As Kaorte says, climate plays a big part. Covering the top will probably help. As mentioned, the humidity will escape upwards. What are you using for heat? Under-tank or bulbs? Bulbs kill the humidity. I have similar issues here. Have to use a bulb to keep the ambient temp up (cold here!) which kills the humidity.

Oh, by the way, I'm sure someone here will correct me if I'm wrong, but I'm sure you only need humidity of 60%-70% for a good shed - not 70%-80%.