Page data
Part of PH261
Keywords Heating and cooling, Thermal insulation, energy efficiency, construction, heating, cooling
SDGs Sustainable Development Goals SDG07 Affordable and clean energy
Published by Kerri Fesenmyer
Nicholas Colbrunn
Published 2007
License CC BY-SA 4.0
Affiliations Clarion University
Page views 543

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.

Definitions[edit | edit source]

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.
  • T=Time
  • 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.

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