moisture building assessment

Figure 1

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

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.

To be continued…

Click here to read our previous post on this topic.

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