Trevor is dead right. the seal is unimportant.
as soon as there is hot air rising through the pipe any not perfectly
sealed joint will only be sucking room air INTO it not out.
Basically once the draft is established the hot air rising up through
the chimney is MOVING AIR. the air in the fireplace room (My rec
roomin my case) is static air that is barely moving at all at best.
Air in motion exerts less pressure thna air not moving so therefor the
non moving air will flow INTO the imperfectly sealed single wall stove
pipe joint and go up the chimney. In itself that is actually
desirable. it is less efficient as a means of heating BUT it makes it
impossible for carbon monoxide to enter the house. Unless i'm stoking
the fire i cannot smell my woodstove because all the airflow having
combustion applied to it is being fed up and out.
I love the fireplace smoke smell but i like living
my driveway and porch smell like my fire my house does not due to the
partially burned products not entering the house itself