A contractor is installing a new high efficiency furnace for me soon to replace an older unit. The air conditioner is newer and the contractor plans to install the new furnace and leave the existing plenum and AC coils in place. He will clean the AC coils. The old plenum has what appears to be fiberglass insulation glued to the inside of the sheet metal. I don't think the contractor noticed the old fiberglass.
Question: Is this old fiberglass a inside the plenum a bad idea and should I ask him to remove it? What is good practice? Should the inside of the plenum be insulated, part of it is not where a previous contractor installed the new AC coils and a humidifier.
Your opinions will be appreciated, Gene