In my experience, the plants are happiest when 40% to 60% humidity is maintained throuout the life of the plant. However, the seedlings and clones do better at about 75% for the first 7-10 days, and a lot of people like to reduce the humidity to 40-45% max during flower to avoid the nasties that accompany high humidity. I discovered that when the room gets above 75%rh, the plants will actually slow down their growth as they aren't able to "sweat" transpire in the warmth. Also the higher yer heat in yer grow room the more they will drink and "sweat" causing the humidiy to quickly jump up. I run my DH all the time but it has a setting that maintains the percentage that I want. right now I am really recycling a lot of water from my DH as all are in flower and its quite warm here. If yu don't have one, get a good digital hygrometer (not expensive) so that you can maintain yer RH for better results. good grow mojo