I experimented with propane and electric heaters in my Florida room (screened-in porch- aka "Tiki Hut: The Home Version") and the Upstate NY winters were too much for them. I switched to an
indoor kerosene heater and now sit out there in shorts and a t-shirt. The heaters are more expensive, but I have the heater to use not just there, but also in the main house in case of an emergency. Power outages are common in our area.
Kerosene can be hard to find, and if you are not burning properly, they can spit soot out, but I get more heat for the money this way.