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Hot, humid climate makeup air conundrum: The moisture and mold risks that developers, designers and installers continue to take

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Hot, humid climate makeup air conundrum: The moisture and mold risks that developers, designers and installers continue to take

Donald B. Snell, PE, CMC | April 18, 2018

Moisture damage around the bathroom exhaust fan from reverse air flow through the exterior wall cap

But why?! Hasn’t there been enough lessons learned through moisture/mold construction litigation in the hospitality, multi-family apartment high rise, student and military housing sectors that show that dumping the make-up air to the corridor is a risky proposition. Apparently not?! This concept of make-up air delivery to a corridor has been and continues to be a living unit moisture and mold risk because the make-air cannot reach each occupied room on each floor for purposes of ventilation, pressurization and make-up air for exhaust. Many times it is intended that this makeup air will reach each occupied room across each rooms door undercut. But it can’t because resistance to airflow, the amount of required makeup airflow to each room, the size of the door undercut, and the internal and external pressures on the rooms and corridor. In addition, test and balance can not accurately measure it.

Condensation in dryer exhaust duct from outside air infiltration.


Most designs often overlook or do not account for air leakage across the living units through planned openings, continuous exhaust,  infiltration and the consequences of moisture, mold, and odor problems that occur as a result. These conditions are described and/or defined as follows:

  • Air leakage through HVAC planned openings such as dryer and toilet exhaust including those with backdraft dampers
  • Continuous toilet exhaust
  • False assurances of living unit positive pressure achieved from return side leaks (e.g. air handling unit (AHU) room plenums or ceiling plenums).


Unbalanced airflows, excessive air leakage and infiltration are not examples of air going where it is intended. Although these designs that dump make-up air to the corridor have benefits that compartmentalize airflow and minimize infiltration from the windward to the leeward side of a building,  they lack best practices for individual living unit moisture control.


What is more surprising, this make-up delivery concept continues to be designed and constructed and it is not permitted by the International and Florida Building Codes. The reason that it is done is less expensive first cost.  Despite violating the IBC and Florida Building Codes local building officials are still allowing it to occur.


HVAC designers, building envelope designers and architects can not design in silos. Developers and contractors also need to be aware of the moisture and mold risks to living units when makeup air is dumped to the corridor. For these reasons, avoid the make-up air conundrum and duct the make-up air directly to each living unit.  If you are not sure of the moisture and mold risks that may reside in your design, obtain a peer review from a moisture consultant. 


Download our free e-book on “The Single Most Important Factor in Reducing the Risk of a Mold and Moisture Lawsuit in Your Next Project.”

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