Don't insulate it too much ... my first shot at a walk-in was ultra insulated and there was no where for the heat to go if it go too hot (thermostats will over shoot to correct temp drops from opening the door)

I've found that if it's an interior room, a Helix 1500 watt system and an oil filled heater are all that you should need ... maybe some light insulation, but that's about it.

-adam