Fireplace chimney blocked by cold air?

[eytonxav]

Thinking back in my memory, the area in your chimney around the flue pipe may contain air that is too cold which can cause a reverse draft. This can occur in chimneys that are on the outside of the home, especially on the north side of the home. Therefore, in some weather situations if other obvious problems are ruled out, it may be necessary to insulate that interior cavity to handle such colder temperatures. This can also occur in chimneys if the height to the chimney is of an insufficient height above the roof.

 

[OldShooter]

I don't think there is any blockage, because on warmer days, the chimney works perfectly. ...

On warmer days, a partial blockage/weak draft will not be a problem.

But, hey, an ounce of data trumps a pound of SGOTI's theories. Just get someone up there to check while you shine a flashlight up from the bottom.