R38 is probably the most commonly recommended level of insulation. When you select insulation you have to base your decision on variables. Homes in different regions need different types and thicknesses of insulation in order to be effective. If you live in the Southern warmer areas you aren't quite as worried about maintaining heat in the winter but keeping it cool throughout the long, warm months. At the same time you can apply the opposite to homes in colder areas.
The type of heating system used is also considered in determining the best insulation for the home. The final variable is what part of the house is being insulated. Walls and attics do not carry the same legal requirements. R38 is a measurement of the thickness and effectiveness that is found in homes in both cold and warm regions. It is at the high end of recommendations for warm regions and the low end for cold regions.
Homes in the warm southern areas that are more concerned with preventing the escape of cooling air but have minimal requirements for the heating season can use this standard insulation. Gas, oil, and electric systems are all applicable to the rating and this is insulation typically found in the ceilings and attics.
Mid range homes that have hot summers and also have cold, although not frigid winters (Mid-Atlantic states) with electric, gas, or oil heat are generally considered well insulated with R38. These areas can depend on this level of insulation to keep the heat in during the cold snaps and keep the cool air inside during times of record heat. This is a standard requirement for all homes and buildings. While it is certainly possible to insulate more thoroughly it is not considered to be essential.
Very cold regions can use this grade of insulation but it is considered to be at the very bottom of the recommendations. Most homes will need to add another insulator or have a higher grade sealing insulator in the ceiling cracks. It is generally considered to be essential to seal attic ceilings as tightly as possible while still allowing for proper ventilation, especially in areas of high winter winds.
Using R38 rated insulation is common because it is effective and typically inexpensive. You can line your ceilings and attic space with it and be protected from cold and heat regardless of the region you live in.