Quick review: Our tiny home started as a sun porch attached to an old travel trailer, built in 1960. From my work on the remodel, most measuring when building it used the "looks about right" method of measuring. There are 16 windows (also from 1960). On the back wall, I purchased an Amish wood building for the kitchen and bathroom. That part is completed 100%.
This past week, our handyman helped finish the ceiling on the porch side. The insulation I use, which is mouse unfriendly, is a product called Reflectix which has gained enough popularity now that it's harder to find. It's bubble wrap that's coated with Mylar on both sides. I also spray a peppermint/garlic oil spray in the air-gap. The mice are pretty upset with me right now.
I'm using Reflectix in it's R-21 configuration which is one layer with an air-gap on both sides followed by another layer, so you have the roof boards, a 1 inch gap, layer of Reflectix, a 4 inch gap and another layer of Reflectix. Both the walls and ceiling are insulated with that method of installation.
The ceiling panels which I had delivered last spring and have since found from photos of tiny homes I've found online is apparently very popular, distressed barn wood. After the panels were up (which were very floppy and a pain to hang), I used 1 x 2 strips over the seams between the panels to finish the look I was after. The wall under the windows is White Oak paneling and the back wall is red brick.
This past week while I was there, we had one 41 degree overnight period. I had the heater set to 4,000 BTU which heated the full 400 square feet. This is a night and day type of difference in energy efficiency.