I insulated the interior of my tiny house with the same insulation I did for the floor. Roxul. I used the R23 for the ceiling because I have 2x6 ceiling rafters and R15 for the wall and the 2x4 framing. I'm pretty happy I used it. It's super easy to cut, doesn't make that horrible Styrofoam squeaky noise that sets me shivering, or make that huge static clingy Styrofoam mess. The weird little corners and shapes around the pipes and wires were challenging but not overwhelming. I really hope I didn't forget to do something in the walls.
Then just like the subfloor I covered the walls with a plastic vapor barrier. I did this because of reasons. Look it up if you want the science. As long as I have good ventilation I'm hoping this is the correct thing to do.
The ceiling was especially fun because of gravity. The plastic wrap helped hold it into place but I ended up with some pretty itchy eyes from stray fibers even with the eye protection.
