If I have a large steel box that is 27ft x 8ft x 8ft and has 7 in thick walls with 4 non-insulated windows each 1ft by 1ft, where does heat escape from inside the box, is it the windows and the walls? So if the inside temperature needs to be 72 F and the outside air is 0 F how much heat would need to be pumped in? I figured that a lot of the heat would escape through the walls and treated it as heat transfer through conduction, but I am getting an answer of about 39 million BTU/hr and this makes zero sense. I attached the math I did, but I think my understanding of the situation is where I went wrong. Also I did the math in MathCAD 15 so the units are corrected and handled for me automatically.