Fireplace flu has 3/4" gap
 

My fireplace has a metal flu plate that lays against a metal frame The frame does not meet the chimney wall There is a gap of over 1/2" letting in plenty of cold air I was thinking of filling the gap with rutland mortar mix or similar insulating mortar I would wire a 32" angle bracket to the frame to keep mortar from slumping, then fill in the gap as shown in this picture

..................||.<Chimney.wall
..................||
frame.V.oooo||
......----OOO||
...../.....oooo||....the.oooo.are.beads.of.mortar.to.fil l.1/2".gap.
..../......----.||
.../.......!.....
../........!.<=temp.angle wired to frame keeping mortar from slumping
./.........!
/...<<<<<.flu.plate


But, rutland mortar says it is to be applied 1/8" at a time with heat curing in between layers

What should I use instead?

Hey Allthumbs, Welcome to the Froum:
I feel like I'm talking to myself (All Thumbs).
Perhaps you could open the damper, put a piece of 1" angle up in the cavity and lay it down over the gap. If you could get a good fit and get it to lay flat on the gap, it would cut out the draft and would be permanent. Almost any kind of grout or mortar will crack, break up and fall through the gap; whereas the steel will be there for years.
Glenn

Same issue with my chimney
 

Thumbs, I have precisely the same issue with my chimney, but with perhaps a 1.5 inch gap that streams cold air.

Were you able to fix the problem? I get smoke coming out of the sides and it is very disconcerting.
Please respond to
desmoid@gmail.com

Regards