The heater, in my back hall I would like to 'turn off' with out actually turning the water flow off. So I need to insulate it. I can remove it, but just insulate it so no heat, or very little 'gets out'

Reason being, that one room, is poorly insualted, and is a major heat loss in the winter... If I could insulate its heater, that would help on my bills as the water in that radiator wouldnt be cooling off the whole system as much...