Air Leakage (Infiltration)

Whenever there is a pressure difference between the inside and outside (driven by wind or temperature difference), air will flow through cracks between window assembly components. The air leakage properties of window systems contribute to the overall building air infiltration. Infiltration leads to increased heating or cooling loads when the outdoor air entering the building needs to be heated or cooled. Air leakage also contributes to summer cooling loads by raising the interior humidity level. Operable windows can be responsible for air leakage between sash and frame elements as well as at the window/wall joint. Tight sealing and weatherstripping of windows, sashes, and frames is of paramount importance in controlling air leakage.

The use of fixed windows helps to reduce air leakage because these windows are easier to seal and keep tight. Operable windows, which are also more susceptible to air leakage, are not necessary for ventilation in most commercial buildings but are desired by occupants for control. Operable window units with low air-leakage rates feature mechanical closures that positively clamp the window shut against the wind. For this reason, compression-seal windows such as awning, hopper, and casement designs are generally more effectively weatherstripped than are sliding-seal windows. Sliding windows rely on wiper-type weatherstripping, which is more subject to wear over time.

The level of infiltration depends upon local climate conditions, particularly wind conditions and microclimates surrounding the building. In reality, infiltration varies widely with wind-driven and temperature-driven pressure changes. Cracks and air spaces left in the window assembly can also account for considerable infiltration. Insulating and sealing these areas during construction can be very important in controlling air leakage. A proper installation ensures that the main air barrier of the wall construction is effectively sealed to the window or skylight assembly so that continuity of the air barrier is maintained.

Infiltration (and exfiltration) occurs due to three factors: poor product design, poor quality control during product manufacturing, and poor construction detailing and/or installation. Test procedures, such as ASTM E283-04 Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen, are used widely in the industry to account for product design, and to some extent, quality control. Testing actual products off the production line at random will ensure that quality control is accounted for in infiltration test results. Infiltration due to poor construction detailing and/or installation can only be accounted for in test results if specific mock-ups of the wall section are designed and tested.

AAMA/WDMA/CSA 101/I.S.2/A440, NAFS - North American Fenestration Standard/Specification for windows, doors, and skylights (Canadian supplement CSA A440S1-09) was developed for window, door, and skylight performance, including permissible air leakage. This specification is generally consistent with building codes used throughout the United States. Generally, operable products with an infiltration rate of less than 0.3 cfm/square foot are required. Architectural grade products utilizing a compression seal are required to have an infiltration rate of less than 0.1 cfm/square foot. Nonoperable products are not expected to have a measurable level of infiltration.

Natural ventilation—the practice of operating windows to provide fresh air and cooling—is distinct from the issue of infiltration/exfiltration as it is based on the controlled movement of air through open windows.


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