I'm growing in the upper midwest. Right now, my ambient temp in the unheated basement is 61 to 63 degrees. I built a 6' x 6' enclosure using 2 x 4s, pink insulation board and reflectix insulation on the inside. I have two 600W HPS lights running 24/7 right now, and the temp inside the enclosure is 75-80 degrees. I haven't sealed up the room at the top where the joists are, but if the temp inside the chamber drops too much that will be my first step. I'm about to start flowering, so I'm not going to have the lights on constantly soon. I will have my daylight period at night though, so temperatures will stay steady even when I lower my thermostat for the night. Supercold days might require a space heater for a few hours. There will be a few of those as I expect to finish flowering around the end of February. brrrrrrrrr.