I have a little greenhouse with just screen and have a fan hung up with a piece of corrugated fiberglass over it to keep it from getting soaked. It is just a cheapy from Home Depot. Seems to really keep the temp down as it was getting horribly hot. Don't know if you could add a fan with a bit of plastic to keep it dry but it really does help. Also helps with drying things out a bit as that has been a huge problem this year.