Insulation is rated with an R-value number. This number equals the thermal resistance of a material to allow heat to transfer from one side of the material to the other. When considering insulation for your home, materials with higher R value ratings will be better at slowing the transfer of heat from your home. In the winter these values slow the heat from moving from the inside of your home to the cold outdoor air, and in the summer these insulation values will slow the heat from moving from the hot outside air to the cooler inside air in your home.
R-1 has a low resistance to heat transfer. This means heat will move through it fast and thereby cause your furnace and air conditioner to use more energy to keep your home warm in the winter and cool in the summer. R-2 means 1/2 the heat loss (or gain) of R-1. R-15 means only 1/15th as much heat as R-1 will get through, and so on. Higher R values thereby cause your furnace and air conditioner or geothermal heat pump system to use less energy to keep your home warm in the winter and cool in the summer.