[Rain Man]

It seems like there's nothing that can fix radiant problems other than to put something in front of it (inside or outside) that would slow it down. But the flow would be based on distance, so I wonder if a fair amount of it is cold air in the flue, since that has a shorter distance to travel.

We had a water line that would freeze in our house whenever the temperature got below zero, and we couldn't figure it out. We assumed it was on an exterior wall, which seemed really wrong. Eventually we mentioned it to a contractor doing another project, and he figured it out. The line was inside an interior wall, but it ran next to a vent pipe that went up and out the roof. In subzero weather, the cold air inside that vent pipe was cold enough that it would freeze the water line that was running parallel an inch or two away from it.

So I bet your flue is part of the problem. If you don't use it, can you put a hard cap on the top of it? Edit: or blow it full of foam insulation or something nonflammable so a cap doesn't kill the next owner.