Elevated Indoor Relative Humidity in New Construction Caused by Whole House Ventilation Systems

To improve indoor air quality, some new construction single-family and condominium structures include mechanical ventilation systems to introduce outdoor air to the occupied spaces. Some of these ventilation systems are designed to provide outdoor air to the conditioned space on a pre-set schedule. These systems can cause elevated humidity, condensation, and mold growth in the conditioned spaces in some applications.

Some ventilation systems do not pre-condition the outdoor air to remove humidity; instead, they rely on the air-conditioning evaporator coil to dehumidify the outdoor ventilation air prior to distribution throughout the conditioned space.  A problem can occur when the system introduces outdoor air on a regular schedule, including periods when the air-conditioning evaporator coil is not energized.  During periods of cool and humid weather when the air-conditioning system is not required for cooling, this ventilation system operates on its pre-set schedule and circulates humid air throughout the conditioned space.  Over time, occupants notice higher humidity, condensation on cool surfaces, and even mold growth.

In Nelson’s experience, the climate is an important consideration for selecting the outdoor air ventilation system, and it is often overlooked. A ventilation system that does not pre-condition the outdoor air may be appropriate in an arid climate. The same system installed in a similar structure in the hot and humid south may negatively impact indoor air quality rather than improve it.  In the hot and humid south, an outdoor air ventilation system that pre-conditions the outdoor air without reliance on the air-conditioning system may be the only appropriate choice.

Nelson’s Mechanical Engineers are expert at investigating and diagnosing causes of elevated indoor humidity, surface condensation, and mold growth.