peter wrote:
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.

The short answer is you don't

BTW if air is going up the chimney, you need to add a working damper to
stop that. Air going up the chimney would actually warm it since it would
be at room temperature.

Moisture condensing would actually warm the brick, it would be moisture
evaporating that would cool it.

In any case I would strongly advise you to seek the help of a expert
professional before trying any home brew fixes.

--
Joseph Meehan

Dia 's Muire duit