A number of factors influence how much insulation a building needs. For new home construction many states have energy codes that define the levels of insulation. For local insulation recommendations you may want to contact your state energy office.
When determining an adequate amount of insulation to add to an existing home, there are a number of factors to consider, including: existing insulation levels (if any) and its condition, ease of access to install the insulation, adequacy of ventilation, fuel costs, climate conditions, and occupant's comfort requirements. While there are guidelines for insulation levels, since every house is different you're only going to be able to add the amount that the house will cost-effectively "fit" into the home. To determine the most practical and cost-effective levels of insulation that will fit into your home you may want to hire an energy auditor to assess your home.
The United States Department of Energy has developed a website that provides information on insulation and includes a link to a zip code insulation calculator that provides a more detailed evaluation of heating and insulation recommendations for specific areas. For example, current recommendations for most of the northern half of the United States suggest an insulation equivalent to an R-value of 49 to R60 in attics and R13 to R21 in walls. Zip Code Insulation Calculator
Climate specific information can be found at the Building America website in their Best Practices handbook series
