My insulation was free. A company I do work for had received marble table tops that were packaged in Styrofoam blocks 3"x 3'x3'. I got about 4 or 5 blocks. I cut them up and placed them in the roof. I used their old convention booth which was a kind of corrugated plastic board covered with a black cloth for the interior walls.
I haven't camped in it yet and don't plan on using it in temps below 30 degrees but besides the insulation, it will cut down on campground noise and rain noise. We'll see.