R-value is the measurement of a material's ability to slow the temperature's wind. The higher the R-value, the slower the wind speed thru that substance. It takes the wind twice as long to penetrate a R20 as an R10. They are not necessarily outside surfaces, but are all of the temperature surfaces between the inside and the outside and include doors, windows, and wall and ceiling insulation. There are basically two types of insulation.
Rigid insulation comes in sheets and is installed below a slab or on surfaces.
Cavity insulation comes loose, in batts, or as a mixture which can be pumped into the empty spots created by the frame's structure. Whatever the type used, the term R-value always refers to the same resistance to temperature penetration. Because the rates and speeds of temperature differ from one geographic area to another, the requirements for home insulation will also vary. The correct amount of insulation for a house is dictated by the severity of the outdoor temperatures that it will be subjected to. Most new homes are calculated as a package, with all insulating factors mixed in together.