Well the answer is it depends. Insulation protects against the outside climate. Depending on how extreme the climate is in your area, the more insulation you will need.
The International Energy Conservation Code (IECC) breaks up the United States into zones based on their respective climate. There is a different insulation requirement for each zone. The IECC uses R-Values to determine how much insulation a zone needs. An R-Value is simply the universal unit to measure the thermal resistance of a material. Drywall even has an R-Value of about R-0.5.
See map and chart below
For Example Zone 5 requires *R-49 in your attic
The most popular option is blown fiberglass. In a zone 5 right around 17 inches of blown fiberglass is required depending on the brand of fiberglass. You can find the R-Value per inch of a material written on the packaging. This is called a coverage chart.
This is the coverage chart of Owens Corning’s Blown Fiberglass L77
Another popular option is blown cellulose. Cellulose has a higher R-value per inch than fiberglass. So to attain an R-49 you will need to have 15 inches of blown cellulose
Sprayfoam insulation typically does not need a high R-value do to the nature of its air sealing capabilities for example an R-30 in an attic within zone 5 should be sufficient enough.
R-30 Closed cell foam = 6.5 inches
R-30 Open cell foam = 11 inches
09/26/2022 Update: The recently passed Inflation Reduction Act has increases tax credits from the $500 amount listed below to $1,200. This is new information that