moisture building assessment

Figure 1

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

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. (See Figure 1.)

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.)

To be continued…

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