20% RH? damn! Wish i had that prob~ i'm in 5th wk of flowering and im having trouble keeping the RH down. how do you get yours so low? window unit?
as for more moisture in the air, are you misting your plants at all? are you using a cool tube? i think i read of someone putting a couple of buckets or big bowls full of H2O in the room (uncovered). put a fan in front of it blowing over the water...