In hotMoldy Cavity humid climates, outside air contains a large quantity of moisture. If outside air is drawn into a building by negative pressures, it travels through the wall system and into the interior space. As the air flows through the wall system and moves past cool, interior surfaces, the moisture in the air condenses and accumulates in the wall cavity.

While it is unrealistic to hermetically seal the building envelope, the magnitude and direction of airflow across the envelope can be controlled by the construction tightness of and the pressure gradients across the building envelope. The interrelationship between the forced air systems in the building construction dictates the pressure characteristic of the building.

Any feature that decreases the air barrier characteristics of the building envelope will increase the likelihood that the HVAC system will be unable to adequately pressurize the building. Because airflow follows the path of least resistance, air can be carried down demising walls of an interior space if they are connected to the exterior envelope. Also, because leaking air can travel long distances through wall cavities, it must be determined whether the HVAC forced air systems are pressurizing not only the interior of the building spaces, but also the interstitial spaces be connected to them case studies of shown that building spaces can be adequately pressurized wall adjacent envelope cavities are depressurized with respect to the outside.

This post was taken from an article titled, “Moisture Problems: Why HVAC Commissioning Procedures Don’t Work in Humid Climates”. by George DuBose and J. David Odom, and Philip Fairey, and was previously published by CH2M Hill.