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Duct sealant problems

Smith19 Member Posts: 95
Hello everyone,
After many weeks of deliberation, I have elected to seal my ductwork with MasterFlow mastic. It works like a charm! However, when it came time to insulate over the mastic, I ran into trouble.

It looks like pretty much all types of foil duct tape do not stick to dried mastic. I can't keep the stuff on the ducts where it meets mastic. Any thoughts on this issue?



    JUGHNE Member Posts: 10,562
    What type of insulation are you wrapping with. Fiberglas or other?
    EBEBRATT-Ed Member Posts: 13,423
    Can't say I am a duct insulating expert but I used to stick the insulation to the duct with a few short strips of tape, then wrap it around the duct and tape the insulation to itself.

    the foil tape I have seen is pretty sticky. I would think it would stick to mastic
  • Smith19
    Smith19 Member Posts: 95
    Can't get it to stick to mastic. I have to areas around the furnace plenum where the foil backed fibre glass insulation has to be stuck onto the duct where it meets the furnace. Thinking I may mastic over the insulation to hold it on.
  • Mike
    Mike Member Posts: 94
    I like using the plastic disks. Screw them through the insulation into the duct. Holds the insulation in place, and keeps it from sagging on the bottom. Then a piece of tape on each exposed disk, then tape the seams. Makes for a nice looking job.
    EBEBRATT-Ed Member Posts: 13,423
    never seen the plastic disks. There are also metal stick pins
    JUGHNE Member Posts: 10,562
    You could also use a termination strip/bar. Pieces of pre-drilled "S" cleat could secure the insulation at the lower end of the plenum.
    This would hold the 4 sides down allowing you to get a start.
    Just take cautions if are drilling into AC coil box.

    The plastic washers with long screws that don't compress the insulation would work, but the job can get pretty busy looking.
    I have used AL washers from the lumber yard, intended to hold down tar paper etc. again not too tight to compress and washer to cut into insulation.