A project was undertaken to develop a computerprogram to predict air temperature and humidity within buildings. Mathematical models were developed for heat recovery ventilators, both with and without moisture recovery, moisture transport through basement walls and floors, window condensation and evaporation, and moisture storage in interior materials. These models were inserted into a commercially available dynamic simulation program that previous!y had only rudimentary methods for estimating indoor humidity. The upgraded program was used to investigate the role of various types of ventilators in maintaining acceptable indoor humidity. This investigation produced recommendations for selection guidelines for residential enthalpy recovery ventilation systems.