Often overlooked, air movement in your grow room is almost as critical as feeding your plants! Good air movement helps regulate temperature and humidity better - you risk having one side hotter/colder than the other without it! Your grow room may not be too hot or too cold...just maybe your temperature gauge is sat in a cold/hot spot.
If you find yourself with a mould and mildew problem, chances are you didn't have adequate air movement and/or you've crammed your space with too many plants that the room can handle. In that case, maybe it best to sacrifice and cut your losses on the most affected plants and get some more fans in there - a diffuse air combined with a duct fan is an ideal option - a little bit more expensive but gives ultimate coverage in air movement.
A nice breeze in your grow room mimics nature where plants will, in response, develop a stronger structure. This, in turn, produces stronger, healthier plants setting up the foundations for more nutrient uptake and bigger yields!