The U.S. Department of Energy recommends every 7—10 years using mineral insulation such as cellulose, fiberglass, etc. There are several reasons: -The first reason is that the R-values will diminish when the insulation settles down, kind of like a sponge. You see, the tiny holes in insulation are what traps the heat or the cold. Once those holes are full they cannot absorb any convection heat, as when a sponge cannot hold any more water. – The other reasons range from traffic, dust, moisture, and workman such as cable or phone installers who have moved through and stepped on the insulation in your attic, crawl spaces, or basement, thereby packing it down and reducing its ability to insulate effectively.