had the same problem and tried buckets of water in the room which did not work.
the only way is to buy a top quality humidifier or one of these mist makers/ pond misters/foggers. i have one and it works great.
just put it into a plasterers bucket filled with water and have a fan aimed at it to blow the mist into your grow space.
humidity of 30% to 40% when you are vegetating can kill your plants as they transpire water more quickly and then the leaves go dry and curl up and eventually they go crunchy.