Increasing emphasis on energy-efficiency has many jurisdictions enacting stricter energy codes. Yet, these same green building codes typically do not adequately address ventilation when a building envelope is designed to both minimize infiltration/exfiltration and maximize thermal efficiency. Our company investigated an apartment complex in Southern California, U.S.A. that was designed 25% more thermally efficient than required by State Code. Within months of occupancy, the first complaints of biological growth at windows and closets occurred. It appeared that the combination of occupancy and the tight building envelope does not permit dilution of interior humidity. We conducted multi-unit blower door tests to find the infiltration rate. We used EnergyPlus by US Department of Energy to simulate the interior environment and to model different forced ventilation scenarios to determine the effects on reduction of humidity levels. The simulations predicted optimal forced ventilation of 15 to 20 CFM per person. We installed computer-controlled, sensible and latent heat generating devices in two mockup units. The result showed marked differences between the repaired and unrepaired units. This paper will focus on the computer modeling, in-situ testing, and recommendations for practitioners regarding humidity control in multi-family housing.