Air Barriers: Increasing Building Performance, Decreasing Energy Costs

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Air Leakage

Air leakage through the building enclosure refers to the unplanned, unpredictable, and unintentional airflow in or out of buildings, and must be distinguished from the intentional and, ideally, controlled flow of outdoor air into a building via either a mechanical or a ventilation system. A building can be very tight in terms of air leakage and have sufficient ventilation; conversely, a building could be very leaky and have insufficient ventilation. In mechanically ventilated buildings, it is desirable to have an airtight building enclosure, which is achieved by connecting materials with high air infiltration resistance into a continuous air barrier system.

In order for air leakage to occur, there must be a driving force and a pathway.The driving force for air leakage is the difference in total air pressure across the building enclosure, with airflow occurring from higher to lower pressure, or from positive to negative pressure. There are three main sources of air pressure difference.

The wind pressure is a significant factor and it is usually high (positive) on the windward side, and low (negative) on the leeward side of the building. When averaged over the course of a year, industry experts estimate that wind pressure is about 10 to 15 miles per hour (0.2 to 0.3 pounds per square foot) in most locations in North America.

Stack pressure (also called chimney effect or buoyancy) is caused by the atmospheric pressure difference between the top and bottom of a building, which is in turn caused by the temperature difference and hence the difference in the weight of the columns of indoor and outdoor air. In cold climates, during winter, the stack effect can cause infiltration of cold air at the bottom and exfiltration of warmer air at the top of the building. The reverse occurs in summer, with air conditioning.

Mechanical pressure is caused by heating, ventilation and air conditioning (HVAC) system pressurization. HVAC engineers tend to design buildings with slight positive pressures in order to reduce infiltration and associated pollution. However, this practice might not be ideal in some climates, such as in cold climates.

Main Sources of Air Pressure Differentials in Buildings

The main types ofairflow pathways include diffuse flow and concentrated pathways. Diffuse flow occurs through leaky materials and assemblies, such as fibrous insulation, uncoated masonry (such as concrete masonry units and brick), or other porous building materials. Concentrated flow occurs through unintended openings. Direct channel flow or orifice flow, which occurs when the air enters and exits in a direct path, has the highest cost penalty because of high energy loss. Offset channel leaks are the most damaging because of longer pathways, which allow for air to change its temperature and to reach the dew point within the building enclosure leading to interstitial condensation. Flow leaks occur between floors and could add to stack effect.

Air Flow Pathways

Moisture Transport

Moisture moves through the building enclosure as liquid water or as water vapors. The difference between the two physical states of water is the size of the molecular aggregates: liquid water exists as large molecular aggregates (up to 100 molecules at room temperature), while water vapors exist as free molecules. Consequently, the transport mechanisms are different for liquid water and water vapors.

 

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Originally published in Architectural Record.
Originally published in January 2006

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