Here are some downsides of can lights that are frequently mentioned by owners, contractors, and government agencies. The "bright" spot in all this is that can lights are fixable; no need to rip them all out and plug the holes.
- Can lights are notorious "air leakers." For one reason, they are located on the ceiling (where heat congregates); for another, these are sizable holes in the ceiling that often have gaps between the wallboard and the light. To fix these gaps, consult EnergyStar (see link on this page) for a free guide on plugging those gaps.
California's Title 24 has strict regulations on this, which requires (summarized) that "the housing must be air-tight type which can be used with an air-tight or non-air-tight trim, you may instead use an Insulation Contact Only (Non-Air-Tight) Housing but the trim you use must be air-tight approved."
- Some can lights cannot be covered with insulation. With the heat generated by can lights, they might start a fire. Even though fiberglass does not burn, the covering paper does. Or, the heat build-up caused by the covering insulation will hamper the efficiency of the can light. Either way, the fix for this is to get a sealed or IC-rated light that does allow for close contact with insulation.
- Traditional can light bulbs drain enormous amounts of energy. Is it possible to change out those power-suckers? Yes, but it is not as easy as screwing out one bulb and screwing in another. It is possible to buy retrofit units so that you can use energy-efficient LEDs in your existing can lights. These LEDs use up to 75% less power than incandescent bulbs, have great light quality, and still are dimmable.