The management of water in all its phases — ice, liquid, and vapor — in buildings is a familiar topic among engineers. If unplanned moisture accumulation in walls, ducts, and other interstitial spaces leads to mold growth, then people with allergies can become sick immediately. Over time, mold can damage the building structure itself. The importance of indoor dehumidification to reduce water condensation and subsequent mold growth is reflected in ASHRAE codes that establish upper limits of indoor air rh. Surprisingly, however, building codes for lower limits are hard to find despite studies showing that indoor rh between 40-60% is optimal for building occupants.