lots of attics and even new construction space is a premium.
one 10" duct with 6" of insulation won't fit in a lot of spaces.
having an attic with lots of space is an exception..most are
tight cramped spaces.
I think everyone agrees that having more insulation on ducts in attics
and crawlspaces is a good idea. that we put ducts & equipment in those
spaces is just crazy.
unfortunately the hvac industry has not found a way to increase R-value that works
in multiple applications.
it takes a lot to bring about industry changes.
good examples are the 2005 move from 10 seer a/c to 13 seer a/c as minimum
efficiency. another is pending May 2013 elimination of 80% gas furnaces
in northern climates.
efficiency is improved in both examples, but it took a long time
for the industry to mandate these changes.
ductwork efficiency has gone from R-2 to R-4 to R-6 & now R-8 in most areas.
in my area installers complain about the difficulty of attaching R-8
ducts to plenum & supply boxes. to be able to attach 6" of insulation to
start collars & supply boxes would require modification of these pieces.
is this for your house..or your sister's house in florida?
best of luck.
The cure of the part should not be attempted without the cure of the whole. ~Plato