By Steven Attrill and David Hamlyn
Modern buildings provide a respite from both the freezing winds of winter and the scorching heat of summer, but not without some notable trade-offs. As season gives way to season, the greater the difference between the conditions out of doors and the controlled climates into which we retreat, the more likely it is we will again witness the manifestation of stack effect in London’s growing number of high-rise buildings.
As the outside temperature drops, for example, those who are staffing lobby reception desks will wish they had brought heavier sweaters as cold draughts infiltrate the lower floors, and occupants of the higher and lower floors will again be treated to a variety of whistles, howls, and moans. On colder days, doors leading into and out of the building may be difficult to open—or to close—and some lift doors will not work properly.
What’s happening and why?
These problems are due to the phenomenon known as stack effect, which occurs in buildings when the outside air temperature is significantly different from the air temperature inside the building. It is most common in hot or cold climates or in moderate climates like the UK where the inside temperature is routinely very different from the outside temperature.
The stack effect happens in part because warmer air is less dense and tends to rise, while cooler air, being denser, tends to sink. In cold weather, for example, joints and openings in the upper parts of a building allow the warmer air inside the building to escape, and the resulting pressure differential draws cooler air in through the lower portion of the building. The building, in essence, wants to behave like a chimney. (More on that shortly.)
Two factors—the height of the building and the difference in air temperatures—determine how forceful the pressure imbalance is that gives rise to the stack effect. Stack effect can occur in any building where the temperature is different between the inside and the outside, but with all other things being equal, the symptoms are more pronounced in taller buildings. The reason for this is that the gradient by which pressure varies with height is different for the warmer internal air than for the cooler external air (considering the cool weather example, which is simply reversed in hot weather). This results in the differences in internal and external pressure being greatest at the top and bottom of the building. Therefore, the taller the building, the further the gradient extends and the greater the forces driving the stack effect. With the current surge in the number of high-rise buildings being built in UK cities, it is no surprise that stack effect seems to be rearing its ugly head much more frequently.
About the stigma
The undesirable symptoms of stack effect generally appear only when the weather gets either cold or hot. The resulting airflow can cause problems related to pressure as well as unrestricted airflow, causing whistling in vents and around doors, slamming doors shut or making them difficult to open, and creating cold drafts and other thermal discomfort. Whistling noises around lift doors are particularly prevalent.
Property owners and managers frequently hesitate to address the problem of stack effect symptoms in their buildings for two main reasons—partly because it is usually a seasonal occurrence, but also because it raises questions about the quality of the design and construction.
The fact is that building features and design can both mitigate and exacerbate issues related to stack effect. Poor seals around doors and windows, for example, can be the result of either construction or manufacturing shortcomings, but maintenance needs also can be a contributing factor. Beyond issues related to components, a building’s overall layout and design can make it prone to severe stack effect symptoms. The lift shafts in high-rise buildings present a ready opportunity for vertical air movement. Direct paths from outside doors to lift shafts provide a great setup for stack effect airflow and all the effects that come along with it. So, just as a brand can become known for its visually stunning grand entranceways, it also can gain a reputation for having stack effect issues, and nobody wants to be saddled with that.
Searching out the culprits
Controlling airflow is ultimately the key to controlling stack effect. Although it normally is not feasible to eliminate the pressure differentials that drive stack effect, reducing the impacts and issues associated with this natural phenomenon is well within reach.
Some engineers and consultants specialize in addressing potential or existing stack effect issues once the building is complete. However, proactively heading off likely problems through consultation early in the design process is always far less costly than remediating them after the fact. An experienced engineering consultant can model a building before it is built and use that model to very effectively mitigate the risks of stack effect.
When stack effect issues must be dealt with in an existing building, the first step is usually a site visit to begin uncovering likely points of infiltration and exfiltration as well as any paths where air can flow unrestricted.
Because the building fundamentally wants to be a chimney, openings—intentional or otherwise—in the upper and lower portions of the building can contribute to the issues we experience as a result of stack effect. The gaps around frameless sliding glass doors are a common source of problems. Wide-open entry areas without vestibules allow air to transfer more quickly and directly to lift shafts, boosting both infiltration and exfiltration rates. The same can be said for vents throughout the building.
Occupant behavior can also exacerbate issues related to stack effect. Merchants propping open swing doors at street level and occupants opening windows at higher levels can both contribute to increased stack effect airflow.
Poor quality facades can contribute significantly to overall structural leakage. Specification of doors that does not take airflow into account—whether infiltration of outside air or airflow within the building—can also lead to problems. Even where doors are positioned can contribute to either controlling stack effect or making it worse. For example, relocating doors from one side of a set of lifts to the other, or repositioning them to a junction in a corridor, can have a large impact on stack effect.
Resolving the issues
Clearly, planning ahead to incorporate measures that control airflow in buildings is the best approach to minimizing stack effect impacts. Including barriers to airflow between external entries and lift shafts, for example, is something that is far more easily accomplished in the design phase than as a retrofit. Changing specifications for doors, façade materials, windows, and vents is also a good way to head off stack effect problems.
In existing buildings, once the major causes of stack effect have been identified, they also can be addressed. The solution may be as simple as modifying building operations and educating occupants, or as significant as replacing doors or adding vestibules. An experienced consultant often can suggest modifications that will at the least reduce stack effect manifestations to acceptable levels. These can include such things as sealing gaps, changing or adding doors, or otherwise reworking internal building elements; making changes to the operation of the building; encouraging behavioral changes by building occupants; or a combination of these approaches.
Regardless, stack effect is a natural occurrence with effects that can be addressed—or avoided—through building science. It’s actually a breeze.