Air Conditioning - to mastic or not to mastic?

Since its pretty cold in my attic now I was wondering if it's a good idea to apply my whole ductwork up there with mastic? My ductwork currently doesn't have any mastic and has an old fiberglass-based duct blanket on it. The air handler and main ductwork are in the attic. The main duct is sheet metal spanning across the whole attic and flex ducts are attached to it from corresponding rooms. The sheetmetal ductwork is blanketed with a fiberglass-based insulation. I remember going up there last summer and cold air was seeping out through the blanket-wrap from various locations. I guess this is the air from the joist connections.

I noticed my attic gets cooled just as fast as my other floors of my house during the summer. I have been thinking of adding mastic for quite a while. I just had a few questions:

1) How long does mastic last on ductwork? I imagine it isn't easily replacable. It's a messy job.

2) Is it a better idea to use foil tape first and then add mastic over the foil tape? This way the matic can be easily removable.

3) Does quality metal foil tape (ie: UL NASHUA) apply as a substitute for mastic? Taping up seems to be a cleaner job than gooping up. But I would suspect that heat and condensation would make the foil tape less effective.

Duct seal or (mastic as you and others call it) is the only way to go. Tape will not hold on if duct is not clean. Get a 2 in chip brush to put it on. No Mess!

A quality Mastic will last as long as your duct sytem. If you have gaps to fill use "fab" vs. Tape. Fab is a mesh like material that will help strenghen the joint once the mastic has dried. You will never need to remove the mastic. Do not run your system for at least a few hours after applying the sealant.
-B

You dont say how old is it all now??? We find that a lot of that old flex is getting holes in it . SO might check that out. In the attic all we use now is the silver kind of flex. Most codes also call for the inside liner to be pull over the take off and a band on it . Then the outside of the flex pull down over that and a band put on it. Then it all sealed with the mastic . Hardcast
makes a mastic also they have a rolled duct sealant Johnstone Supply has that.

yeah i like that Hardcast 601 duct seam sealer comes in a small plastic bucket and goes on easy. It is just the right viscosity with a 1 1/2 or 2 in chip brush. You will have to strip all that old duct wrap insulation off to do the job properly so be sure to get enough ductwrap to reinsulate plus you will need staple gun/staples and enough foil back tape for the duct wrap seams. Have Fun

Just don't go crazy with the mastic. I have come across air handlers with the access panels and screws all covered over.

That’s why ductless is the future, anyway remember to clean the area to which you are applying mastic just like tape if there is dust covering the joint you are sealing the mastic wont adhere to the surface even though it hardens on the top the seal might not be air tight (manufacturers installation instructions).