Ever notice a breeze in older buildings even when it is not windy? Many buildings leak air, which means their owners are losing money. The Charlotte Mecklenburg School System (CMSS), the 19th largest in the U.S. with a $1.3 billion operating budget, recently decided to take action to identify school system buildings that might be exhibiting air leakage.
In early 2014, CMSS approved an architectural/engineering guideline change proposal to incorporate air barrier testing for all new buildings, additions, and major renovations. Using this guideline, CMSS’s plan was to decrease the air leakage from 0.25 cubic feet per minute (cfm) per square foot of exterior enclosure for buildings built in 2014 to 0.15 cfm per square foot of building enclosure for buildings built in and after 2018.
To help the school system determine sources of air leakage in one school built in 2015, Terracon performed air barrier testing in general accordance with the U.S. Army Corps of Engineers (USACE) Engineering Research and Development Center (ERDC), Air Leakage Test Protocol for Measuring Air Leakage in Buildings, using a proprietary (Retro-Tec) Blower Door Testing system.
For effective use of test equipment, the building had to be divided into five areas to be tested separately. Splitting up the building into separate areas proved challenging due to the amount of ducts and air transfers which crossed firewalls and had to be sealed. All doors and mechanical ducts were sealed to prevent extraneous air leakage, which can disproportionately alter the results of the test. The flow through the building enclosure was recorded at 10 different pressure points. These points were placed on a graph which created a straight line that was then used to calculate the flow through the building enclosure at 75 Pascals, a unit of pressure in metric units.
Smoke pens, devices that emit a constant stream of smoke that can be used to show air movement, were used on the interior of the building to focus in on the source of leaks during the pressurization phase of the test. A stream of smoke was moved across different surfaces with potential for air loss, such as front windows, doors, electrical outlets, roof and wall intersections, and other areas. Terracon also used infrared cameras to help detect air leaks. By repeating this process, Terracon was able to identify significant sources of air leakage. For this facility, the exterior doors and the intersections of the exterior walls at the roof deck were the major offenders. In addition, large gaps were exposed at the door locks and at the thresholds.
The allowable leakage rate of 0.25 cfm per square foot for an exterior enclosure area meets the target standard. The amount of leakage area this equates to is a single opening of approximately 36.7 square feet.
Other areas were identified, however, which could be easily repaired, providing the client additional savings in energy consumption. For an idea of how much these repairs could save, The National Institute of Standards and Technology issued a report stating that an office building with a total floor area of 24,200 square feet (approximately 30 percent of the floor area of the school building tested by Terracon) in the same climate zone saved more than $3,000 annually when air infiltration into the building was eliminated by 83 percent from its baseline. Equating this to the floor area of the entire school building, the expected annual savings is approximately $10,000.
By identifying the most efficient way to conduct air barrier testing for this type of school building, Terracon, in partnership with CMSS, was able to show results and recommend cost saving as well as energy conservation solutions, setting a baseline for future energy performance reviews within the system’s buildings.
Jeff Poe is an engineer in the facilities division in the Charlotte, N.C., office. He has experience in building enclosure testing, roofing design, above-and below-grade waterproofing, and contract administration.