For a datacenter with a single controlled area the ideal range for relative humidity (RH) is in the neighborhood of 35% to 50%. Here are some data points: 1) Static electricity is minimized when RH is at or above 35%. 2) RH below 25% can cause embrittlement of hygroscopic materials such as paper. 3) RH above 65% can cause mold growth and metal corrosion. 4) Humans are most comfortable when the RH is between 20% and 60%. 5) RH above 50% in cold weather can cause condensation inside of outer walls (or on equipment itself if the facility has an external door or window that opens directly into the controlled area). Sean Donelan wrote an informative post on a related subject a few year back: <http://www.cctec.com/maillists/nanog/historical/9810/msg00276.html> -Richard