Been reading about block off plates and insulation.........

I don't think so, but we shoved some in there anyway. I doubt that the benefit we get from it will come close to making up for the bruises and soot we inhaled, but it seemed like a good idea, to block air flow. Ours still leaks a little bit anyway, but I have no problem with heating up the pre existing triple walled metal chimney as there is bound to be moisture that needs to be purged.

Wouldn't that be like saying I don't need insulation because I have a roof ???? The insulation will trap air pockets and will slow/stop the transfer of heat/cold from either side of the plate. Now asuming the plate is metal, which is an very good heat transfer mechanism, the insulation will have a benefit. Now, you just need some pads and a dust mask so that you don't suffer the way Milt did.

hearth.com is a wealth of information.

Try reading here as to the why's and how-to's: https://www.hearth.com/econtent/index.php/wiki/Why_damper_seal_is_needed/

I still don't think it is worth the hassle. As my unit was a prefab, I used the original damper as a block plate, cutting a six inch hole in it. Most of the dead air space in the original chimney is filled by the insulated flex liner. The additional insulation will not keep the pipe warm if the fire is out. There is a 6 inch open pipe that allows cold air all the way down to the stove and the insulation does not help that at all. At the top, the original pipe is air tight. It will all get cold, and it all gets hot and that amount of insulation will not change that. If there is no air flow, nothing is changed. Use your efforts to put new weather stripping on the back door or caulk something.

Of course, if you have a 12 INCH tile and a couple of square feet of cold air on your flue blocking plate, and your fireplace opening is open, it would probably make a difference. Masks and pads do not help much.