I am on my third hatch this season in an LG with a computer fan. We heat in the winter with a pellet stove= very low humidity. After evaluating our three hatches' outcomes, I have decided that a dry hatch is wonderful as long as the humidity doesn't go below 20%. So, I ignore the humidity unless it goes below 20%. If it does, I add water and tell myself the hen got caught in a rainstorm. The humidity goes up for a day or so ( to maybe 35-40) and then wanders down into the 20s again. By that time we have a rainstorm or the weather warms and the natural humidity keeps the incubator in the mid-20s or maybe into the mid-30s.
The cooler hatcher that DH made has a plastic tray that I put three sponges in and then drape a terry cloth fabric into the tray and up the side of the cooler , letting the lid catch the fabric and hold it up.
I fill the tray up to just below the level of the sponges. With 3 vent holes and a fan, I was delighted to find lots of air circulation and steady 65-70% humidity. I could put a hose thru a vent and add warm water to the tray without opening. We could feel the edge of the towel sticking out and find it still damp: still putting off humidity. Capping/uncapping the vent(s) controlled the humidity beautifully.
Your mileage will certainly vary depending upon your particular driving habits.
