Amazing write-up,thank you from one Yankee to another. I will put a small stove in the basement to heat the Basement since I has no other heat source down there. I will put a air tight insert on the main floor. I have a wide open large room on the third floor/attic space. So any interior chimney would come stright through the middle of it. I think I will have too go the exterior route, but use a metal enginnered flue and insulate the chase around the flues. If I was building a house new I would have put in a interior chimney for sure.