If the hot side of the tank is fairly warm you will not need to add an above heat source. Matter of fact I reccomend against any light source for heating. As long as he has an undertank heating and he can hide in that area (Sometimes snakes will choose cover over heat) he should be plenty warm. Check to see if the hotspot is warm to the touch or hot. I would hope more on the hot side if the room gets as cold as you said it does.

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