.....I've recently finished transforming a gutted detached garage into my dedicated listening room.
It is extremely well insulated with R-13 insulation between the studs and a layer of both 5/8" drywall over 1/2" quiet rock all around, except for the ceiling which is only 1 layer of 5/8" and the R-13.The space above the ceiling should have at least an R-30 insulation rating, R-40 being better yet. Most of your heat in the room is going up through the roof.
Lissnr
Jim