Is a lip necessary on an HVAC ceiling register boot for use with flexible ducting?
New answer thanks to updated question.

Yes, the lip around the rim of the opening I'd call it a "ridge" but I dunno if that's right) on the boot is there to help ensure a firm connection.  It's to keep the zip tie on the inner core from slipping free.  This is necessary (the keeping the duct snug, not nessarily the tie) if your ducting is going to be pulling away from the boot due to gravitational stresses - such as when the boot is above the duct and the duct can be pulled down, such as along a wall or below a ceiling or floor.  If your boot points down and the duct sits on a horizontal surface, you're good - no extra tension should happen.

Important note - neither mastic nor tape are intended to be structural - they don't serve to hold the ductwork in place, they serve to seal the ductwork so that there are no air leaks. 

So the question really is - how do I properly secure my flexible ducting to the boot when there's no lip on the collar?  Answer:  Self-tapping screws.


Get yourself a box of those (the hardware aisle or the ductwork aisle).  When you go to mount your ducting, pull the outer insulation back up away from the core, pull the core over the collar, and use TWO screws on opposite sides of the duct (180 degrees from each other - noon and 6'o clock (or 1 and 7, 3 and 9, you get the idea) to ensure that neither side of the duct can pull free.

Use the inner to snug up the connection (don't over tighten!), seal with mastic (I'd recommend tape though cause of the insulation layer...), slide the insulation up as far as it will go and snug it up with the outer zip.