I am in the midst of an insert install in my downstairs fireplace. I have the 6" lining in place and had to remove the damper, no block off in place but I'm working on it, but that's another post. My current problem is now that it has turned cold I've had a fire or two in the upstairs fireplace which is a normal masonary flue fireplace that I am intending to leave as is. Since sealing the top of the downstairs flu well with a top plate and stuffing the pipe full of towels at the bottom, I am getting significant amounts of smoke coming in the downstairs flue when I burn the fireplace upstairs. I thought sealing the top of that flu would prevent any smoke from reverse flowing down the flu. So, oh wise forum members whom I owe so much to, say it ain't so but could I have cracks between the flues allowing smoke to pass from one to the other? Please advise.