived technical digest on stack effect notes that wind and mechanical supply and exhaust systems also contribute to pressure differences across building components. Although the document is historical and should not replace current design guidance, its explanation of the underlying physics remains useful.

This interaction explains why the same entrance door may operate normally one day and become difficult to open during a cold, windy evening.
 

Why Stack Effect Is Stronger in Ontario Winters

Two main physical factors increase stack pressure: building height and temperature difference.

A taller building provides a greater vertical distance over which the indoor and outdoor pressure profiles diverge. A larger temperature difference produces a larger air-density difference. When an indoor temperature near 21 C is paired with a very cold outdoor temperature, the winter pressure range can become substantial.

The 2024 Ontario Building Code supplementary guidance on measures for fire safety in high buildings recognizes stack effect as particularly relevant in Canadian buildings because of cold winter conditions. The guidance also illustrates the neutral pressure plane and the inward flow at low levels and outward flow at high levels.

Stack effect usually becomes more noticeable as outdoor temperatures fall. That relationship makes cold-weather investigation especially valuable. A building may appear well balanced during a mild autumn inspection but develop serious pressure complaints during a January cold snap.

Summer can produce reverse stack effect when the indoor air is cooler and denser than hot outdoor air. In Ontario, the summer indoor-outdoor temperature difference is generally smaller than the winter difference, so reverse stack effect is often less pronounced. Wind and mechanical airflow can still create significant summer pressure issues.
 

What Residents and Building Operators May Notice

Stack-effect symptoms often vary by floor because the direction and magnitude of pressure change over the height of the building.
 

Lower-floor symptoms

Below the neutral pressure level, operators may notice:

• Cold drafts through lobby and loading doors

• Whistling around exterior doors or openings

• Doors that are difficult to open or that close abruptly

• Cold air entering parking, service and garbage-room connections

• Lower-floor suites drawing air from corridors or adjacent spaces

• Increased heating demand near entrances and perimeter zones

• Odours entering from loading, parking or waste areas

Open entrance doors can become a major air path. As cold air enters at the bottom of the building, it must be heated, increasing the load on lobby heaters, make-up air equipment and the central heating plant.
 

Mid-building symptoms

Near a neutral pressure level, exterior pressure differences may be smaller, but interior air movement can still occur. Occupants may experience:

• Odour movement between suites and corridors

• Unpredictable airflow at suite doors

• Uneven bathroom or kitchen exhaust performance

• Pressure changes when elevator or stair doors open

• Comfort changes caused by wind or fan scheduling

The neutral pressure level can shift as mechanical systems and exterior openings change, so the floor with the fewest symptoms may not remain the same throughout the day.
 

Upper-floor symptoms

Above the neutral pressure level, operators may see:

• Warm air leaking outward through facade and roof-level openings

• Noise or air movement around elevator doors

• Apartment or balcony doors behaving differently than those on lower floors

• Condensation or frost near cold leakage locations

• Odours transported upward through shafts or other vertical paths

• Upper-floor overheating caused by heat and air movement from below

Warm indoor air carries moisture. If that air enters a cold wall, roof or window assembly, water vapour can condense on cold surfaces. The risk depends on indoor humidity, outdoor temperature, assembly design and the specific leakage path. Stack effect does not guarantee condensation, but it can provide the airflow that transports moisture to a vulnerable location.
 

How Stack Effect Affects Energy Use

Uncontrolled air movement creates several energy penalties.
 

Infiltration heating load

Cold outdoor air entering lower floors must be heated to indoor conditions. If the air is excessively dry, humidification may also be required in buildings that use active humidity control. Lobby heaters, perimeter heating and make-up air coils may operate longer to offset the infiltration load.
 

Exfiltration of conditioned air

Air leaving at upper levels carries heat that the building has already paid to produce. This loss may occur through roof penetrations, facade leakage, mechanical openings and doors.
 

Uneven ventilation

Central ventilation systems can deliver very different effective airflow by floor when stack pressure combines with duct pressure. Some areas may be overventilated while others receive too little outdoor air. Overventilation wastes heating energy, while underventilation can contribute to indoor air quality complaints.

