Air Barriers: Increasing Building Performance, Decreasing Energy Costs

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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.

Thepressure 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 byair 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 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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Originally published in Architectural Record.
Originally published in January 2006

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