We insulated a 12 X 32 building from the inside.
It is my wifes future sewing cottage. This insulation is so much easier, to, install than roll fibreglass. No itch either.
As we, worked it was 95 degrees outside and the window A/C was struggling.
Once the insulation was on the ceiling the A/C was able to keep it at 72 degrees.
I bought extra to put on our covered porch. It drips, when the weather cools.
Have not installed that yet.