We are going to be doing a total gut rehab on our kitchen and would like to get a low profile steel radiator to replace our current cast iron one. The room is 12 x 12 and faces the North and will have new insulation a new door and a new thermal window. The current cast iron radiator does not really make the room hot, but it is also about 80 years old and the room has little or no insulation. Using a btu calculator I determined we need 6400 btu's. If we went a steel radiator at 5347 btu's, would it be sufficient?