Home depot sells 1 inch thick 4x8 sheets of foam insulation. Works wonders in summer to keep the cool in and winter to keep the heat in your room. I use my garage and it gets hot as hell in summer and cold as Alaska in winter.
That, and a decent oil-filled heater. I have a noma heater that is oil-filled. I grow in an isoleted space in an unisolated garage, detached from my house, in a very cold climate. I insulated the place well. I can keep a minimum of 20 Celcius temperature at night when lights are off and -20 Celcius outside. It comes with 3 power settings and 5 temperature settings. 300/600/900 watts, 18/21/24/27/30 celcius. I keep mine at 300 watts, 27 celcius. During light on, heating is not a problem, but current draw is an issue because of my 1000 watts lamp, fans and so on. At 900 watts, the heater trips the breaker when light is on. At 600 watts, it does not. I keep it at 300 because it is all I need and I like to keep me some wiggle room.
Hope it helps,
Good luck!!!
I bought it for 40$ instead of 80$ at Canadian Tire... They have them at Walmart too... may be under the brand name Garrison, who bought Noma or was bought by them.
Anyhow, it was a very convenient and unexpensive heating solution for me... The only problem I foresee, and may be avoidable if you look carefully, is that it is completely electronic, with being off by default when powering. If unplugged and replugged, it has to me manually set to start, to the disired temperature and power setting. This means that if I left for 3 days, and power went out, even for just 1 minute, on the first day I am gone, they are not getting any heat at night until I come back.
This is my only worry, so if you would consider my suggestion, I suggest you try to find a unit that has mechanical settings instead of electronics.