The purpose of insulation is to resist the loss or gain of heat energy between the inside and outside of the home. Materials such as fiber glass, mineral wool, cellulose or foam are placed in the walls, ceilings, foundations, ducts, etc. Missing insulation can lead to excessive heat loss or heat gain driving up your energy costs. Missing insulation can also cause other issues such as mold.
The image to the right shows about 2-inches of insulation over an interior ceiling. It should have at least 10-inches!