Yes Nobile types will go to zero. But keep them dry during the winter rest. There should be culture sheets available for them somewhere. Actually they are fantastic for cool growing and in some climates will take full sun as well during the winter season.
I'd like to go back and correct an misunderstanding Randy has: a larger greenhouse - which typically will have a lower surface-to-volume ratio - is far more temperature stable than is a small greenhouse.
It sounds to me that you simply need more heating capacity.
Why is that theory wrong though? An Aquarium for instance has a more stable temperature the larger it is. So why wouldn't an orchid house or shade house?
It would take more time for a larger facility to change it's climate than a smaller one surely?