The ceiling isn't insulated, but it has a flat 3/4" plywood floor.(not open to the rafters, there is a second story)
I really like the electric idea, no flammable fuels, simple installation. But if its going to take over an hour to warm up, depending on what I'm doing, I'd be done already.
My 100,000 btu kerosene salamander heater takes about 15/20 min to drive the chill out of my garage. I don't need to work in my shirt sleeves, but when my hands go numb from handling tools... I have to do something.
Usually I would like to bring the temp from 35/40* to 55/60*
Because of being built into the bank I must get some "ground heat". The garage typically never gets below freezing, even if its in the teens outside for a couple of weeks.
I really didn't want to have to dig a trench and lay pipe for Ngas..... But that might be a better route. You can definitely get a lot more BTUs from a similar sized heater with gas.
I did find a monster 51,195btu 63amp forced air electric ceiling heater..... But I think the cost, and I only have a 100amp service to the garage, wouldn't be worth it.