Getting ready for the install of my new Osburn 2200 insert (going out to purchase this weekend if everything goes to plan) and i've got a question about the install. I am installing in an existing masonry fireplace. I know that I need to install a block-off when I remove the existing damper from the flue. I have asked several local dealers and the ones who actually confirmed that I need a blockoff (not everyone said it was necessary) told me to "just jam some kaowool, or something like that in there". Is this good enough, or should I fabricate a metal plate and seal it to the existing flue? I guess another way to ask that question is how critical is it that that block-off is air tight?