Multifamily buildings with good insulation, great windows, and decent ventilation systems--what's not to like? Well, unfortunately, over the past five years, we have been getting calls from multifamily building owners who are battling persistent summertime humidity problems in their units. Comfort complaints, sweating ductwork, waterlogged drywall, and mold on furnishings, clothing, registers... you name it. We will discuss how we went about these investigations, and the tools we used to diagnose and sleuth out the problems. When solving these problems, unfortunately, there was no ‘magic bullet’ or single cause that we could pin down: it was often a ‘perfect storm’ of factors, that combined to create the issues. From air conditioner sizing vs. cooling load, to poorly configuring cooling systems, to outside air ventilation system problems, to catastrophic air leaks. We will look at them all, and let you know options on how to identify and fix these issues.