A quick way to see if you need more insulation is to look across your attic floor. If your insulation level is at or below the top of the floor joists, you probably need to add more insulation (but remember to seal the air leaks before you insulate). If you can’t see the joists because the insulation is above them, then you should measure the insulation depth with a ruler to see if you have enough. Once you know total depth, you can estimate the R-value if the insulation. For fiberglass and cellulose insulation, you can estimate about R-3 per inch. The goal is a minimum of R-30 in the Southern US and R-49 in the Northern US. If you are more than R-10 below those numbers, adding more is probably cost effective. Even if you have plenty of insulation, it may not be performing well due to cold air leaking through the insulation into the house. So make sure to look for signs of air leaks and seal them up as you find them. Visit the Seal and Insulate with ENERGY STAR for more information.