Mechanical pipe insulation systems are installed on chilled water pipes that operate at below ambient temperatures inside buildings. Water vapor is drawn from the surrounding ambient air toward the pipes and the thermal conductivity of the pipe insulation systems is affected by ambient temperature and water vapor ingress. This talk provides an overview of the findings from two recent ASHRAE funded research projects on this topic. The speaker shows cases in which pipe insulation thermal conductivity increased with moisture ingress. This talk also illustrates the importance of including appropriate insulation thickness safety margins.