Adding cups of water is not an efficient way to add humidity, especially if you have a big fan sucking all of that air out of the grow area. That humidity in the air that's created by the cups of water will get sucked out of the grow room along with the rest of the atmosphere.
Depending on the size of the grow, you could make a humidity tent that sits around your plants, but doesn't cover them entirely, like a humidity dome would for a flat of clones or seedlings -- that might hold far too much humidity (depending on how fast the air is sucked out of the room by that fan of yours -- there's a couple of dynamically related variables here). Something needs to limit the amount of air moving out of the room in order to keep the humidity sufficiently elevated to nominal levels. That's a project though. Not difficult if you like DIY though. Just have to enclose the environment enough by adding or removing enough material (from the plastic humidity tent you would place around your plants) to ensure adequate ventilation that doesn't completely negate the added moisture from the cups of water.
That's my ramble.
<3dj.bath
