This article replaces an earlier version which depicted the practice of sealing openings in the housing of a recessed fixture. This practice is not authorized by manufacturers or code and could cause the fixture to overheat.
Recessed lights have received plenty of attention as a potential source of energy savings through the use of CFLs and, more recently, LED lamps. But even when equipped with the most efficient bulbs, can lights are a major source of undetected air leakage in most homes. Whether they are providing a pathway for air movement into floor systems or attics and ultimately out of the house, or drawing outside air into the house, they can affect air quality, comfort, and utility bills. This air movement is difficult to see or feel directly, but it can be easily detected using a smoke stick, especially in conjunction with a blower door.
Although newer homes may have fixtures rated for insulation contact (IC-rated), most insulation is not enough to stop air leakage through the fixture. (In fact, you can often see evidence of air leakage in patches of dirt and dust that has collected on batt insulation that has been laid over these fixtures.) Where you have access to the attic, you can insulate and air seal in one step by creating a "top hat" to isolate the fixture, sealing it to the surrounding materials, then insulating around it. A number of manufactured products are available to provide clearance for insulation around the fixture, but you can also build a box using scraps of rigid insulation or drywall.
This video from Green Homes America provides a good overview of the air leakage issue with recessed lights and shows an actual top hat installation.
Sealing From Below
In homes where attic access isn’t available or is difficult—and for recessed fixtures mounted in first floor ceilings—a top hat and insulation is not an option. Merely caulking under the trim ring is ineffective, because the fixture housing itself contains numerous slots and other openings through which air can escape. To completely seal recessed fixtures from below, you would need to plug all of these voids. But this practice could create a fire hazard, particularly in older recessed fixtures that are not equipped with a thermistor that shuts down the power supply if the fixture over heats. Moreover, manufacturers consider this an unauthorized modification of the fixture that will void the warranty and possibly create a fire hazard, even when the fixtures are thermally protected.
Nevertheless, there are any number of online videos from weatherization contractors demonstrating various techniques for sealing recessed lights from below. In some the sealant is fire-rated foam; in others, it is a non-hardening, non-combustible duct sealant. In most cases the contractor cautions that incandescent lamps should be replaced with more efficient and cooler CFLs, but that’s the extent of any warning about the fixture overheating. Given that the potential liability far outweighs the savings from the air sealing, the safe bet is to avoid modifying the fixture in this way.
Air flow appears to play a role in cooling recessed fixtures, but the message is mixed. For example, one code official we heard from said it was okay to seal the space between the recessed housing and the drywall. But caulking an eighth-inch joint around a 6-inch fixture is the equivalent of plugging air flow through a 2-square-inch hole. That stops a significant percentage of the air flow in a typical (non-airtight) can, yet neither manufacturers nor code officials seem to see any overheating hazard in sealing this joint.
Does any of this have any bearing on the top hat treatment? If by design air flow is essential for cooling the fixture, then why is the top hat solution acceptable? The fixture cover is sealed to the ceiling below, so it effectively stops the flow of cooler air through the fixture. And because insulation is installed over top of it, heat is trapped inside. Despite conduction losses, it seems likely that this would raise the temperature of the fixture well beyond what would be the case for a fixture installed in a first floor ceiling joist bay with no air sealing or insulation.
It’s pretty clear that, when it comes to energy retrofits, recessed fixtures are problematic, particularly older models. They are a significant source of energy loss, and in many attic ceilings they interrupt both the thermal barrier and air barrier.
It’s also clear that sealing fixtures from below by filling the openings inside the housing creates a risk of overheating, whether or not the fixture is equipped with a temperature-sensing limit switch. This practice also voids the manufacturer warranty and most likely violates code.
Finally, although the top hat treatment appears to be acceptable for modern recessed fixtures equipped with thermal protection, overheating still seems possible, especially in models lacking thermal protection.
If you have access to the attic during a retrofit, replacing older fixtures with newer, air-sealed, IC-rated models, is a safe, if more expensive, solution to air-sealing. If you lack access to the attic, another option is to retrofit with an LED insert, such as Cooper’s RL7 LED module (about $65 on Amazon), which incorporates an air-sealing gasket. Finally, you can always eliminate recessed fixtures altogether in favor of surface-mounted lights, but this could involve replacing and refinishing drywall and some rewiring.