Which of the following can contribute to humidity issues in a multifamily building?

Under-ventilated bathrooms are indeed a significant contributor to humidity issues in multifamily buildings. Bathrooms generate moisture through activities such as showering and bathing. When these areas lack adequate ventilation, the excess humidity accumulates, which can lead to a range of problems including mold growth, structural damage, and poor indoor air quality. Proper ventilation helps to expel this moisture-laden air outside, maintaining a comfortable and healthy environment within the building.

In contrast, well-insulated attics generally help to control humidity levels by preventing unwanted heat transfer and maintaining stable temperatures throughout the building, which can reduce the potential for moisture buildup. Low occupant density and items stored in garages typically do not directly influence indoor humidity levels in the same way that under-ventilated spaces do. While high moisture contents can arise from certain activities related to occupant density or from the materials in garages, they are not primary contributors to humidity issues as insufficient ventilation is.