Research summarized by Lawrence Berkeley National Laboratory on ventilation characteristics in Canadian mid- and high-rise residential buildings found that corridor supply flow, suite-door leakage and inter-suite transfer air can make actual suite ventilation difficult to predict. The findings reinforce the need for measurement rather than assumptions based only on design airflow.
 

Compensating setpoint changes

Residents and operators may raise thermostat or supply-air setpoints to address a draft that is fundamentally caused by infiltration. Upper floors may then overheat, prompting residents to open windows. The additional high-level opening can strengthen the overall low-to-high air path and increase infiltration at the bottom.
 

Fan energy and maintenance

Increasing fan speed without a measured strategy may consume more electricity without correcting the dominant leakage path. It can also create excessive pressure at some floors, noise, control instability or higher filter loading.

An energy and sustainability assessment can help distinguish ventilation energy, envelope losses and heating-plant performance when planning improvements.
 

Why Odours and Contaminants Move Between Floors

Air carries whatever is present along its path. In a poorly compartmentalized building, stack-driven flow can transport cooking odours, tobacco or cannabis smoke, cleaning-product vapours and air from parking, loading or waste areas.

Possible transfer paths include:

• Gaps around plumbing and electrical penetrations

• Unsealed service chases

• Elevator and stair shafts

• Corridor-to-suite leakage

• Common exhaust shafts

• Gaps around doors

• Ceiling and wall cavities

• Open windows and balcony doors

Odour complaints do not prove the source is on the same floor. Air may enter the building at a lower level, move vertically and exit into an upper corridor or suite.

This is why odour investigations should record the source if known, affected floor, time, outdoor temperature, wind and fan status. Airflow direction at doors and penetrations can be more informative than odour intensity alone.

Fire and smoke movement are life-safety matters governed by the building design and applicable code requirements. Normal ventilation corrections must not interfere with fire separations, stair pressurization, smoke-control systems, fire dampers or other protective features.
 

The Role of Corridor Make-Up Air and Ventilation Systems

Many high-rise residential buildings use rooftop equipment to deliver tempered outdoor air to corridors. Depending on the building's age and design, this system may be called corridor supply air, ventilation air or a make-up air system. Ambient Mechanical's project gallery includes examples of make-up air units and other commercial HVAC equipment.

Its intended role must be confirmed from the design documents. Corridor air may support ventilation, corridor conditions and pressure relationships, but it should not automatically be assumed to provide all replacement air required by every suite exhaust fan. Actual transfer depends on the building design, door and enclosure leakage, exhaust operation and code requirements.

A make-up air unit can be running while delivering much less air than expected because of:

• Dirty or incorrectly installed filters

• A slipping belt or worn drive

• Incorrect fan rotation

• Closed or failed dampers

• Blocked intake screens

• Coil fouling or freezing concerns

• Variable-frequency-drive speed limits

• Sensor or actuator faults

• Duct leakage

• Unbalanced floor outlets

• Inappropriate scheduling

• Fire or smoke-control interlocks that have not been restored correctly after testing

Ambient Mechanical's commercial HVAC maintenance and service can include make-up air equipment, fans, dampers, heating components and controls that influence high-rise pressure.
 

Why simply increasing make-up air may fail

More supply air can reduce negative pressure at lower levels in some cases, but it can also:

• Overpressurize corridors on certain floors

• Increase air transfer into suites

• Move odours along unintended paths

• Raise heating and fan energy use

• Change exterior door forces

• Shift the neutral pressure level without solving the main leakage path

The correct goal is not maximum airflow. It is a controlled pressure and ventilation relationship supported by measured supply, exhaust and transfer airflows.
 

Suite and Central Exhaust Systems

Bathroom fans, kitchen exhaust, laundry exhaust and central shaft systems remove air from the building. Commercial tenants, amenity spaces, pools and parking garages can add large exhaust loads that may not operate on the same schedule as the residential ventilation system.

Problems can occur when:

• Exhaust fans operate while replacement-air systems are off

• Central exhaust shafts are unbalanced

• Roof fans run at incorrect speed

• Suite fans discharge less or more than expected

• Exhaust grilles and ducts are blocked

• Dampers stick or fail

• Tenant improvements add exhaust without coordinated replacement air

• Garage or loading-area pressure communicates with residential areas

The investigation should compare measured exhaust and supply quantities at the same time. Design drawings provide a starting point, but they do not confirm current performance.
 

