It sounds to me that you are talking about the backfaces. They don't render. If you are using a model program such as milkshape, you need to create the interior, such as a box for example and reverse the faces. You then can put it inside, , snap and weld the vertices...etc etc. and create the inside. If you are using something like Deled(probably not since some are 3ds files) if you hollow something it creates the interior for you. Just some ideas. Depending on your modeller, it should be a process similar to this.

Hope that explains it.
opps, edit.....just saw you use animat8or. the problem is your models, and the process is probably something similar to the above.