R-Value is not the Total answer to insulation.
Heat naturally flows from warm areas to cooler areas, regardless of direction.  In winter, heat flows from the inside of a building to the outside and in the summer high heat from roofs and walls travels from outside to inside.  This flow of heat can never be stopped completely, but the rate at which it flows can be reduced by using materials which have a high resistance to heat flow.
( R value  =  resistance)

Obviously an important step in the creation of an energy efficient house or building is to control heat loss or gain, which accounts for 75% of the total energy loss of a home.As was previously stated, heat will flow in any direction where a temperature difference occurs. Therefore all areas which separate the interior of a house or building from the outside or which separate heated spaces from unheated (or air conditioned) spaces need to have a high resistance to heat flow, in other words, they should be insulated!

How does insulation work?

Insulation is any material that slows the rate of heat flow from a warm area to a cooler one. The more the rate is slowed, the better the insulative qualities of the material. Its ability to resist heat flow is measured as an R or RSI (metric) value, the higher the R - value, the more the material will resist the flow of heat.  In order to be effective, insulation materials must be able to reduce the transfer of heat by the three ways we just discussed, conduction, convection and radiation

Choosing an insulation

The R - value is not the only consideration when choosing insulation yet it is the first thing most consumers ask, "What is the R value" Conventional insulation materials like fiberglass, cellulose, rock wool and Styrofoam, no matter how thick, have almost no ability to block radiant heat energy which can account for as much as 93 percent of summer heat gain and up to 75 percent winter heat loss in conventional structures.These products are only designed to slow down (resist) conduction heat energy only. Insulation once saturated with heat will simply allow remaining heat to pass through. Ever notice in the summer, its 10 PM or so and you touch one of the walls in your home that butts up to the outside, the sun has been down for hours but the wall is still Hot!  That's because the wall has been absorbing heat all day and is passing it through the wall to the inside of your home.

Remember... R-value means "resistance", if a product resists, it does not stop radiant heat transfer. R-value material only deals with conductive heat transfer. Other factors to consider when choosing an  insulation are the materials fire, mold, insect, vermin and moisture resistant properties, as well as its cost and ease of application.

The R values required by Standard 90.1 are based on equivalent energy performance. For example, in Tulsa, OK, Standard 90.1 requires an R 8.3 frame wall or an R 4.3 mass wall for some buildings. These requirements are based on the fact that an R 4.3 mass wall is as energy efficient on an annual basis in an occupied building as an R 8.3 frame wall, in this particular climate. The benefits of mass walls vary by climate and can be influenced by factors such as temperature "swings" (differences between the high and low outdoor temperatures during the day), by solar radiation and wind near the building, and by how the building is designed, operated, and maintained for comfort to be energy efficient.