Building Automation and Control Sequences

A building automation system can help connect pressure complaints with equipment operation. Useful trend points may include:

• Outdoor temperature

• Wind data, if a reliable sensor is available

• Make-up air fan command and proof

• Supply-air temperature

• Damper command and position

• Variable-frequency-drive speed

• Filter differential pressure

• Central exhaust fan status

• Parking-garage fan status

• Building pressure sensors

• Low-temperature and freeze-protection alarms

• Equipment overrides

Fan proof alone does not confirm airflow. A fan can be enabled while a damper is closed, a filter is blocked or a belt is slipping. Periodic field verification remains necessary.

Controls should coordinate normal ventilation schedules and provide useful alarms without overriding equipment safeties. Fire alarm and smoke-control sequences must remain consistent with the approved design and should only be tested or modified by qualified parties.
 

Building Envelope and Compartmentalization

Mechanical balancing cannot fully correct a building with large uncontrolled air paths. Interior compartmentalization reduces the vertical connectivity that allows stack-driven airflow to act over the full height of the building.

Potential leakage locations include:

• Service penetrations through floor slabs

• Plumbing and electrical risers

• Garbage and laundry chutes

• Elevator-machine and hoistway openings

• Stair and service doors

• Roof access doors

• Loading-dock connections

• Wall-to-slab interfaces

• Window and curtain-wall transitions

• Mechanical shafts and duct penetrations

The objective is not indiscriminate sealing. Many assemblies have fire-resistance, smoke-control, drainage, ventilation or movement requirements. Firestopping and repairs must use appropriate listed systems and be completed by qualified contractors where required.

The Canadian multi-suite building guide published by CMHC explains that ventilation performance must account for the uncontrollable effects of stack and wind. Its discussion of mechanical and electrical systems in apartment and multi-suite buildings also describes infiltration at lower levels and exfiltration at upper levels during cold weather.

For existing properties, a coordinated HVAC retrofit and design review can bring together ventilation upgrades, controls, shaft conditions and envelope recommendations instead of treating each complaint in isolation.
 

A Practical High-Rise Stack Effect Investigation

A reliable assessment should be planned for weather that produces the problem. Measurements taken during mild conditions may not represent peak winter operation.
 

Step 1: Build a complaint map

Record:

• Floor and specific location

• Time and date

• Outdoor temperature

• Wind direction and speed if available

• Door, window and loading-dock status

• Odour type or comfort symptom

• Make-up air and exhaust status

• Whether the issue changes when an elevator arrives or a door opens

Mapping complaints by height often reveals a low-level infiltration zone, a relatively calm middle area and upper-level exfiltration concerns.
 

Step 2: Review documents and operating intent

Collect available ventilation drawings, air-balancing reports, control sequences, commissioning records and fire-safety information. Identify:

• Make-up air and exhaust design quantities

• Which shafts serve which floors

• Fan schedules and interlocks

• Pressure-control points

• Renovations that changed walls, doors, ducts or exhaust loads

• Previous temporary fixes and overrides
   

Step 3: Inspect the equipment

Check make-up air units, central exhaust fans and related systems for serviceable faults. Inspect filters, belts, fans, dampers, coils, drives, sensors and intake or discharge obstructions. Confirm fan rotation and actual operation.
 

Step 4: Measure airflow

Measure supply and exhaust quantities using appropriate instruments and methods. Compare floors rather than relying only on a total fan reading. A correct total airflow can still be distributed poorly.
 

Step 5: Measure pressure by height

Technicians may measure pressure differences across:

• Exterior entrances

• Vestibule doors

• Suite doors

• Stair doors

• Parking and loading connections

• Garbage or service rooms

• Relevant vertical separations

Measurements should be recorded with outdoor conditions and equipment status. Door-opening force may also be evaluated where operational or accessibility concerns exist, using methods appropriate to the application.
 

Step 6: Test operating scenarios

Where safe and permitted, compare pressure under controlled scenarios such as:

• Normal occupied operation

• Make-up air enabled and disabled

• Selected exhaust systems enabled and disabled

• Loading or entrance doors open and closed

• Different fan speeds within approved operating limits

Life-safety systems and required ventilation must not be disabled casually. Testing should be planned with qualified personnel and restored to normal operation afterward.
 

