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The materials ability to resist the heat transfer by conduction is known as thermal resistance or R-Value. Materials with high R-values have greater insulating capabilities than materials with low R-value numbers. Most construction materials have easy to find R-values, such as insulation you may find at a local hardware store may have R-values ranging from R-19 to R-32. As it would be expected the R-32 insulation has better insulation capabilities than the R-19, but the R-32 is usually more expensive.
R-values can be expressed using basic thermodynamic properties of heat transfer, conductivity and amount of material in question:
Sine R-values are "A measure of the resistance of the material to heat flow is the thermal resistance R, often called its R-value given by R=delta/k. The higher the R-value is, the better are the insulating properties of the material. Using this notation, the rate of heat flow is given by
Qc/t = 1/R*A*change in T
- R is a function of both the type of material and its thickness.
- Qc=Heat transfer
- AL/k = R-value of insulation
R-value according to Physics by Cutnell and Johnson:
- The R value expresses in a single number the combined effects of thermal conductivity and thickness.
- Larger R values reduce the heat per unit time flowing through the material and therefore mean better insulation. It also convenient to use R values to describe layered slabs formed by sandwiching together a number of materials with different thermal ratings and different thicknesses. The R-values for the individual layers can be added to give a single R-value for the entire slab. It should be noted, however, that R-values are expressed using units of feet, hours, F and BTU for thickness, time, temperature, and heat, respectively.
According to the Kansas State University Extension Service the R-value of soil is about 1 per foot.