The measure of the effectiveness of insulation is its resistance to heat flow, or R-value.  The higher the R-value, the better the energy efficiency.  The Department of Energy has established minimum R-values based on zip codes.  R-values are additive.  As an example, if a home has an R-19 insulation in the ceiling and the owner wants to get up to a R-30, adding more insulation rated R-11 will achieve the target.