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Researchers Investigate Air Quality of Portable Classrooms

side view of portable building

Oct.18, 2016

Professor Kerry Kinney and Associate Professor Atila Novoselac have completed a study of the air quality inside portable classrooms. The research team collected a detailed set of building ventilation and microbiological measurements from school portables and made suggestions for low-cost methods to improve the indoor air quality in these increasingly common structures.

Over one third of U.S. schools use portables, also known as temporary buildings, to provide classroom space when there is a capacity shortage. The research team is particularly interested in understanding the relationship between ventilation factors and the microorganisms and pollutants that are found in the air within these portables.

Their study, funded by the Alfred P. Sloan Foundation, found that while portables all tend to look the same, they can vary substantially in construction, design and operation. They also found that even though portable classrooms are different from conventional classrooms, the indoor levels of carbon dioxide, formaldehyde and volatile organic compounds in the older portables were similar to those found in conventional classrooms.

Fourteen portable classrooms situated in a hot and humid climate were investigated, with nine classrooms selected for detailed examination. The HVAC system, building, and environmental conditions were characterized in these classrooms. The classrooms were similar in size with a suspended ceiling or a small attic space; a crawl space also existed beneath each portable. Each portable building contained two classrooms, divided by an interior wall.

In some cases, the attic spaces were vented to the outdoors. When combined with loose-fitting suspended ceiling tiles, this vented attic creates a large leakage path of indoor air to the outdoors and vice versa. Open vents may allow wildlife or unwanted pests to enter into the attic space. The leaky ceiling and attic space also make pressurization of the occupied space difficult.

A properly operating mechanical ventilation system could provide the necessary pressurization of the conditioned/occupied space while controlling the source of fresh air and preventing air from crawl spaces or attics from entering the classroom. However, this was not the case in many of the tested classrooms, where insufficient pressurization allowed wind and temperature differences between the indoor and outdoor environment to drive uncontrolled airflow from the attic and crawl spaces into the classroom.

Microbial samples collected during the study indicate that some of the microorganisms found in the indoor air were also found in the attic and vice versa suggesting that there is communication between the spaces.  Not surprisingly, though, human-associated bacteria were the most commonly detected taxa in the indoor air of these classrooms, which are typically occupied by 20 to 25 students.  Also, positively pressurizing the classroom with fresh outside air increased the fraction of outdoor microorganisms in the indoor air, although human-associated taxa persisted.

The HVAC systems also differed substantially from portable to portable, with most being undersized and/or improperly configured for fresh air delivery. A frequent finding was that the fresh air intake system was disabled by closing the fresh air damper or by taping over the external air vents; anecdotal evidence suggests that this was done to overcome insufficient cooling capacity in the HVAC units. Only one portable in the study had a fresh air intake system with sufficient capacity to operate as intended.

close up view of duct tape on AC unit

A taped-over fresh air intake vent on a wall-mounted HVAC system.

Only a few of the classrooms investigated in this study met the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard requirements for ventilation; the majority of classrooms had a slightly lower ventilation rate than the standard’s recommendation. The results also show that buildings with a vented attic allowed for more uncontrolled infiltration due to weather-related pressure changes than their counterparts with non-vented attics. This resulted in greater overall ventilation rates for vented vs. non-vented attics.

Many of the indoor air quality indicators found in portable classrooms were similar to those found in regular classrooms.  When comparing overall ventilation rates and indoor CO2, VOC and formaldehyde concentrations, the portable classrooms were similar to standard school buildings. It is important to note that for standard classroom buildings the air quality is managed through controlled ventilation, since interior classrooms of a large school building cannot directly exchange air with the outdoor environment. This differs from the infiltration ventilation mechanism prevalent in portable buildings, where every surface – walls, ceilings, and floors - leads to the outdoor environment.

The leakiness of the portable building shell combined with their proximity to the outdoor space results in overall ventilation rates similar to those found in typical school buildings, and thus similar environmental conditions. However, this proximity to the outdoor environment also brings with it other issues. For instance, evidence of water intrusion due to weather was commonly found in portable classrooms.

close up view of water damage to wood

Damage to wood paneling due to water intrusion.

The results collected to-date suggest that several relatively easy solutions can be implemented to improve ventilation conditions and minimize uncontrolled ventilation in portables.  These include:

  • AC units should be sized for expected loads, which eliminates the need to close or disable the fresh air vents.
  • The HVAC system should be designed to deliver fresh air independent of cooling load through the use of a dedicated ventilation system. 
  • Attics should be unvented, with insulation installed against the rafters rather than on top of the suspended ceiling; this will allow for positive pressurization of the occupied space as a means to control infiltration. 
  • Regular maintenance of the portables should include verification of positive pressurization (2.5 – 5Pa) and proper ventilation (typically >3 air changes per hour); this will prevent uncontrolled infiltration that could transport undesirable pollutants into the occupied space. 

Portables are here to stay as a cost-effective solution for schools trying to meet student enrollment demands. Since portable classroom buildings often remain in place for extended periods of time, it is important that they be treated the same as permanent school buildings.

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