While rated R-Values are important, how much air is allowed to pass through an insulation material can effectively raise or lower its efficiency. This is called Air Infiltration. It can be felt in some fiberglass-insulated houses by putting your hand in front of an outlet on a cold windy day, and seen by doing the same with a lit match.
The bottom line is that any insulation that is installed in batts or rigid boards has the common problem of not forming a tight seal around all crevices, cracks, holes and other microscopic breaks in the structure. There are gaps that remain in between boards or batts that, even when professionally installed, allow air through. This lowers the effective R-Value of the entirety of the insulation.