Even if it's insulated, you'll need some source of heat unless the average temp outside is above freezing. The insulation just reduces the amount of heat loss, so reduces the amount of energy required to keep the area at a given temp.
As far as what to use, Watts are Watts, or in other words all electrical heaters are all exactly equal in efficiency, including heat lamps, light bulbs and heaters, so pick based on cost, safety, and lack of moving parts. (This assumes you're heating an area.) After you insulate, I'd suggest trying one or two 100w lightbulbs.
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