disagree on this topic. I base my study on my own cabin in MO that is is a small valley and the chimney is all to specs. My smoke comes back into the house via the fire box when its still and quite. When the wind is blowing or a slight breeze the fire box draws perfectly, never ever did that window thing. That would defeat the entire process on heating the place, but then my cabin is not that tight as todays homes are so that could be the issue there. But, mine does back draft.
Oh, yes, one other friend of mine in the same situation as there are many valleys in MO, has the same problem.