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.
WHAT IS THE R-VALUE?
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).
WHAT IS THE U-VALUE?
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.