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Air Leakage Testing

Q: What is air leakage testing on exterior walls??

A: Air leakage testing is now an energy code requirement which focuses on the performance of the exterior envelope. It is cited in both the 2018 International Energy Conservation Code, Section C402.5, and in 2019 ASHRAE 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, Sections and One of these is required in most states.


Controlling air leakage is very important. Moisture-laden air that leaks into a wall cavity can cause damage through condensation. Air leakage decreases the thermal efficiency of the exterior envelope, creating increased energy consumption to compensate for the heat loss. Air leakage introduces pollutants into the conditioned space.


The IECC mandates buildings be tested for air leakage per ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization. The scope of the test is to “quantify the air tightness of the building envelope.” The building envelope is defined as the walls, floors and roofs that separate conditioned from non-conditioned spaces. Options are available that waive the requirement for actual testing. Washington and California go beyond the IECC and mandate physical testing.


The test essentially establishes a pressure differential of 0.3 inches of water column using fans between the adjacent spaces, and then sets a minimum air leakage rate. That rate cannot exceed 0.4 cubic feet per minute per square foot of the envelope. For large buildings there is a more appropriate test method: ASTM E3158, Standard Test Method for Measuring the Air Leakage Rate of Large or Multizone Buildings.


For E779, the first step is to establish a single zone to test. The zone be accomplished by closing doors to adjacent spaces. Fireplace dampers should be closed, and the balancing dampers in the HVAC system should not be adjusted. A uniform air pressure should be maintained throughout the zone.


The temperature in both the interior and exterior should be monitored as well as wind speed and direction. The test procedure itself is not suited when it’s windy or the temperatures outside are greatly different from those on the inside. The ideal range for temperature is from 5ºC to 35ºC, and for wind speed it is 0 to 4 mph. The height of the building can become problematic for what is termed the “stack effect” where the height of the building can induce vertical air movement. The height of the building along with the building’s height above sea level are to be recorded.


Envelope pressure sensing devices are installed at strategic locations along the exterior envelope. These locations should not be near building corners or any unique architectural expression. A specially designed fan assembly is connected to either an exterior door or opening.


Initial flow measurements will be taken with the fan openings blocked. These measurements will define a baseline and will be subtracted as the fan is ramped up to create higher pressure differentials. The standard states that the range of induced pressure differential will be 10 to 60 Pa, and they should be applied in 5 to 10 Pa increments.


ASHRAE research revealed the most common location for air leakage is at the intersection of roofs and walls. Other areas include soffits, roof overhangs and in locations where conditioned space is adjacent to non-conditioned space within the same structure. In 2014 the Center for Energy and Environment published an air leakage investigation on 26 existing institutional and commercial buildings constructed between 1936 and 2009. All were built prior to any air barrier requirement. The distribution of actual leaks was as follows:

  • Wall/Roof Intersections – 59%
  • Exterior Doors – 17%
  • Windows – 7%
  • Soffits – 4%
  • Mechanical Penetrations – 4%
  • Other – 9%

The average size of the gap between adjacent elements varied with the location:

  • Wall/Roof Intersections – 0.095 inch
  • Exterior Doors – 0.068 inch
  • Windows – 0.033 inch
  • Soffits – 0.164 inch
  • Mechanical Penetrations – 0.099 inch

The relative size of these gaps may make them hard to discern during construction. From a contractor perspective, the locations cited above are quite possibly beyond the scope of their work. In all cases, the leak is at a location where one trade encounters another. However, it may be the contractor’s problem should a forensic analysis of the leakage reveal that their wall is the source of the leak, even though it’s the interface that is compromised.


To guard against this situation, the contractor should understand the building science behind air movement and leakage. Further, the building code to which a project will follow is very important. This knowledge should then be applied to the construction techniques the contractor employs. Training of the staff, along with a review of the contractor’s quality program, would be warranted. On every project, the scope of work should be clearly called out with open communication between all the trades involved with sealing critical interfaces.

Robert Grupe is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.

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