I have an upflow furnace in a hall closet with a coil on top from the late 80's. The coil is in a poorly constructed sheet metal plenum with an insulating blanket around the outside of the plenum. There is no type of insulating material on the inside of the plenum.
The blanket is in pretty bad shape and the front is uncovered sheet metal since all of the insulation has been removed a few times for service of the coil. The heating vent pipe runs right in front of the plenum so it won't be easy to get a blanket back in place, which is probably why it the blanket torn up so bad.. Any ideas on how to best insulate this plenum? Can I buy an insulating material and 3M spray adhesive to properly insulate the inside?