HVAC question about ductboard

[Donald]

I have a 2x2:return duct where a 12" flex duct is attached in horizontal. The connection is not tight and I can see some yellow of the ductboard fiberglass. What will stick best to the bare fiberglass. Mastic? Foil tape?
Obviously replacing that section of duct would be best but for now I would like to try and repair.

 

[irad]

There is a duct tape with UL code 181B-FX. This is not the aluminum foil adhesive backed tape but a silver Mylar tape.

 

[Donald]

What's is a good brand of high quality duct mastic (paste)? Not the cheap stuff.

 

[PowerSurge]

Design Polymerics mastic.

 

[Donald]

So any tape that says 181B-FX?