Because I have to use a heat lamp to get the temps correct I'm also battling with with my humidity. Something I have found that helps me is to cover half the screen top with a wet towel. I only have 1 working humidity gauge (I still need to get digital ones for I'm using dials), but it reads at 65-70% for me on the cool side. Hot side is probably still down to 30-40% because of the lamp, though. The towel is more so a temporary fix until I can find a more permanent solution, but it might help you with your shed until you find a better solution as well. Hope that helps.