Nick, if you make a hole in the duct so air cold leaks out, you will be
wasting energy! Also the cold leaking air will chill the inside of the
vapor barrier Because the insulation will not stop the air flow, only
act like a filter, which will chill the outside of the vapor barrier,
causing it to sweat. You are back to the original problem again. Now
the duct is sweating again, ruining the ceiling again. Seems like you
are going in circles.

In addition, I can see you have never actually worked on ducts in a
tight attic. Insulating with R-6 uses about 1.5 inch wrap. R-19 is
about another 4.5 inches. So if the duct is six inch, the outside of
the original duct with R-6 is 6" + 1.5" + 1.5" = 9" diameter. Now add
R-19 to that which is 10" more. (5" on the left side and 5" on the
right side.) Now the duct is 19" in diameter. Hope you have a BIG
attic. Especially when the duct gets to the register near the outside
wall where the roof slopes down to mneet the wall. I can just
visualize you laying in that rock wool insultion in the attic trying to
wrap that duct and getting all itchy. Then I visualize you falling
through the drywall ceiling. I hope you land on a nice soft couch
instead of the floor. If you had ever actually worked in a tight, hot
attic, laying in the insulation, You would not propose such a thing.

By the way, you are using duct wrap, are you not? Standard insulation
would not meet code when used to wrap a duct. Code also says to follow
manufacturer's instructions. If your R-19 bat is not listed for use on
ducts, you could get in trouble with the building inspector. The
thickest duct wrap I have seen is about 3", or R-8. 2.25" is rated
R-6. So R-19 DUCT WRAP would be more like 6.75".

This is rapidly getting out of hand. If you still think it is a good
idea, go up in a tight attic tomprrow with some R-6 duct wrap tomorrow
and put 3 layers on. Then you will see what hapens when theory meets
practice.

Stretch


In short