The Physics Of Air And Moisture Transport Through The Building Enclosure

To gain a proper working understanding of the role of air barriers in building enclosures, some basic terminology and fundamental physics must first be defined. The building enclosure, also known as the building envelope, refers to the part of the building that physically separates the interior conditioned space from the exterior environment. Its main function is to control all loadings due to separation of the two environments, the flow of mass and flow of energy. Air barriers are an important component of the building enclosure.

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 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

Main Sources of Air Pressure Differentials in Buildings

The main types of airflow 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

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.

Liquid water

The main source of liquid water for above grade walls is rain, which can infiltrate behind the exterior cladding and be driven into the building enclosure by four main forces:

Gravity can draw water down through openings and cracks, and into the construction assembly.

Capillary forces act like a sponge sucking water through small cracks and pores. Smaller cracks result in greater capillary forces.

Rain droplets can pass through openings in the exterior cladding, driven by the momentum of falling rain drops.

The pressure differential can push or suck water through openings and cracks, into the construction assembly.

There are three basic types of exterior wall design, from the standpoint of rain penetration control.

• Face-sealed (barrier) walls rely upon every seam and crack to be face sealed. This design requires detailed workmanship and continuous maintenance, and is most vulnerable to rain infiltration. This design is effective only in areas with low wind and rain exposure. Examples of barrier walls include non-drainage Exterior Insulation Finish Systems (EIFS) and face-sealed curtain walls.

• Concealed barrier walls rely on multiple layers for rain penetration control. In contrast to face-sealed systems, these walls include a drainage plane within the wall assembly that functions as a second line of defense against water intrusion. The drainage plane is usually a water resistive barrier membrane. This design is effective in areas with moderate wind and rain exposure. A typical example of a concealed barrier wall is the drainage stucco system.

• Drained cavity or rain screen walls rely on two layers and a drained cavity space for rain penetration control. This design is similar to the concealed barrier system in that it provides two lines of defense, but it offers additional features, such as capillary breaks between porous materials, freer drainage, and venting or ventilation to limit average relative humidity (RH)) outside of sheathing. This design is most effective in rain penetration control and should be used in areas with high wind and rain exposure. Examples of rain screen walls include brick-veneer cavity walls, furred-out clapboard walls, and drainable EIFS walls.

Water vapor can be transported through the building enclosure by air currents and by vapor diffusion. Air currents could carry significant amount of moisture vapors into the building enclosure. A continuous air barrier will control airflow, hence the moisture migration through air currents. Air-transported moisture must not be confused with vapor diffusion.

For water vapor diffusion to occur there has to be a driving force and a pathway. The driving force for water vapor diffusion is the difference in water vapor concentration or difference in vapor pressure across an assembly: water vapors flow from an area of higher concentration (higher vapor pressure) to an area of lower concentration (lower vapor pressure). The ability of materials to allow vapor diffusion is measured by vapor permeability, which is expressed in perms: the higher the perms, the higher the vapor permeability.

The 2003 International Building Code (IBC) classifies building materials into vapor permeable (greater than five perms) and vapor non-permeable (less than one perm). Vapor non-permeable materials are called vapor barriers or vapor retarders. Other terms often used to describe vapor permeable or non-permeable materials are "breathable" and "non-breathable," respectively.

"Breathability is often associated with air flow, rather than moisture vapor flow," notes Maria Spinu, Ph.D., Building Science Manager, Dupont Building Innovation. "The use of this terminology may have contributed to the confusion between an air barrier versus a vapor barrier function." While the two functions could be performed by a single material, providing an air and vapor barrier, the needs addressed are quite different. Air barriers retard airflow, which is the result of air pressure differences. Vapor barriers retard water vapor flow, which is the result of water vapor concentration differences.

Experts estimate that the amount of moisture vapor transported by air currents can be 100 to 200 times higher than the amount transported by vapor diffusion, and can account for more than 98 percent of all water vapor movement through the building enclosure.

In summary, there are three main moisture sources, which could lead to water problems in buildings: bulk water, air transported moisture, and vapor diffusion. "The three moisture sources do not contribute equally to the wetting of the building enclosure," says Spinu. Liquid or bulk water infiltration is usually the largest wetting source for above-grade walls, followed by air transported moisture, which is significantly higher than the amount of water vapor transported by diffusion. It is generally accepted that the buildings will occasionally get wet; however, moisture problems in buildings will occur if wetting exceeds drying. Consequently, in

Experts estimate that the amount of moisture vapor transported by air currents can be 100 to 200 times higher than the amount transported by vapor diffusion, and can account for more than 98 percent of all water vapor movement through the building enclosure.

order to prevent moisture problems it is essential to protect the enclosure against wetting and promote drying. Although moisture movement by diffusion cannot be discounted as a wetting source, it should not be the primary focus for moisture intrusion control; vapor diffusion, however, is critical for drying.

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