The Role of Moisture and Airflow in Building Design, Construction & Commissioning

This article is compiled from a series by Liberty Building Forensics Group examining the relation of moisture building assessments and building science as a component to design, construction, and commissioning. This content has been approved for re-publication by AEC Quality .com.

Moisture Building Assessments: A Retrospective Look at Building Design and Operation

Many of the decisions made during the building design process can either create or lead to problematic situations with airflow – even when approved building codes are followed.

One such example can be seen when mechanical closets for air-handling units (AHUs) have outdoor accessibility.

The AHUs located in these closets are intended to circulate air conditioning or heating to the living spaces which they serve. It’s assumed that the air will automatically circulate and go where it is intended.

This doesn’t always happen, however, because problematic building designs often don’t account for excessive air leakage; infiltration; and consequential moisture, mold, and odor problems. Here are several important factors about airflow to consider:

  • Excessive air leakage can occur when wall materials, assemblies, and whole wall systems do not meet the leakage rates in accordance with the American Society of Testing and Materials (ASTM).
  • Excessive air leakage can also occur at air handling unit casings.
  • Infiltration can be caused by unconditioned airflow due to wind pressure and mechanical effects.
  • Warm, humid climate conditions tend to exacerbate the consequences of moisture problems within a space.

Imbalanced airflows, excessive air leakage, and infiltration are examples of air not going where it is intended. Although designs such as outdoor-accessible mechanical closets have benefits such as maximizing space and functionality, they lack best practices for moisture control. The consequences of moisture and mold problems are even greater when these mechanical room closets are designed as return air plenums. (See Figure 1.)

Understanding Where the Air Comes From, Where It Goes, and How It Impacts the Living Space

Air will take the path of least resistance. Infiltration and air leakage across the mechanical closet and air handling unit represent a path of least resistance.

This type of infiltration: 1. Depressurizes the mechanical closet, 2. Reduces the amount of return air from the living space, and 3. Results in a positively pressurized living space (as seen in Figure 1).

Normally, positively pressurized spaces are a best practice condition, but in this case, the air handling unit cooling coil is not sized for this amount of infiltration and becomes inadequate in dehumidifying the supply air being distributed.

This condition results in an elevation of relative humidity, which translates to the contact of warm, humid air with cold surfaces in the air handling unit. These are benchmarks conducive to elevated condensation.

Building Symptoms and What Tenants Will Typically Do to Tolerate the Problem

The tenant, in an attempt to achieve more comfortable space conditions, will typically lower the thermostat set-point, causing these spaces to cool further while the relative humidity remains high.

This increases conditions conducive to condensation and mold on cold surfaces in the space. The mold then gives off odors known as volatile organic compounds (VOCs), which cause the tenant to mask the damp odors with disinfectant.

At this point, the tenant recognizes that it is too damp in the space and attempts to clean the mold and also installs desiccants to help minimize the dampness. The symptoms continue until the tenant has had enough or the problem gets too big to control.

Building Science Approaches for Controlling Airflow Pathways

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

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.


The information provided in this article 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.

About the 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.

Mr. Snell is a heating, ventilation, and air conditioning (HVAC) consulting and building systems professional with nearly 25 years of experience in forensic building investigations related to moisture control, indoor air quality (IAQ), building science and construction management.

Mr. Snell is a Forensic Consultant on projects requiring litigation support, moisture and related issue assessment, evaluation, remedial HVAC concepts, and life-cycle cost analysis. This experience includes both forensic and non-forensic projects.

Liberty Building Forensics Group® (Liberty) is the leading building forensics firm responsible for solving some of the world’s largest, most complex building failures – particularly those involving moisture & mold problems. Boasting a combined 200 years of experience in solving construction and design deficiencies, Liberty’s experts bring to the table knowledge gleaned from over 400 projects to solve your building needs and green building moisture problems. Besides addressing your technical problem, our team also helps you resolve any business challenges you face as a result.

The original posts comprising this article can be found at Liberty’s website: Part 1, Part 2, Part 3, Part 4.

Image courtesy of lorenkerns