Due to its density, compacted earth acts as thermal mass, meaning that it stores heat and releases it again slowly. Compacted soil is more of a conductor of heat than an insulator. Soil is stated as having an R-value of about 0.65-R per centimeter (0.08-R per 1 inch), or 0.25-R per 1 inch. Variations in R-value of soil may be attributed to different soil moisture levels, with lower R values as moisture level increases. The most superficial layer of earth typically is less dense and contains the root systems of many different plants, thereby acting more like thermal insulation, meaning, it reduces the rate of temperature flowing through it.