I have modeled a rather small room, and when trying to get the right camera angles I can't fit the camera inside the room without getting too close, so I wan't the walls and ceiling only to be visible if the camera is placed inside the room. Understand?
Make it so the camera can look through the ceiling and walls from the outside, but they still appear in the cameras view when the camera is on the inside looking out?
This is kinda same idea as @Duarte Farrajota Ramos but it is working in Cycles (viewport/camera).
Your mesh normals should point to the inside of the room as in the image below.
You will need to assign different materials for floor and walls and ceiling.
Floor material. Could be anything you want.
Wall and ceiling material. Should look like this and in the place of red Diffuse you can and anything you want. Remember to check if Transparent shader color is pure white. Hex value #FFFFFF.
The solution I found that actually worked was, scroll down in material properties, and check Backface culling.
Hmm, this is I guess the same as another answer. Anyway, here goes:
Use the "Backfacing" output of the "Geometry" node as the mix factor between your shader (whatever it is) and a transparent shader.
No matter what direction you are looking from, anything with front-facing normals will be transparent. (As you can see from the picture, suzanne and a point lamp are inside a solid sphere, but in the rendered view you can see everything inside the sphere plus the back wall of the sphere.)
Note, however, that in an enclosed space all of your illumination is going to have to come from inside, and/or windows/openings!
