Lighting fixtures recessed in the ceiling need to have a way to get rid of all the heat generated by the lamp(s). Fixtures are designed with openings to allow warm air to rise by convection out of the fixture, into the ceiling space. Otherwise, heat builds up in the fixture and the lamps burn out prematurely. The only luminaires that do not have to take this precaution are those with low-wattage bulbs or that are surface-mounted. The implication for air-conditioning is that when the fixture is off, warm air continues to be lost through the fixture by convection. In the heating season, this exfiltration from the occupied space can account for up to one-fifth of the heating load. The alternative is to use surface- mounted fixtures or those that communicate with the ceiling space only, with no flow-through from the occupied space; or fixtures with low-wattage bulbs that need a minimum of ventilation.
This heat loss is beneficial in the cooling season since it gets rid of the heat from fixtures before it enters the conditioned space and must be removed mechanically. However, if the fixture communicates with a ceiling space, it permits exfiltration and infiltration.
Consequently, the ceiling air leakage results in more energy loss than is gained by allowing the fixture heat to escape, especially in those situations where the light is in only occasional use. The only exception is for luminaires in the ceiling of a multistory building, where the fixtures project into a between-floors ceiling space that does not communicate with the exterior. Those on the topmost floor should be modified to minimize airflow while still permitting sufficient room for the luminaire to cool.
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Excerpt from Energy Conservation Projects.