Tips on Room Converson for CWD

Hi I want to start my build but I'm having trouble figuring out what to do about my walls. First off my area is 12'Wx7'Hx6'D it will have a pre-molded 4'x5.5' pond with heater and pump lots of live potted plants and plenty of light. I was thinking of lining the whole thing with high density foam board 1'' thick to keep in heat as it gets cold hear in the winter. I need some ideas as how to make light weight artificial rocks or something to do my walls with. Grout or concrete would be far to heavy and also keep in mind non toxic and able to stand 60-80% humidity over a long period of time any ideas?

You could always mold them out of Styrofoam or great stuff and then coat them with grout.

Wow, that is going to be a fantastic sized enclosure.
Here's a link that might help with some ideas on different things.
For the background. There's a few different routes you could go.
Of course I think the best way is as already stated, styrofoam coated with grout or cement mix.
The benefit of doing this is you can create multiple ledges for basking.
This will be very time consuming but will look and function great.
Or you could cover a wall with lattice and grow vines up it. Still gives a lot more climbing area and looks very pleasing.
This is perhaps the best I've ever seen.
Built by Dave (Crocdoc) of Australia.

Syrpopor makes a good base for false rock backgrounds. It's easy to shape with a knife and heat gun. Then if you really want to save weight paint it the color you like or use a water proof clear coat colored with pigment powder ( save painting the backgrond as you"ll need to seal it later anyway). An coat this while it's wet with bed a beast/sand or a mix there of.

Maybe you can get some ideas from this:

Water Dragon "Summer Palace" Construction - Reptile Information - Caresheets Database