Boxes should be insulated that same as the rest of the duct. They're just as subject to sweating as any other duct in an attic. The boxes should also be sealed to the sheetrock ceiling to prevent water stains from forming around the diffuser.
If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!
short of changing the boxes to internally insulated boxes,
wrapping them in ductwrap is the only other option
that I'm aware of.
as stated above ductwrap should be installed all the way
to ceiling. ductwrap around box, not compressed and
then on the other end of the supply box,where duct connects, the
ductwrap should be installed over mastic seal of joints
of connection to supply duct. (yours are sheetmetal ducts?)
ductwrap stapled with duct wrap stapler and seam covered
with fsk tape.
make sure hvac company understands not to put new
duct wrap over old duct wrap. two vapor barriers
will cause condensation within the insulation.
sheet metal ducts have long seams, joints where 90 degree ell's
are installed. these should be sealed with mastic as much as
possible. then at supply plenum where ducts connect should
also be mastic sealed.
where plenums both supply and return connect to heating
equipment should also be mastic sealed.
Return air should be mastic sealed.
then from inside the house, mastic seal supply boxes to sheetrock
ceiling. attached is photo of mastic sealed supply box
have hvac company use only mastic to seal these areas.
both paint on mastic in a gallon bucket applied with a brush
and mastic tape (not duct tape..not foil tape.. mastic tape)
can be used.
bucket mastic should be UL 181 rated.
I perfer hardcast brand #1402 mastic tape above
all other brands.
insulating without air sealing does not stop
condensation.
best of luck
The cure of the part should not be attempted without the cure of the whole. ~Plato
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