The higher the R-value, the better the insulating power. That's why it makes the most sense to have these areas with ample, high efficiency insulation. But insulation isn't just for controlling heat. Resistance works in reverse where cool air is concerned, keeping the warm air from flowing into spaces you're trying to keep cool. That's why home owners in warm climates need to know their home's R-factors in order to keep a lid on their air conditioning bills.
Manufacturers clearly mark the R-value on the types of insulation they produce.
Home owners can often use various types of insulation together to obtain a high R-value. Standard R-values differ based on what part of the home you're trying to insulate. But in the South, R-19 in the attic should be ample. But if a home is built using 2 x 6 studs, there's enough space in the wall cavity to insulate up to a value of R-21. A frequent cold air leak can be where the house meets the foundation.