There are three parts to a recessed lighting fixture: housing, trim and bulb. The trim is the visible portion of the light. It is the insert that is seen when looking up into the fixture, and also includes the thin lining around the edge of the light. The housing is the fixture itself that is installed inside the ceiling and contains the lamp holder. There are many different types of bulbs that can be inserted into recessed lighting fixtures, with the amount of heat generated by the bulb being a unique consideration.
UL 1598 recessed housings generally fall into one of four categories.
IC or “insulation contact” rated new construction housings are attached to the ceiling supports before the ceiling surface is installed. If the area above the ceiling is accessible these fixtures may also be installed from within the attic space. IC housings must be installed wherever insulation will be in direct contact with the housing.
Non-IC rated new construction housings are used in the same situations as the IC rated new construction housings, only they require that there be no contact with insulation and at least 3 in (7.6 cm) spacing from insulation. These housings are typically rated up to 150 watts.
IC rated remodel housings are used in existing ceilings where insulation will be present and in contact with the fixture.
Non-IC rated remodel housings are used for existing ceilings where, ideally, no insulation is present. However, these also require that there be no contact with insulation and at least 3 in (7.6 cm) spacing from insulation. Sloped-ceiling housings are available for both insulated and non-insulated ceilings.
The main feature of the housing is designed to ensure that no flammable materials come into contact with the hot lighting fixture. Badly-housed downlight can be a fire hazard, though all newer ones contain a self-resetting thermal switch for safety.
The housings come in various sizes based on the diameter of the circular opening where the lamp is installed. The most commonly used sizes are 4, 5 and 6 inches in diameter, with 4 inch IC New Construction units less readily available at present. Smaller housings (2 and 3 inch) are also available for specialized uses.
The housing can also be "Air Tight", which means it will not allow air to escape into the ceiling or attic, thus reducing both heating and cooling costs.