I'm trying to figure out what recessed lights to purchase. The ceiling room has an attic directly above with insulation in between. It appears that IC-rated cans are necessary here, correct? How does that relate to "airtight" cans? Can one be both? Which are appropriate or necessary?
IC or Insulated Ceiling or Insulation Contact are required for can lights that touch the insulation. You cannot use a non-IC can by moving or cutting the insulation back the 3 inches required to use a non-IC can. Inspectors see insulation and they expect to see the silver cans, which is what an IC can would look like. The airtight cans are designed to use a cone that you would snap the socket into and peel of supplied foam gaskets to make the can airtight. On any other can you look up and see the inside of the can, and on airtights you look up and see only the trim and bulb.
Airtight cans can be IC and non-IC. The design of the cans and the gasketed trims help keep air conditioning or heating from leaking out of the home through holes in the can.
I guess overkill is appropriate and what ever the code calls for in your area is what is necessary. The prices should not be much different and even in areas like where I live, the ATIC cans are a little cheaper than the IC cans, and the same should hold true on the non-IC cans. I suggest using the airtight because it will make your house tighter.
Airtights come in new construction and remodel cans both 4 inch and 6 inch. Low-voltage 4 inch cans do no have an ATIC remodel can though, just non-IC and airtight.
Remember that the lamps and trims used in the Non-IC cans might not be rated for the ATIC cans. You have to give up some heat and that will come from lamp wattage. The inside of the can should tell you what you can use.
Edit 8/27/2016: Now with the new LED cans you can get 4 inch retrofit ICAT rated can. Actually on most LED cans, they are mostly ICAT cans, because of reduced wattage. With the exception of some dimming quirks, which has nothing to do with the cans, unless you are matching existing, I would go with LED.
Again, good luck.