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
