Light fixtures contain a foil backing on the inside of the base. The foil backing is attached to a thin layer of insulation. The combination of the foil and the insulation deflects the heat from the light bulbs away from the ceiling where the electrical wires are located. Removing the foil insulation may cause the fixture to operate in an unsafe manner. There may, however, be a need to remove parts of the foil insulation around screw holes or edges to make the base fit.
Inspect the fixture base where the foil backing is located and determine exactly where the foil needs to be cut. In the case of the foil interfering with the edge of the base, dry fit the base and determine how much of the foil backing you must remove to make it fit properly. In the case of screw holes, locate exactly where the screw holes are on the base and where you need to cut the foil so it does not obstruct the screws.
Score the foil backing with a utility knife in the cut location. You may need to run the knife over the area a couple of times to cut through the foil. In the case of a screw hole, score a square section around the hole.
Cut into the insulation below the foil with the knife. Cut as close to the base surface as you can.
Pull out the cut area of foil and insulation with your hand. Pull the insulation straight off the base to avoid ripping off insulation from an area you do not intend to remove.
Scrape the surface of the base with the edge of the utility knife blade to remove any residual insulation or adhesive.