Poor Make-Up Air is A Major Risk We Haven't Learned to Stop Taking

Expert Contributors:

Haven’t there been enough lessons learned through mold and moisture construction litigation in the hospitality, multi-family, student and military housing sectors to show that dumping make-up air in the corridor is a risky proposition? Apparently not! This concept of make-up air delivery to a corridor has and continues to be a mold and moisture risk because the make-up air cannot reach each occupied room on every floor for purposes of ventilation, pressurization and make-up air for exhaust.

Our firm has been writing and teaching about this issue for decades. Yes, literally decades. Still, in our hot and humid area of the world, we find new buildings being designed this way and ending up with mold and moisture problems. That is why we feel that this article that was originally written a few years ago deserves to be refreshed and reissued.

A typical scenario can be found in a recent luxury senior living complex that had moisture and mold problems in the residential living units. The building uses continuous exhaust for the living unit bathrooms and some general exhaust in the kitchen. Large, 100% dedicated outside air units that supplied the outside air to the corridors also exist on each floor. Through the use of pressure mapping, our firm determined that the living units were not receiving sufficient air from the corridor to overcome the amount of exhaust from the living units. This resulted in living units that were depressurized when compared to the outdoors. Even though the walls had interior finishes that were forgiven, condensation and mold damage ensued. Further, mold growth occurred on surfaces in the living units.

mold damage around bathroom exhaust in senior living community
Moisture damage around a bathroom exhaust fan from reverse air flow through the exterior wall cap.

Why Are Buildings Still Designed This Way?

Many times, it is intended that make-up air will reach each occupied room across each room's door undercut. However, the make-up air can’t reach each room because of resistance to airflow, the amount of required make-up airflow to each room, the size of the door undercut, and the internal and external pressures on the rooms and corridor. In addition, the test and balance can’t accurately measure it. Surprisingly, this make-up delivery concept continues to be designed and constructed when it is not permitted by the Building Codes. The reason that it is done is because it costs less. Despite violating the International and Florida Building Codes, local building officials are still allowing it to occur. But that discussion is for another day, another article.

Many designs can overlook or do not account for air leakage into a building through planned openings, the use of continuous exhaust, and local pressure problems because of return or supply side duct leakage.

A few examples of these design conditions are:

  • Air leakage through HVAC planned openings such as dryer and toilet exhaust, even those with backdraft dampers
  • Continuous toilet exhaust causing negative pressure in the living unit despite make-up air delivery to the corridor
  • False assurances of living unit positive pressure achieved from return side leaks (e.g. air handling unit room plenums or ceiling plenums)

The designs assume that if make-up air is provided to a building, regardless of where it is delivered, the make-up air will overcome the adverse effects from pressure problems. Overall, these designs cause infiltration, with the result being moisture, mold, and odor problems. Unbalanced airflows, excessive air leakage and infiltration are examples of air going where it is not intended to. 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 do not provide individual living unit moisture control.

The Way Make-Up Air Should be Supplied

HVAC designers, building envelope designers and architects can’t design in silos. Developers and contractors also need to be aware of the moisture and mold risks to living units when make-up air is dumped to the corridor. For these reasons, the make-up air should be ducted directly to each living unit at the correct quantities.  If a designer is not sure of the moisture and mold risks that may be inherent in their design, we recommend obtaining a peer review from a moisture consultant.