As buildings are constructed increasingly air-tight to reduce energy consumption for heating and cooling, they require mechanical ventilation (fans) to bring in outdoor air to maintain indoor air quality. Ventilation has a strong effect on comfort, indoor humidity levels, and durability. One type of ventilation technology that reduces the energy needed to heat or cool the outdoor supply air is known as an energy recovery ventilator (ERV). This piece of equipment transfers both heat and moisture between exhaust air and supply air streams; however, the amount of moisture transferred between air streams is a complicated function of indoor and outdoor conditions and ERV material properties. Manufacturers typically provide information only for summer and winter extreme conditions. A Forest Service study at the Forest Products Laboratory in Madison, Wisc., combined (1) in situ field measurements using an ERV in the Forest Products Laboratory’s Research Demonstration House, (2) laboratory measurements of material properties, and (3) computer modeling to characterize the moisture transfer through the ERV under a wide range of conditions. This study provided a physical basis for understanding and optimizing ERV performance and for understanding the impact of the ERV on indoor humidity conditions.