Attic Insulation and Ventilation
In our part of the country, you should have a minimum of 10″ ( around R-30) in the attic. You also need venting from the sofits to the ridge, to get good circulation. This helps to keep the warm air in the house during the winter and warm air out of the attic during the summer. This house had 4″ ( about R-12) insulation. The shingles were 6 years old but looked around 15 due to all the heat in the attic during the summer. This also prevents Ice dams in the winter.