If you follow the clearances by code, go ahead and fill it in. I don't think your savings will amount to much.

I had huge amounts of cold air coming in through recessed cans. Fortunately all was IC rated, so I had cellulose blown in that space. Made a big difference but there had been no insulation at all there before. Do you feel cold air coming in the can? If so, I would add insulation. Looks like you are pretty well set already.

"Fortunately all was IC rated, so I had cellulose blown in that space."

Cellulose and fiberglass are not very effective at stopping air leaks.

One way to spot leaks in either is to look for the dirty spots that dust in the moving air has left on the insulation.

That would drive me nuts and I'd probably swap them out or secure some insulation behind them without touching the housing.

Just wondering - is it possible to make those lights air-tight by taping off any adjustment holes & grooves in the can with aluminum tape?

Also, could rock wool insulation be used if it were in contact with those hot cans? Would that be a fire hazard or not?

I retract my consideration of rockwool against the light. Dumb. Sorry about that.

"I retract my consideration of rockwool against the light."

It is not the thermal insulation that is the problem, but the electrical insulation on the wires and sockets.

One of the requirements for IC light is a temperature sensor that shuts the light off if it overheats.

They are normally simple bi-metallic strip switches.

I cannot tell you how many time I have had complaints of can lights blinking on and off after the owner installs larger than rated bulbs.
The extra heat raises the temperature enough for the thermostat to shut the lights off, then back on when they cool, then off when they heat up, then on...

go to cooper lighting with the model number of the IC Halo brand light you have installed.
purchase an air tight insert that installs from inside of the house. This is a reflective baffle insert that seals the air leakage from the attic.
use a cfl.
then insulate with conventional insulation.
the insert will stop the air flow, the insulation can be in contact with the can. it is the air leakage through the insulation that causes it to not perform.
best of luck.

Ecomech -- it is against the rules of this forum to promote your own business.

As for air coming through the cellulose, it has not been a problem. We had a huge windstorm the other night, so I put my hand up to the recessed can lights and could not feel any air at all. It felt pretty much the same temperature up there as in the rest of the room. There must be a good foot of cellulose above the lights, if not more, so unless the wind were to blow it aside, I don't see how it could blow through. It is "dense packed." I'm far from an expert, but just reporting on my first-hand observation.

housebound, IC can's can't be externally sealed with foil tape. this is why there are products that safely retrofit these lights.