Addressing Building Air Tightness and Understanding the Function of Air Barrier Systems
This article addresses infiltration and exfiltration air pressure forces that are exerted on buildings, and addresses the benefits of air barrier specification in building design.
The three main sources of air pressure flow through a building are Ambient Air Pressure, Stack Effect and Mechanical (HVAC) pressure. Architects must address the combined effect of these three primary air pressure sources when designing an air barrier system. Specifying the correct air barrier system will ensure sufficient air tightness is achieved to provide excellent internal air quality, reduce building energy consumption and extend the life of roof and wall building materials.
Ambient Air Pressure
The most significant air pressure on a building is exerted by outdoor weather and changing weather systems, known as ambient air pressure, wind pressure and barometric air pressure. Ambient air pressure includes the forces that wind and climate cycles have on a building through its interaction with the building's roof and walls. Wind impacting a building's roof, walls and corners exerts pressure, measured as pounds per square foot (psf), on the structure of a building. This air pressure is equalized when air transfers in (infiltration) or out (exfiltration) by passing directly through building materials or by moving through gaps and holes caused by dissimilar material connections. In buildings without an air barrier, pressure draws conditioned air out of the building or can force ambient (outside) air into the building. This natural equalization is a critical component that must be addressed during the design phase to ensure a sufficiently robust air barrier is selected that can withstand the geographic, land elevation, and seasonal pressures that are expected during the building's life.
The next most significant source of air pressure exerted on a building is known as the Stack Effect (or chimney effect). This form of air pressure force is created as air rises and falls within the building's interior environment. Stack effect creates pressure on building materials as different air pressure centers within the building work to equalize with each other, and with the exterior climate. Simply put, cold air is less dense than warm air, and air pressure varies throughout a building depending on how indoor air naturally settles or rises. Stack effect can create significant interior pressures at the top of a building (pressure from the interior of the building acting outward) pushing building materials away from the wall. When you combine wind effect, stack effect, and HVAC fan pressures, the resultant force can be extremely large. For example, if you look at the top floor of a tall building on a windy cold day, on the leeward side wall (opposite of wind direction), when the mechanical systems are on and provided a positively pressured environment, the combine pressures must be considered when designing an air barrier system. A mechanically attached building wrap would have a hard time staying on the wall, let alone preventing air from escaping from the building. This type of stack effect air flow can lead to major problems including mold growth, and wall material deterioration because of water vapor transmission and condensation. Air Barrier continuity (known as the air barrier plane) together with proper flashing membranes that connect the roofing and foundation waterproofing membranes are the primary defense against stack effect air pressure.
Mechanical (HVAC) Air Pressure
Mechanical air pressure is pressure that is built up when Heating Ventilation and Air Conditioning (HVAC) system exchange a buildings internal air with external air. HVAC or fan pressure can be a significant contributor to unaddressed wind and stack air pressure problems. Mechanical air pressure can create a high air pressure center in a building that exerts forces on building components, as air works to escape into a lower pressure exterior environment. In colder climates mechanical air pressure is particularly relevant when indoor air is heated and becomes a contributing factor in air pressures like stack effect. Air Barriers, working as a primary component of an effective Building Envelope System?, are the most reliable way for architects to address the three types of air pressure forces exerted onto buildings. Architects must make precise calculations for the differences in air pressure forces a building can expect given the variable factors like:
Geographic Region, Weather and Climate Conditions
Building's intended (primary) use
Primary building structural components
Secondary wall components drywall, plywood, ceiling components, etc.
Continuity between all building components
HVAC system requirements
Air barrier manufacturer's and their sales representatives are skilled at helping architects select and specify products which can address a variety of needs to ensure proper internal air quality is maintained once the construction is complete.
Article Courtesy of:Henry Company; 909 N Sepulveda Blvd, Suite 650; El Segundo, CA 90245; 800-598-7663 www.Henry.com