This photo was taken at the Public Service Company of Oklahoma, which was plagued for years with serious mold problems in the air-handling units. (Photo courtesy Dr. Richard Shaughnessy, Tulsa University.)
Rooftop air-handling units can contribute to indoor air quality in a variety of ways. Recent information offers more insight into how to make rooftop unit installations more moisture resistant while passing along fewer biological contaminants.

At the American Society of Heating, Refrigerating, and Air-Conditioning Engineers’ (ASHRAE’s) IAQ Conference in San Francisco, CA, Larry Schoen, P.E. and principal of Schoen Engineering, Columbia, MD, discussed “Controlling Moisture in Buildings’ Mechanical Systems.” He covered several interrelated aspects of mechanical systems, including plumbing, piping, ductwork, and condensation, much of which would be familiar to hvac contractors.

But perhaps most interesting was his information on the interactions between the roof system and rooftop/exterior hvac systems, and how these can result in moisture infiltration.

During his “nuts and bolts, or pipes and ducts perspective,” Schoen discussed the roof system (including the membrane) and how it is sometimes breached when air handlers are installed on the roof. Exterior ducts also allow the entry of moisture to the system, where it can help breed biological contaminants.

After four weeks of exposure to UVC light, as shown here, the mold problems were abated. (Photo courtesy Dr. Richard Shaughnessy, Tulsa University.)


“In some cases,” Schoen writes, “ducts on the exterior of a building, often the roof, supply air to the interior. If the ducts carry heated or cooled air, there is an insulation system present and it is often sealed with a brushed mastic and sometimes a reflective coating.

“Our experience,” he continues, “is that such coatings remain waterproof for a maximum of five years. The fact that personnel commonly walk on ducts on roofs further compounds this problem.”

The compromised insulation can result in condensation in hot, humid climates, when the duct contains cooler air than the outdoor air. “In cold climates, condensation can occur inside the duct.” In climates with a lot of precipitation, rain, ice, and snow can collect on the surface of the duct, and can lead to leaks in the duct and the entrance of moisture to the building.

“To prevent this,” advises Schoen, “treat the top and side surfaces of the duct [with] an appropriate roofing material and not a brush-on product. We have found that rigid insulation board covered by a welded seam roof membrane to be a system that works well.” It costs about three times as much as alternatives, Schoen says, but also “performs leak-free four to 10 times as long.”

An alternative would be for the seams in the metal duct to be welded and no insulation applied. In either case, “The top of the duct and insulation system (when present) should be crowned to reduce ponding.”

Schoen points out that roofs that are installed with hot asphalt are more forgiving of human traffic, such as hvac service and maintenance techs, since those roofs have some ability to reseal after puncture from materials such as metal tools or shoes. “Heavy foot traffic dictates the use of walkways and pavers on top of the roof surface,” he writes.

Regarding rooftop units, “Roofs are more likely to fail at equipment curbs,” says Schoen, “where flashing is more vulnerable than in the flat expanses where the roofing material is continuous. The weight of rooftop equipment can further depress the roof structure, resulting in ponding in a location where leaks are common.”

Moreover, installers need to realize that “Hvac becomes part of roof membrane,” Schoen says. “Put a roof membrane over the top of the air-handling unit” in order to keep the roof membrane intact.

“The devil is in the details,” says Schoen. “I believe in some cases it’s our job to lead and to explain.”


On the inside of rooftop units, research was recently published to prove that fungal contamination of hvac systems is a year-around condition, and that it can be effectively reduced through proper application of UVC.

A research study conducted in a 286,000-sq-ft office building in Tulsa, OK, has found that some UVC light fixtures (fixtures that use the ultraviolet C band) can effectively reduce fungal contamination within air-handling units (AHUs).

The study, led by Dr. Estelle Levetin, a nationally recognized mycologist with the University of Tulsa Faculty of Biological Science, looked at eight air handlers. Four were test units treated with UVC lights manufactured by Steril-Aire, Inc. (Cerritos, CA); the remaining four were control units, with no UVC treatment. The research team took air and surface samples from all eight air handlers in the spring and fall of the same year, both before and after the cooling season.

Surface samples were taken from the air handlers’ insulation prior to the cooling season, before the UVC lights were turned on. According to the report, the samples contained nearly identical mean concentrations of fungi — approximately 212-213 x 103 colony-forming units (CFU)/ sq cm.

After cooling season, fungal concentrations in the untreated (control) air handlers increased to 2,240 x 103 CFU/sq cm (more than tenfold). In the UVC-treated air handlers, concentrations decreased to 30.5 x 103 CFU/sq cm. “This is nearly a 99% reduction in the contamination of the insulation surfaces,” states the company.

Similar results occurred with the air samples. Sampling in the spring showed mean concentrations of fungi to be similar in the control and test units — about 21-22 x 102 CFU/cu cm. After cooling season, the control units with no UVC experienced a tenfold increase in fungal concentrations, to 239.52 x 102 CFU/cu cm. The UVC-treated air handlers experienced nearly a tenfold reduction in fungi, to 2.98 x 102 CFU/cu cm.

Dr. Levetin concluded that “Germicidal UV irradiation may be an effective approach for reducing fungal contamination within AHUs.”

Forrest Fencl, Steril-Aire president and ceo, commented that “Statements from the operators of the facility used in the study and hundreds of others support these findings.

“Building operators consistently report that the use of UVC yields a noticeable difference in air quality, while enabling them to reduce or eliminate costly cleaning of coils, drain pans, and plenums,” he said. “Also, because there is no mold or organic buildup to compromise performance, air-handling equipment runs more efficiently and energy costs are reduced.”

For a reprint of the study, “Effectiveness of Germicidal UV Irradiation for Reducing Fungal Contamination within Air-Handling Units,” contact: Steril-Aire, Inc., 11100 E. Artesia Blvd. #D, Cerritos, CA 90703; 562-467-8484; 562-467-8481 (fax); (e-mail); or visit (website).

For more information on ordering proceedings from the IAQ Conference, contact ASHRAE, 1791 Tullie Circle N.E., Atlanta, GA 30329; 404-636-8400; 404-321-5478 (fax); (website).

Publication date: 01/28/2002