This calculator is useful on a modern house but with an old house, it is very difficult to get accurate values for most of the input data. For example, we have some 3 feet thick brick walls, some 16" thick and even some 2 feet thick interior walls. I used infra red camera pictures of all sides and then worked on the worst areas of heat loss. After a year, repeated the process and again, worked on the worst areas. Same for years 2 + 3. It can be a never ending process but eventually, you reach a point of diminishing returns. A hand held IR reader does a good job finding small leaks and infiltrations too. To return to the post topic, the exterior windows were one of the biggest initial losses although as much from drafts as from direct heat loss.