Step 7: Identify dominant paths

Use pressure, airflow, smoke-pencil or other suitable diagnostic methods to identify major air paths. Tracer smoke must be used safely and must not trigger alarms or expose occupants. Thermal imaging may help locate cold infiltration areas, but it does not measure airflow by itself.
 

Step 8: Correct and verify

Complete repairs in a controlled sequence, then repeat the relevant measurements during comparable weather. Verification should confirm that the original complaint improved without creating new pressure, ventilation or odour problems elsewhere.
 

Corrective Measures That May Help

The appropriate solution depends on the dominant cause. Common corrective categories include the following.

Restore equipment performance

• Replace loaded filters

• Repair belts, bearings or fan components

• Restore failed dampers and actuators

• Correct fan rotation or drive settings

• Clean blocked intakes, coils and grilles

• Repair temperature or pressure sensors

• Restore the intended schedule
   

Rebalance supply and exhaust

• Measure floor-by-floor airflow

• Correct excessive or deficient branches

• Coordinate central supply and exhaust quantities

• Review suite, amenity and commercial exhaust loads

• Confirm garage and loading areas do not create unintended transfer
   

Improve controls

• Coordinate ventilation schedules

• Add reliable status and alarm points

• Trend outdoor conditions against complaints

• Review pressure-control logic

• Remove undocumented overrides

• Recommission sequences after equipment or tenant changes
   

Address major leakage paths

• Repair damaged exterior seals

• Correct service-penetration leakage using appropriate systems

• Restore door closers, gaskets or sweeps where permitted

• Seal uncontrolled shaft and floor openings with code-compliant assemblies

• Improve vestibules or entrance airlocks where feasible
   

Consider localized entrance improvements

Vestibule repairs, suitable door operators, revolving doors in appropriate commercial contexts, localized heat and entrance-layout changes may improve lobby conditions. These measures should be selected based on pressure measurements, accessibility requirements, egress and the building's architecture.
 

Plan a coordinated retrofit

Older high-rise buildings may need a larger program involving make-up air replacement, variable-speed control, exhaust rehabilitation, envelope work and improved compartmentalization. Treating the mechanical and architectural scopes as one pressure-management project generally produces a stronger result than isolated repairs.
 

Common Approaches That Can Make the Problem Worse

Some quick responses can move the complaint rather than solve it.
 

Raising the make-up air fan speed without measurements

This may improve one entrance while overpressurizing another floor or increasing airflow into suites.
 

Increasing heating setpoints

More heat may mask a cold draft locally, but it does not stop infiltration. It can also overheat upper floors and increase window opening.
 

Sealing one door aggressively

Closing a major path may redirect air through another suite, stair or service space. Door changes can also affect closing, accessibility, egress and fire protection.
 

Disabling exhaust fans

Required exhaust supports indoor air quality and moisture control. Turning it off may create different health, odour and condensation problems.
 

Balancing during mild weather only

A system balanced without considering winter stack pressure may perform differently during the conditions that cause complaints.
 

Using Winter Data for Capital Planning

Cold weather is an opportunity to collect information that cannot be reproduced easily in summer. A winter investigation can establish:

• Pressure profiles by floor

• Supply and exhaust performance

• Major leakage locations

• Door-operating concerns

• Equipment capacity and control limitations

• Relationship between outdoor temperature and complaints

• Which corrections belong in maintenance and which require capital planning

This evidence supports better decisions about make-up air replacement, variable-speed drives, controls, envelope repairs, entrance upgrades and ventilation redesign. It also creates a baseline for measuring results after the work is complete.

For condominiums and rental properties, a phased plan can help boards and owners prioritize immediate repairs while preparing larger items for reserve or capital budgets. Ambient Mechanical supports multi-unit residential HVAC systems, including ventilation, boilers, chillers, controls and retrofit planning.
 

Preventative Maintenance for Better Winter Pressure Control

Before cold weather arrives, property teams should:

• Replace or inspect make-up air filters

• Verify fan belts, drives and bearings

• Exercise outdoor-air and isolation dampers

• Test heating coils and freeze-protection controls

• Confirm fan schedules and proofs

• Review central exhaust operation

• Inspect intake and discharge openings

• Check pressure sensors where installed

• Remove temporary overrides

• Review prior winter complaint locations

• Confirm entrance and vestibule doors close correctly

• Schedule cold-weather testing for unresolved issues

A planned review is more effective than waiting for the first extreme-cold complaint. Ambient Mechanical's full range of commercial HVAC services can support seasonal maintenance, diagnostics, automation and system upgrades.
 

