I have a brick fireplace that is brick both inside and outside (the
chimney). It is stone cold to the touch from inside, and is warmer than the
wall from the outside. I surmise heat is leaking out of this thing due to
air going out the chimney, and also due to moisture condensing in the brick,
causing it to conduct heat more (and causing the brick to get even colder,
and creating more condensation).

How do you insulate this? I found a lot of windows and floor/ceiling
insulations at hardware store, but didn't see any fireplace insulation.
Specifically, is there such thing as moisture barrier for the bricks, to
prevent them from absorbing moisture? Yet, it has to be "fire-friendly" in
case I decide to use the fireplace afterall --meaning it should not burn,
smoke, create foul odor, turn into goo when heated. If it vaporizes quietly,
that is ok.