The higher the R value of an object, area or material, the more resistance to heat conduction there is, and thus the higher insulative abilities. This is especially relevant to building materials and structural aspects to a building or home. Roofs, walls and windows should have high values of insulation.
The greater the number or the higher the material has the capacity to conduct heat, or the higher it’s thermal conductivity, means the r-value will be lower.
R values are a requirement to building and some will vary based on climate. In the U.S., there are four different types of insulation: Foam-in-place, Rigid Foam, Loose-fill, and roll and batts. Foam-in-place insulation offer flexibility because it can be used in tight spaces to control leaks around areas like windows, or it can be used for an entire home. Rigid foam is another form or insulator, that has a higher R-unit of per unit of thickness. Loose-fill insulation is blown into place and comes in the form of loose fibers, while rolls and batts are insulators that are flexible and come in the form of fibers, like fiberglass.
The thicker the insulation, the more heat retention ability it has. Therefore, doubling layers of insulation will double the R-value, which means that the room will be warmer. Adding an extra layer of insulation with a lower R-value won’t do as much. For instance, while important for shielding against elements like weather, adding siding or sheetrock will add R-value, but not as much as proper insulation. That said, squeezing what ought to be two layers of loose fill into a space meant for one layer will increase the R-value, but it won’t double it.