Insulation is installed by either batts or preformed blankets, blown-in or a combination thereof. The actual recommended amount will vary based on your home's unique needs, but a general rule of thumb may be around R-38 to R-50. If you are adding to existing attic insulation, then the insulation R values add together to achieve the desired number. For example, if you currently have R-20 insulation in your attic, adding R-30 will result in a total value of R-50. For more specific information and for more ways to use your attic to save on heating and cooling bills, contact a reputable insulation company such as Efficient Attic Systems.