It would make a whole lot more sense to insulate. You pay for that ONCE, as opposed to ongoing electric bills forever and ever from a higher-wattage bulb, plus the risk factor.
"Need" aside, I would not personally put a heat lamp in a 3x3x4 coop, because you will not really be able to have 18" clearance from flammables (walls, shavings) in all directions. And even if the lamp were poised in the exact geometric center of that coop, with as much clearance as possible from flammables (although it will not be quite 18"), it will not be out of the way of chickens' heads, and thus there is a pretty good chance of them bopping their heads into the hot bulb or hot guard surrounding it and getting burned. Plus the cost of electricity and fire risk. Really, if you *have* to heat (and I think the majority of people who heat, don't need to) a lower-wattage bulb and more insulation is a much better way to go IMHO.
Good luck, have fun,
Pat
"Need" aside, I would not personally put a heat lamp in a 3x3x4 coop, because you will not really be able to have 18" clearance from flammables (walls, shavings) in all directions. And even if the lamp were poised in the exact geometric center of that coop, with as much clearance as possible from flammables (although it will not be quite 18"), it will not be out of the way of chickens' heads, and thus there is a pretty good chance of them bopping their heads into the hot bulb or hot guard surrounding it and getting burned. Plus the cost of electricity and fire risk. Really, if you *have* to heat (and I think the majority of people who heat, don't need to) a lower-wattage bulb and more insulation is a much better way to go IMHO.
Good luck, have fun,
Pat