Three years ago, I built a small building in my back yard for my snakes. It's a 12x12 shed built to match my house: vinyl siding, shingled roof, etc. Inside, I framed and screened a third of the room for my lone big snake, a 15-foot burm. The snake has a 12-foot by 3-foot walk-in enclosure in which to live. On the other side of the room, I have an 8-foot Neodesha cage that I use periodically depending on what rescues I have (most recently, I had an 8-foot boa in there).
I have no problems with ventilation, but I do have two small windows that I can open if I need to. I'm in South Georgia, so my weather isn't too bad, but it can get fairly cool here in the winter. For the colder months, I have a propane heater that works wonders. I can keep the room about 40 degrees warmer than the outside temperature with the propane heater. When it drops into the 20's, I supplement the propane with electric heaters and a pig blanket. In the summer, the temp rises to nearly 90 on its own.
My burm quits eating from October until February, but he did that when he was inside, too, so my heating/cooling strategy seems to work. I love having the room, and wouldn't have it any other way. I have had as many as 18 snakes inside my house at one time, including six over 10 feet (the largest being a 16-foot retic). My snakes have been outside now for several years, and I have no desire to have a big snake in my house ever again. I'll take a few photos of the building over the weekend and post them soon.
RP