A fire stop for use with a light fixture in a suspended ceiling or wall. It prevents flames and smoke from passing through the light fixture and spreading throughout a building by moving above the ceiling or behind the walls. The fire stop has a lightweight enclosure that defines an interior space disposed above the suspended ceiling, or behind the wall. Contained within the interior space are a portion of the light fixture and a layer of intumescent material. The intumescent material forms a material with a relatively low thermal conductivity that substantially fills the interior space, impeding the movement of flames or smoke, when the intumescent material reaches at least a pre-determined activation temperature.