I am installing a wood stove insert in my fire place. I am putting a 6" DuraFlex liner down my chimney to connect to the insert. I am going to insulate the chimney using Perlite poured in from the top because my chimney does not have enough clearance for a insulating wrap. My question is in regards to pouring in the perlite. I have about a 20" x 5" opening where I took my old damper out in order to run the pipe. What should I put there in order to hold the perlite in the chimney? Should I put sheet metal with a 6" hole for the pipe or a solid piece of steel. I would greatly appreciate any advice on this.