Orchids dropping leaves

[ImABrat]

Hi...I've been growing orchids for several years. Last year, I had them in a shade house until the cold weather and have brought them in the house. They have been in the house now since maybe the beginning of November or maybe earlier but not much (north Florida). I have them in front of a northeast window on shelves with clear plastic wrapped around the shelves and just recently added clear plastic to the front of the shelving to keep them moist because they were drying out too much with the fireplace and heater running.

The problem that I am seeing more recently is my orchids are dropping their leaves. There is not one type of orchid but many...vandas, oncid, phals, etc. The leaves started dropping before putting on the front sheet of plastic, so I don't think it has to do with that. The leaves are green and they look just like any healthy leaf on an orchid. I mist them daily sometimes twice a day. I also misted them lightly in the evening because of the dry before putting on the front plastic. I water about every week to week and a half sometimes 10 days and use fertilizer every other watering.

Any ideas what could be going on? I just don't know what's happening to them. I expected some to die, which I have had a few do because of the change and them being young, but this problem I just don't know what to think.