I have left my cattleyas outside in the shade house until temperatures hit frost... That means several nights in the mid to upper 30's and even more in the 40's. No problems. It can be a problem if it is raining and cold.
If you have your greenhouse covered but not heated, I really wouldn't worry about it. At least not much. Worry about getting the heat turned on, but the plants can handle it for a while.
It is impossible for me to tell all the members of this forum how impressed I am with their free knowledge. In the short time I have been a member I have learned so very, very much and for this I will always be grateful to this forum!
Thanks buys