The Stack Attack

Air pressure imbalances from the stack effect can wreak havoc on high-rise healthcare facilities

The third in a series of blog articles on the promise of Vertical Hospital Environments.

It’s not unusual for architects to specialize in high-rise buildings, while healthcare architects traditionally design low-to-midrise buildings. As hospitals become larger and taller, issues that previously were of minor consequence become major design considerations. Mitigating the stack effect in high rise healthcare projects is an example of the new challenges we discover at the design nexus of high-rise buildings and healthcare facilities.

In tall buildings, as the temperature drops in the winter, the heat inside is turned up. That warm air wants to rise through the building, to find its way up and out. The warm air is less dense, it’s buoyant. That upward drive creates suction on the lower floors. If you open an exterior doorway on a lower floor level, outside air comes rushing in due to this negative pressure. The outdoor cold air wants to replace the warm air rising through the building. This is intensified in taller buildings. The taller the building, the greater the temperature differential, the greater the driving force. Consequences of this stack effect can wreak havoc in a high-rise building.– air quality expert Jason Slusarczyk of Novus Environmental

Control of internal air pressures in hospitals is critical for the safety, health and comfort of patients, staff and visitors. Controlled air pressure creates a protective environment for patients who may have compromised immune systems. Conversely, negative pressure environments are needed to protect caregivers and visitors from the spread of infection. Hospitals need both of these protective environments.

What is the Stack Effect?

The stack effect occurs in taller buildings when warmer air inside migrates upward, drawing cooler outside air in. This creates negative pressure areas on lower floors and creates a positive pressure condition at the top floors of a building. Think of the stack effect as a tall chimney with air coming in the bottom and going out the top. [See sketch below.]


For hospital environments, the uncontrolled stack effect can be disastrous. The stack effect in high-rise buildings can mean a loss of conditioned air, uncomfortable cold air coming in at lower floors, undesirable whistling of air blowing around doors, doors that are difficult to close or open and infiltration of undesirable odors. In a high-rise hospital, comfort, safety and air quality issues are heightened tremendously.

Recently, I spoke with air quality expert Jason Slusarczyk and Senior Microclimate Specialist Bill Waechter of Novus Environmental about the stack effect. Jason and Bill are frequently called in to assess air quality and advise on stack effect issues throughout North America.  They broke down stack effect issues for high-rise healthcare facilities into the following categories.

  1. Odor Infiltration
    Unwanted air coming into a building brings the outside with it. Odors (diesel fuel, street fumes, garbage) originating in the parking garage from idling vehicles, loading docks, ambulance drop-off areas and mechanical exhausts can be pulled through building openings on lower floors and distributed throughout the building.
  2. Air quality
    Air moving up through the building can cause many side effects. Negative air pressure in lower floors can play havoc with exhaust hoods for cooking areas and can dissipate unwanted cooking odors and street fumes throughout the building.
  3. Loss of containment and security
    Automatic doors that won’t close due to air pressure in the building can result in security breaches. Wind pressure can keep elevator doors from functioning properly. At higher level building openings, where air wants to escape, there can be significant wear and tear on door hardware.
  4. Sound quality
    Whistling is the disruptive symptom of air moving through doorways and other openings. While it often occurs on lower levels, it can happen anywhere in a building.
  5. Fire safety
    Uncontrolled air can cause rapid movement of fire smoke or fire itself. Sensitive pressure relationships within the smoke management system can be altered disastrously.
  6. Issues magnified in high-rise hospitals
    In a healthcare environment, cross contamination is of extra concern. Isolation room environments become harder to secure; bacteria can travel upward through air shafts, compromising patient health. Cold drafts are an issue for patient comfort. The larger number of penetrations for mechanicals and pedestrian traffic in healthcare structures as well as heavy loading dock activity make air quality control of healthcare facilities an extraordinary challenge.
  7. Mold
    Let me add another critical issue to the list: Mold. The extreme positive air pressures introduced by the stack effect in tall buildings can result in an unintended migration of humidity to exterior walls. This moist warm air from the highly humidified hospital environment can cause frost to build up on the back of exterior wall panels.  When this frost melts in the springtime it becomes trapped moisture which can become mold.

Balanced building pressure – seeking solutions
I asked Tom Vannetta of commissioning agency E Cube, Inc. how to predict the impact of Stack Effect on tall hospitals. Mr. Vannetta says hospitals are typically ‘balanced” at one period of time during the completion of construction and this does not account for the seasonal changes which impact building pressures. He suggests that external air pressure monitors added during construction could help establish a predictive pattern for a building that could be built into the BMS (building management system).

Scant research has been devoted to the challenge of mitigating the stack effect in high rise healthcare environments.  The healthcare design community might have some solutions for certain aspects of the stack effect, but it has little data to guide design teams working on tall hospital environments.

Possible stack effect research topics:

  • The relationship between stack effect-related air movement and placement of critical patient functions within the functional building “stack”
  • Methods for controlling stack effect air movement via sealing of elevator/stair and other shaft openings
  • Methods for controlling moisture migration to the exterior wall, including exterior wall system selection
  • Planning strategies to place functions with high humidification needs (NICU, Surgery, Birthing Units, etc.) in in-board areas with minimal exterior wall exposure
  • Methods to predict changes in internal building air pressures due to climatic conditions and development of settings for the BMS that can offset this effect.
  • Methods to isolate high humidity areas with internal vapor barriers

Addressing the stack effect phenomenon in the design of commercial high rises is not uncommon, but the application of this knowledge with research specific to healthcare has yet to emerge. Informing and educating all stakeholders in the design of taller hospital environments (owners, facilities managers, design team members and contractors) in the issues around the stack effect will be a step in the right direction. One thing is for sure, we need further collaboration between high-rise architects and healthcare designers in mitigating the harmful effects of the stack effect. Healthy air is critical in high-rise healthcare. Let the conversation begin.

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