Multifamily buildings with good insulation, great windows, and decent ventilation systems – what's not to like? Unfortunately, over the past five years, we have received calls from multifamily building owners who are battling persistent summertime humidity problems. 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 these issues. When investigating these problems, unfortunately, there was no ‘magic bullet’ or single cause that we could pin down: it was often a perfect storm of factors. From air conditioner sizing vs. cooling load, to poorly configured cooling systems, to outside air ventilation system problems, to catastrophic air leaks - we will look at them all, and discuss how to identify and fix these issues.