Thats a real High Wattage for a 20gal.. Your going to have trouble with Humidity for sure and if you cover the screen that wattage is going to bang out Hot Hide temps. You dont want to dry the snake out. I bet if you check the substrate in the enclosure directly under that CHE it will be a easy 85 after a few hours because my 60 watt was above the cool hide and was a min of 78. If its to hot just raise your CHE higher above the screen or use tour Dimmer until you find the right temp. The enclosure is going to cool a little at night when the house cools but thats natural anyway.