R-value refers to resistance of heat transfer. We want our homes to stay a steady temperature. So, if the temperature outdoors is different from the temperature in your home, the temperatures tend to coalesce through heat transfer. Air is a very poor heat conductor. Materials trapping and isolating air in cells limit the heat transfer. The greater the limitation, the greater the R value, so R-11 has lower insulative properties than R-15.
R-value measures insulation's heat resistance properties by volume. When you compare two different R-values, like R-11 and R-15, the later provides greater performance if you're comparing the same volume of the two grades of insulation. Consequently, using more R-11 may give you more insulation.
There is sometimes confusion about the relationship between R-value, thickness and volume. This may occur because of the different calculations for R-value for loose fill, rigid insulation and batting. For loose fill, list the R-value per inch of insulation. On rigid insulation and insulation bats, the R-value same calculation applies, but the R-value on the insulation already calculated the thickness.
R-11 has about 73 percent of the insulative efficiency of R-15 when comparing two of the materials. This doesn't mean that one house insulated with R-11 will have 73 percent less insulative properties than a house insulated with R-15. The houses may be very different. If, for example, one house has a wide brick chimney, the chimney itself is a poor insulator and will drop the overall insulative efficiency of the house. The same dynamic may apply to other building materials. The size of windows, exterior sheathing and roofing materials also determine your homes overall insulative efficiency.
While generally, equal thicknesses of R-15 will give you more insulation than R-11, air leaks may all but eliminate this relationship. A poorly built or poorly inadequately insulates house will have air leaks. The air leaks may undo the advantages of R-15 over R-11. Therefore, the difference between these two grades is minimal if the insulation is not properly installed.