this pic is what it looks like from inside the attic.
supply box is picked up off attic floor by screws
from supply grill.

I can fit my fingers between the bottom of the box
& the sheetrock of the attic floor. this box should
fit tightly to attic floor.

don't know if you can see the sheetrock dust around
the box or not.

while these pics are of different houses, the cure is the
same..in third pic to follow.

and last but not least...the fix!

3" mastic tape...not foil tape, not duct tape,
not asphalt tape.
hardcast 1402 mastic tape.
strong tape with thick mastic.

this will keep the box from lifting up when supply
grills are re-installed.
depending upon the size of the gap..you can cut the
3" tape into 1.5" strips. tape from sheetrock (staying
within area covered by supply grill) into the supply
box. inside the supply box is a metal lip for the tape
to seal to. put extra piece in each corner. press well
to seal. make sure surfaces are clean & dry.
tape isn't easily re-positioned.
you can trace around supply grill with a pencil
to show you boundary to stay within.
the tape will pull the paper off the sheetrock,
so position it carefully.
re-install the grill.

if ducts are in floors, ceilings or walls...this works
on all. most cuts are oversized, just hidden behind
the supply grill.
same applies for bath fans, remove the cover &
seal the gap.
with recessed lights...it gets more complicated.
hidden under trim piece is the oversized cut.
seal the same way with strips of mastic tape.

I learned this after getting many calls for "mold"
on grills. in 99% of the cases it wasn't mold at all.
just that condensation had formed, dust had settled
on the condensation & accumulated on the supply
grills.

I've been using this method for 15 years now to
seal supply boxes.
our biggest heat gain/loss is the ceiling.
using the right materials to improve the air barrier
is a diy, fairly easy thing to do.

here is the sealed supply box.
best of luck

Good information rater and simple enough the average homeowner or their handyman should be able to do the job.
Now that weather is cool enough to alow working in attics,how about instructions in testing for leaky ductwork.

thanks klem1, the beauty of this is that it is done from
inside the house. and that some diy'ers
will apply it to their own houses.

the average duct leakage is 30%. that is
a lot.
sealing the box to the
opeing into the house is important.
it is something that anyone with a drill & ladder
can do, it will last, provided they use the specified tape.
other tapes I've used haven't stood the test of time.
I'd estimate % of duct leakage for supply boxes to be
as much as 1/3 of that leakage to be supply box to penetration into conditioned space.
the other 2/3 distributed more or less (because every
house is different) between duct take off at supply plenum & equip/plenum connections.
flex connections leak as do hard duct connections.
I'm not really biased as to type.
in terms of air leakage, hard pipe ducting has more
leakage if it wasn't mastic sealed when installed.
flex leaks less simply because it is a continous air
barrer (exceptiond being torn/defective air barrier of flex).

the duct equip return sealing would be attic work...
and weather is starting to cool off, even here in
La.
I have to say I'm enjoying it every day!

There's heat then there's Ms and La heat,none of which should be confused with Tx heat.
Critique this method aimed at your typical somwhat lazy couch potato that wants to know if leaky ducts are bad enough to cut into the monthly beer allowance. Not a scientific analisis mind you but one that will let you know if mucho dollars is being wasted.
Close all windows and doors,turn off gas appliances and close fireplace damper or stuff a rag in it. Turn cooling (cool air is easier to feel drafting) and go to gas water heater flue bonnet to see if you feel air flowing out of or into flue pipe. Go to each outside entry door and crack it about 2",stand first inside then outside with face next to cracked door opening. Air rushing in = supply leaking. Air rushing out = return leaking. The greater the air flow the greater the % leakage. If you don't know how to track down leaks,tips are available for the asking or you can call in a pro with the knowledge their fees will be offset by energy savings and added comfort.