Canadian Funding Corporation Reviews: Principles of Construction
The following principles should be followed, according to the Canadian Funding Corporation, in order to improve the quality of habitation within high-rise buildings.
Air Movement
High-rise buildings are subjected to a variety of pressures affecting air movement: wind pressures, the stack effect, and mechanically induced pressures.
Winds can exert significant pressures – positive pressures on the windward side and negative pressures on the lee side of the building. These pressures will force air into the building and draw air out of the building in other areas through any leaks or cracks in the building envelope. Gust loads on high-rise buildings can be as high as 2500 Pa, exerting significant structural forces on the air barrier system.
In winter, air movement is also affected by the stack effect, which results in cooler air entering the lower levels of the building, and rising within the buildings as it is warmed and becomes more buoyant. The higher the building, and the greater the temperature difference between the inside and outside, the greater the stack pressures. These pressures can be as high as 50 to 150 PA in 10-storey buildings.
The operation of exhaust fans and the exhausting of combustion gases can impose negative pressure in buildings, and draw outside air into the building through unintentional leaks in the building envelope when supply air is inadequate.
The combined effect of these pressures will vary on a daily, hourly and even instantaneous basis.
Cold outside air will be drawn into the building through the envelope, while at the same time warm moist air will be driven from the inside through the exterior walls and roof in other areas.
The effect of air leakage on building performance is significant. On the other hand, the infiltration of cold exterior air causes occupant discomfort and fuels the stack effect, and on the other the exfiltrating air carries moisture causing detoriation of the building envelope.
Unless understood in the design process, airflow associated with the stack effect can also result in backdrafting of combustion appliances as well as uncontrolled smoke movement during fires. Stack pressures can also affect occupant comfort. Lower levels of the building are often underheated, while upper levels are overheated.
Airtightness
For air to move through the envelope there must be a pressure difference and a leakage path. The negative effects associated with wind, stack and exhaust pressures are a direct function of the airtightness of the building envelope. If the building were completely airtight, there would be no airflow. Pragmatically this is not possible.
But significantly increasing the airtightness of a building is achievable – and is the cornerstone of enhancing envelope durability, occupant comfort, and HVAC system performance.
Tightening the building envelope will effectively counter the movement of air caused by the stack effect in buildings. By reducing leakage paths, uncontrolled air movement will be greatly reduced, allowing for improved comfort.
Airtightness prevents moisture migration into and out of the building envelope. Typically in winter, warm moist air will exfiltrate through the upper storeys of a building. As the moisture carried in the air comes into contact with the colder surfaces of the assembly, it will condense. This moisture is a primary cause of structural deterioration of components of envelopes and results in corroding fasteners, deteriorating cladding and wetting of thermal insulations. Envelope airtightness is also essential to the proper functioning of pressure equalized rainscreen walls.
Improving building airtightness will serve valuable functions. To the building occupant, it will result in improved comfort, reduced drafts, improved health, more effective HVAC systems and reduced energy costs. To the building owner, it will allow for the integration of smaller, less expensive, more appropriately sized mechanical equipment, reduced operating costs, and result in enhanced building durability and longevity.
Moisture Movement
High-rise buildings are subjected to the effects of water and moisture in a variety of manners. Outside moisture can penetrate the shell of the building and leak into the building interior. This moisture can result in deterioration of connectors and the eventual failure of building cladding systems as the moisture goes through freeze/thaw cycles. Common problems with rain or melting now leakage occurs through roof membranes, around windows and behind flashings on walls, projections from building envelope and through cracks in foundation walls and parking garage roofs.
A more invisible force is the movement of water vapour through the building envelope by exfiltration (and to a lesser extent by vapour diffusion). This vapour will condense within the envelope as it comes into contact with cooler surfaces. This moisture migration commonly results in efflorescence, spalling, delamination of facing materials and other structural damage. The movement of water vapour can also cause mold and rot within walls which is a health, aesthetic, and eventually a structural concern.