It's a wonderful idea. There are problems with greenhouses that often aren't discovered until they're in use.
Overheating, difficult to control without some sort of ventilation, and the green house will probably not hold in
much heat overnight. Rocks and water may help hold in heat, but it's still going to be cold in the morning without
a heat source.
I have a green house I use for veggies, and it took some experience to figure out how best to use it. The amount of money
you put into it makes a difference, I made my building about 12 feet long, 6 feet wide and 8 feet tall, and covered
the whole thing in wire screen with a door, essentially a big box, and then set a gable roof on top to help with rain.
The sides are covered in clear plastic. I made the whole thing myself, it depends how handy you are with tools. I know someone
with a conservatory that raises butterflies, I've only seen pictures of it, but I would guess he's built in some better solutions
for escape. Think about what you see at the zoo- a lot of work raising tropical animals.
I thought I could use it for butterflies or moths, and it is useful, but only for natives, I'd have a look at You-tube if I was going to build an animal specific enclosure, butterflies need a lot of space, so the bigger the better.