How Ambient Mechanical Can Help

Ambient Mechanical helps condominium and apartment property teams investigate recurring winter drafts, uneven ventilation, odour transfer and pressure-related equipment concerns across Toronto and Southern Ontario.

Our team can assess make-up air units, central exhaust, dampers, heating coils, fan performance, BAS sequences and hydronic systems that influence winter comfort. Where the issue extends beyond equipment maintenance, we can coordinate airflow findings with controls and retrofit recommendations to support a whole-building solution.

The objective is not simply to increase fan speed or heating output. It is to identify how stack pressure, wind, mechanical airflow and leakage interact, then correct the highest-impact problems without creating new concerns elsewhere.

If your high-rise experiences cold lobbies, difficult doors, odour migration or floor-to-floor comfort differences during cold weather, contact Ambient Mechanical to plan a winter building-pressure and ventilation assessment.
 

Frequently Asked Questions

Why is stack effect worse in winter?

Stack pressure increases as the temperature and air-density difference between indoors and outdoors grows. Ontario winters can create a much larger difference than typical summer conditions. Building height also matters because the pressure difference develops over the vertical distance.
 

Why do stack-effect complaints change by floor?

Winter stack effect generally draws air inward below the neutral pressure level and pushes air outward above it. Pressure magnitude also changes with height. Interior shafts, mechanical systems and leakage distribution can create multiple pressure zones, so symptoms do not always follow a perfectly simple pattern.
 

Can a make-up air unit eliminate stack effect?

No. A make-up air unit can influence building pressure and support ventilation, but it cannot remove the temperature-driven density difference. Its effect depends on capacity, distribution, exhaust airflow, controls and envelope leakage.
 

Should the make-up air fan simply run faster in winter?

Not without measurements and a review of the control intent. Increasing airflow may reduce lower-floor negative pressure but overpressurize other areas, increase energy use or move odours. Supply and exhaust should be measured and balanced as a system.
 

Why are lobby doors difficult to open during cold weather?

Negative pressure at lower levels can pull exterior doors inward. Wind, open doors elsewhere, exhaust fans and elevator movement can add to or temporarily change that pressure. Door hardware should be checked, but the underlying building pressure also needs investigation.
 

Can stack effect move cooking or smoke odours between suites?

Yes. Stack pressure can drive air through gaps, corridors, shafts and other connected paths. Wind and exhaust systems may also contribute. The source of an odour may be several floors away from the affected suite.
 

Does sealing suite doors solve the problem?

Not necessarily. Door leakage may be part of the air path, but modifying a suite door can affect ventilation, door closing and the performance of a fire-rated assembly. Any change should be evaluated in the context of the building design and applicable requirements.
 

Can stack effect cause condensation?

It can contribute by pushing warm, moisture-laden indoor air through upper-level envelope leaks. If that air reaches a cold surface below its dew point, condensation or frost may form. Indoor humidity and the construction of the assembly also affect the risk.
 

When should stack effect be tested?

Testing should occur during weather that reproduces the complaints, usually a sufficiently cold winter period. Baseline airflow and equipment maintenance can be checked at any time, but the pressure profile may be much weaker during mild weather.
 

Can a BAS identify stack effect?

A BAS can help correlate outdoor temperature, fan commands, damper positions, pressure sensors and complaints. However, it may not show actual airflow or leakage paths. Field pressure and airflow measurements are normally required for a complete diagnosis.
 

Is stack effect only an HVAC issue?

No. It involves the building envelope, interior compartmentalization, doors, shafts, ventilation, exhaust, controls and occupant behavior. Effective solutions often require mechanical and building-envelope coordination.
 

Can stack effect be completely eliminated?

The physical pressure effect cannot usually be eliminated in a heated high-rise during cold weather. Its consequences can be reduced through compartmentalization, controlled ventilation, balanced airflow, maintained equipment, coordinated controls and attention to major leakage paths.