R-value is a measure of a material’s thermal resistance – the degree to which a material allows or does not allow heat to be conducted through it. In our context it applies to the insulation used to keep houses warm in the winter and cool in the summer. It’s measured both by the thickness of a given insulation material and by the structure of that material, so 2 different types of insulation with the same thickness can have different R-values. Comparing the R-values of various types of insulation is one of the most important ways to determine what type of insulation to use, and how much.
If a home had no insulation at all, the thermal resistance would be measured by the amount of air within the wall cavity, the air would have an R-value of 1 per inch (extremely inefficient). Fiberglass, a common type of insulation, ranges from an R-value of 2 to 5 per inch, depending on the type. Although R-values are measured by thermal resistance per inch (like an R-value of 3 per inch), the reference to thickness is usually not used (so instead we just talk about an R-value of 3).
As we’ve said before R-values and insulation make up only one of the two parts of the “envelope.” The other equally important component of the envelope is Air Sealing.

Recommended R-values vary depending on where in your home the insulation is to be placed. To find a the correct R-value based on your home’s construction and location visit Department of Energy’s ZIP-Code Insulation Calculator

Unfortunately, most houses built today are constructed with 2×4 framing, and you can only fit so much insulation in that small of a space. 2X4 framing limits the potential R-value (for Fiberglass Batt) that can be fit into the wall R-13. If you are constructing your home or doing major remodeling, an excellent step would be to install 2×6 studs in your exterior walls. This will allow you to fit more insulation between the interior of your home and the cold or heat outside. Your home will feel more comfortable and you’ll save on energy bills. Check out the following chart, which lists some common types of insulation and their approximate R-values.

R-Value Table

       Material					R/Inch
       Fiberglass Batt				3.14
       Fiberglass Blown (attic)			2.20
       Fiberglass Blown (wall)			3.20
       Rock Wool Batt				3.14
       Rock Wool Blown (attic)			3.10
       Rock Wool Blown (wall)			3.03
       Cellulose Blown (attic)			3.13
       Cellulose Blown (wall)			3.70
       Vermiculite				2.13
       Autoclaved Aerated Concrete		3.90
       Urea Terpolymer Foam			4.48
       Rigid Fiberglass (> 4lb/ft3)		4.00
       Expanded Polystyrene (beadboard)		4.00
       Extruded Polystyrene			5.00
       Polyurethane (foamed-in-place)		6.25
       Polyisocyanurate (foil-faced)		7.20

Table Source: http://www.coloradoenergy.org/procorner/stuff/r-values.htm