This is one of a new series of posts based on real questions I have been asked. Feel free to add to the answers or include your ideas and experiences.
Someone asked for recommendations for plants in an unheated conservatory. They had some cacti and an aloe but wanted something ‘leafy’. They had discovered that Dracaena marginata and orchids would not survive.
An unheated conservatory is a tricky place to grow plants because it is often too hot for plants (and people) in summer and then it can be icy cold in winter. If there is any heat then if it is connected to the central heating then it can be turned off at night. This restricts the range of plants that will survive let alone thrive. I would suggest a strelitzia (bird of paradise) which will survive extreme heat in summer and temperatures down to 0c in winter if kept quite dry. I remember one that once got too big and was eventually put in an unheated polytunnel. It rooted into the soil so could not be moved and was left out for years and, though it was not a thing of great beauty it flowered like crazy.
Other options are Cycas revoluta, which is palm-like and very tough and Phoenix canariensis which is hardy in some areas. Both will put up with extreme heat in summer as long as they are watered and tolerate low temperatures in winter if more or less dry. Oleanders (Nerium) should also cope but be aware that they are poisonous if eaten. Red spider mite is going to be a possible problem.
Orchids are a varied bunch and cymbidiums may put up with the winter cold but should be put outside in summer because they will not like the extreme heat. Making cultivation more complex, pleione orchids would be OK in winter and will bloom in spring but would not tolerate the heat in summer, and will need to be outside as soon as temperatures start to rise.
Many conservatories are put on south walls and these become ovens in summer. Any other situation is an improvement. A solid or insulated roof will make the structure better for plants and people.