mike wrote:
I've got this red brick fireplace taking up half a wall.
Capped the chimney decades ago, but it's still siting
there conducting heat outa' the place thru the bricks.
How would one go about calculating the "R-value"
to estimate the heat loss. Any useful rules of thumb?

Trying to decide whether to put insulation over the bricks
on inside or outside or both.


Thanks for the inputs.
I realized that I have no idea how a fireplace is constructed.

I did open up all the unheated areas and opened the heater registers.
The gas consumption went up by about 20%, which sounds reasonable
given the area added includes the patio door and fireplace.

I was surprised that the temperature of the bricks was the same as
the adjacent wall. Measured with IR thermometer, so there may be
emissivity issues not yet investigated.

The exhaust fan over my stove can pull 5 Pascals of vacuum on the house.
I turned it on and went probing with my trusty incense stick.
There's a small amount of air coming out of the fireplace,
but it's not coming down the flue. The cap works.
It's coming from the bottom near the front. Need to pull out
the homebrew heat exchanger and look under there.