Mold was growing on the personal property of tenants and on building surfaces throughout a 1,200 unit Miami, Florida apartment building. We were asked to determine the cause of the mold.
Our investigation found high relative humidity throughout the building. Humidity levels increases as we approached the grills in the corridors where ventilation air was added to the building. Evaluation of the rooftop unit that supplied ventilation air found that it had been damaged by a recent hurricane, and was no longer dehumidifying ventilation air. This was the primary cause of the high humidity condition. In addition, study of airflows, and measurement of pressure differentials found a contributing problem. Bathroom exhaust fans were operating twenty-four hours a day, causing the apartment units to be at a negative pressure relative to outdoors. This negative pressure caused humid outside air to be drawn into the apartment units, and into wall cavities.
Resolution of the humidity problem required repair of the roof top ventilation unit, and to correct the negative pressure in the apartment units, individual exhaust fans were installed in bathrooms and discharged into the original exhaust duct. The original exhaust fan was removed and replaced with a variable speed fan controlled so that the amount of exhaust air being drawn from the building matched the needs of the individual exhaust fans in each bathroom. After repairs, we took measurements in the building to verify proper humidity control, ventilation, building pressurization, and air flow.