My chimney, circa 1985, lets a lot of heated air out of the top floor causing the "fireplace room" to be ten degrees colder than the rest of the house even though I never have used it as a "fireplace".
The chimney metal valve is shut (you can't "see" daylight looking up the chimney) but the chimney still has a breeze going through it (and it makes a lot of metallic noise in the wind). The chimney on the outside is concrete looking but it kind of looks metallic on the insides plus it has a metal covering around it at the top for about 2 feet above the chimney concrete.
I thought about putting a temporary fiberglass or styrofoam cover in the bottom shaft of the chimney but, of course, if someone were to start a flame, that would be a bad idea (even though it has never been used).
I ask you here if there is a standard solution to putting a temporary plug in the chimney from the bottom so that all the heat isn't lost going up it.