This post by Don Snell is the fourth in a series of four examining the relation of moisture building assessments
and building science as a component to design, construction, and commissioning.

solving mold and moisture problemsLet’s discuss some building science approaches for controlling airflow pathways with mechanical closets, while achieving reasonable design solutions for space and functionality.

Sealing a mechanical closet to minimize infiltration, and sealing wall systems and air-handling units to reduce air leakage, won’t likely mitigate these moisture problems because they are multi-factorial in nature.

The solution lies in taking a multi-disciplinary approach early in the design phase to identify the sources and pathways for moisture problems.

One component in this building science approach is controlling the airflow direction. For moisture control, the direction of airflow is more important than the magnitude. A space under a slight positive pressure with cool dehumidified air is better than a negatively pressurized space subjected to infiltration and air leakage.

Fairey, Chandra and Moyer (2007) indicate that a pressure difference as little as -2 Pascals (Pa) with respect to the outdoors can lead to moisture problems, while a pressure difference as small as +2 Pa with respect to the outdoors minimizes conditions conducive to mold. *

Additionally, another building science approach utilizes adjacent space exfiltration in lieu of infiltration when incidental air leakage exists.

Again, the building design component that created conditions conducive to moisture problems was the outdoor accessible mechanical closet for the AHU. Utilizing approaches such as adjacent space exfiltration and controlling the direction of airflow are examples of minimizing moisture problems and can be done while achieving reasonable design solutions for space and functionality.

Note: The information provided in this blog series was intended to create awareness of the risks of building design choices. The approaches presented in this discussion are some components based on research and experience, and do not constitute complete building solutions to this building design example.

Click here to read our previous post on this topic.

* Source: Philip Fairey, Subrato Chandra, and Neil Moyer, “Mold Growth,” Florida Solar Energy Center, last modified 2007, accessed July 25, 2012,

Author Donald B. Snell, PE (Georgia, Virginia, and Pennsylvania), Certified Mechanical Contractor (Florida), and Senior Mechanical Consultant with Liberty Building Forensics Group, has provided moisture and IAQ-related forensic building investigations on more than 200 buildings. For more information, contact Donald B. Snell at