A quick way to see if you need more insulation is to look across your attic. If your insulation is level with or below the 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 probably have enough and adding more will likely not be 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 ENERGY STAR Home Sealing
for more information.