This addendum clarifies and adds requirements related to indoor humidity and the building envelope as they relate to potential indoor air quality problems. Among other changes, it revises the humidity control requirements currently described in Section 5.10 by modifying the upper relative humidity limit and eliminating the lower limit. A lower humidity limit is no longer included because the data supporting such a limit are very limited and not entirely consistent. An upper relative humidity limit is now a design requirement (not simply a recommendation as in the current standard) for mechanical systems with dehumidifying devices and controls since high indoor relative humidity in conditioned spaces has been associated with conditions that can lead to microbial growth. Building pressurization requirements to minimize the infiltration of moist outdoor air (which can cause condensation on building surfaces during cooling operation) have also been added. In addition, the issue of condensation on cold interior surfaces and contaminant transport from attached parking garages is addressed.