The winter heat in my apartment is supplied by old PTAC (Packaged Terminal Air Conditioner) units, using resistance heat (it's too cold for the heat pump to work). A lot of cold outside air leaks through the insides of these units. I was wondering whether putting insulation on the exterior side of each PTAC unit, so that the whole unit is completely insulated from the outside, is a good idea.
Obviously, I'd remove that rear insulation when the cooling function is needed in the summer.
My goal is to reduce heating bills.
EDIT (additional information):
I've tried to find the manual online but I couldn't (probably the model is too old), here's the closest I could find: https://islandaire.com/pdf/units/EZSeries42manual.pdf