TN#16 Thermal Mass, Resistance and heat flows in walls


The concepts of Thermal Resistance and Thermal Capacity are explained in laymen’s terms.

Heat flows by three mechanisms: conduction, convection, and radiation.

  • Conduction is the molecule-to-molecule transfer of kinetic energy (one molecule becomes energized and, in turn, energizes adjacent molecules).  A cast-iron skillet handle heats up because of conduction through the metal.
  • Convection is the transfer of heat by physically moving the molecules from one place to another. Hot air rises and heated less dense water rises.
  • Radiation is the transfer of heat through space via electromagnetic waves (radiant energy). A campfire can warm you even if there is wind between you and the fire, because radiation is not affected by air.


With buildings, we refer to heat flow in a number of different ways. The most common reference is “R-value,” or resistance to heat flow. The higher the R-value of a material, the better it is at resisting heat loss (or heat gain).


U-factor (or “U-value,” as it is often called) is a measure of the flow of heat—thermal transmittance—through a material, given a difference in temperature on either side.  The units are Watts  of energy passing through a square meter of the material for every degree difference (W/m2°C). This is measured in a ‘hot box ‘ test (ASTMC1363) at that time when a steady state of heat flow has developed.

R-value and U-value are mathematically the inverse of one another: U = 1/R. See Technical Note 6 for the calculation of these properties. Materials that are very good at resisting the flow of heat (high R-value, low U-value) can serve as insulation materials.


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