Greenhouse Recuirements for Orchids

[NeonKraken]

Hi, I'm planning on building a greenhouse for my orchids in my backyard but I would like to spend as little money as possible. I need it to function as a place for my orchids year round. Though I won't be able to build it up against my house, I have a wide range of places in the backyard, about 20 by 10 ft areas. So, my question is: What are the minimum revuirements for a year round orchid greenhouse? thank you.