Is mastic needed?

[Bob Cooper]

If i use snap lock hvac pipe with strategically places screws and AL tape, is there a need to coat the seams and joints with mastic or is that just an over kill?

[Ole Anderson]

AL tape will seal it fine, I used it on the long joints too. Consider 1/8" x 1/8"pop rivets in lieu of screws, less for curls to get caught on. Where you can't get tape to lay down like wyes and bends, use clear silicone.

http://www.sawmillcreek.org/showthread.php?158878-DC-Steel-Duct-Install&highlight=

[Jim Andrew]

Don't caulk joints too good, hard to get the pipe apart when you make changes. I'd go with aluminum tape. It is much better than duct tape.