I sealed all holes in my 6 cans with metal duct tape about 12 years ago. Never any problem. I did use 45 watt lamps rather than the 60 watt rated. If you don't have a dimmer,you could use over 100 watt equivelent flouresents.

it isn't something I'd recommend.

but...if you do... then use only LED or CFL's bulbs
from here on out.
most recessed light has a heat sensor when
the heat gets to a certain point it shuts off.
closing the holes that lets the heat escape
will trip the sensor.

there are inserts that install from inside
the house to make IC lights ICAT.
(insulation contact...insulation contact air tight_

take model number & brand name,
go to mfg's website & search for
ICAT inserts.

once the inserts are installed, use a mastic
tape to seal the gap between the housing
& the sheetrock. removing the trim gives
you good access to this other leakage site.
once the trim is replaced, it hides the
mastic tape used to seal oversized cut in sheetrock.

best of luck.

Thanks for the info. I'm going to try the tape since it is a lot cheaper than the inserts. I am using LED lights so heat should not be an issue.

I get an "attic smell" coming through sometimes so I know these need sealed up.

I plan to caulk my other fixtures where the housing touches the drywall.

Thanks.