Recessed ceiling lights (can lights) are beautiful design elements, but they are thermodynamic nightmares. Every can light is essentially a 6-inch hole punched directly between your 72°F living room and your 140°F attic.
The Fire Hazard
Older house lights are "Non-IC" (Insulation Contact). The metal housing gets incredibly hot when the bulb is on. It is a code violation to let insulation physically touch these cans. If a DIYer accidentally blows fiberglass over a Non-IC can, the trapped heat will ignite the paper backing of the drywall or the dust in the attic.
Insulation Contact Approved
Modern LED cans are "IC-Rated." They run cool. An insulation crew can safely construct fire-rated "Ten-Gallon Hats" over them and bury them completely in 14 inches of fiberglass, effectively sealing the 6-inch thermal hole without burning the house down.
Even IC-Rated LED lights bleed massive amounts of conditioned air up into the attic through the gaps around the trim. A true The Very Good Home Company air-seal involves carefully injecting expanding foam around the perimeter of the can housing from the attic side, locking it permanently to the drywall before burying it.