Air leakage is the uncontrolled movement of air, with its heat and moisture, into or out of the building.
Conditioned air from the occupied space may migrate into the envelope or on to the outside: this is exfiltration. Unconditioned exterior air can infiltrate into the building. Both infiltration and exfiltration can cause damage to the building envelope, because both upset the balance of temperature and vapour-pressure that characterizes a welldesigned wall.
In winter, exfiltration carries moist indoor air to the exterior, where it can condense inside the wall assembly: This leads to moisture damage, mold growth, and other undesirable consequences.
Summer infiltration has the same effect in air-conditioned buildings, and winter infiltration creates cold surfaces inside the building that can cause condensation to form — leading to damaged interior finish and poor air quality from mold growth. For more information on the effect of air leakage on wall performance, see the article “Keeping Walls Dry,” available from the CMHC website Continuing Education for Architects http://www.cmhc.ca/en/inpr/bude/himu/co edar/index.cfm).
There is also a cost associated with heating or cooling the uncontrolled outdoor air to maintain the desired interior conditions. In fact, air leakage accounts for as much as one-third of the heating bill in a residence, and windows and their connection to the wall are major sources of air leakage (operable windows are most prone to leakage). Therefore, most windows have seals and weatherstripping to prevent air leakage where the glass is installed in the sash (or the frame, in a picture window), and where the sash meets the frame in an operable window.
The glazing seal (between the glass and the frame or sash) is commonly a two-sided tape made of a butyl compound, although some glazing seals are made of flexible vinyl. The vinyl seal is either a U-channel (often called a glazing boot) that slips into the sash (or directly into the frame, in a picture window) and accepts the glazing assembly to form a watertight, airtight seal or a finand- bulb assembly that is co-extruded onto a PVC glazing stop (see Figure 3a and b).
There is greater variety in sash seals in an operating window. Most windows have two seals: one to stop air infiltration (the primary air seal) and one to prevent moisture penetration. The moisture seal should be outboard of the air seal in a properly designed window. The primary air seal is often a vinyl bulb seal or foamed gasket (often vinyl-coated to prevent moisture damage); although a double-fin vinyl seal is sometimes used. Figure 3b shows a bulb seal acting as the primary air seal between the frame and sash of an operable window.
The moisture barrier should be designed to prevent moisture (for example, wind-driven rain) from penetrating the window and reaching the room side, but should also allow water to drain away from the window to the outside. Trapped moisture can cause damage to wood windows, which can lead to indoor air quality problems.
The moisture barrier should not be airtight, as operation of the window would create pressurization between the two seals that could drive trapped moisture into the window frame. The moisture barrier in Figure 3b is a double seal: the accordion seal prevents water entry into the sash pocket, and the polyester sweep seal acts as a rainscreen (while allowing incidental water to weep out of the sash pocket). For more information, refer to the study called Water Penetration Resistance of Windows: Manufacturing, Building Design, Installation and Maintenance Factors, available from CMHC. Moisture-barrier design should account for freezing (which can form ice dams) dirt accumulation, and painting, all of which can plug weep holes and prevent drainage.
Sweep seals are often used as a moisture barrier, and are in fact preferable to bulb or fin seals for this application: in cold temperatures, the moisture on the sill freezes the seal in place, and vinyl seals will tear more readily than sweep seals when the window is operated.
Sweep seals are also used in place of bulb or fin seals in sliding windows, because the latter interfere with the operation of the window. The use of sweep seals as a primary air seal is the main reason sliding windows have relatively high air leakage problems.
Window air leakage is characterized by the A rating in the CSA A440 Standard. Levels are A1, A2 and A3, with A3 being the least air leakage. This rating describes the measured rate of air leakage at a standard air pressure difference (75 Pa, which is approximately equivalent to wind blowing directly at the window, at a speed of about 40 km/h or 25 mph). A standard-size window is tested in accordance with a standard procedure (ASTM E283) under positive and negative pressures, and the A rating is based on the average of the test results.
The A rating values for operable window units only are:
A1 = 2.79 m3/h per metre of crack length
A2 = 1.65 m3/h per metre of crack length
A3 = 0.55 m3/h per metre of crack length
There is also a “Fixed” rating at 0.25 m3/h per metre of crack length, applied to nonoperable windows, and a “Storm” rating (used for storm panels only). In this context, “crack length” is the length of a crack through which air is expected to leak. For an operable window, it is the length of the sash perimeter; for a non-operable window, it is the perimeter of the glazing sightline.
Note that compliance with any building code based on the National Building Code of Canada requires that all windows must meet the minimum requirements of CSA A440. This means at least an A1 rating, which is indeed a bare minimum: a window that just meets the A1 level will leak enough air to move the curtains — even when the window is closed. The Energy Star program requires a minimum A2 level of performance — recognizing the importance of reducing the air leakage in achieving energy-efficient performance.
Although windows do not have to be certified as meeting the A440 ABC levels, manufacturers must be able to demonstrate (if requested) that their window meets the requirements of the standard. Most manufacturers have their products tested to ensure that they meet this requirement.Reviewed by Martin Lapedus. Therefore, a test report is usually available, and should be requested. This is discussed in detail in the section “Specifying ABC Ratings.”
