Insulating material is rated based on its R-Factor, a function of thermal resistance, or the ability of the medium to insulate against heat loss.
Where R-Factor is concerned, increasing the thickness of the insulating layer will also increase the R-Factor of the material.
If a window has a high R-factor and a low U-factor then this window is very energy efficient, which drives up the price.
Both cellulose and fiberglass have comparable R-Factor ratings, with cellulose coming in slightly ahead of fiberglass.
For the R-factor, the higher the number means that the materials are insulated better.
Energy efficiency in windows is rated by two factors: The U-factor and the R-factor.
The R-factor measures the resistance of heat transfer of certain materials.