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Insulation is some material that is used to reduce the rate of heat transfer.

An R-Value is a calculated value used to measure and compare the insulative values of different materials. R = Temperature Difference × Area × Time ÷ Heat Loss The temperature difference is measured in degrees Fahrenheit, area in square foot, time in hours, and heat loss in BTUs.

Material, for each inch R-Value
High density EPS foam 4[1]
Soil 0.25–1[2]
Sand 0.08–0.11[2]
Concrete 0.07–0.52[3]