What is meant by K-Factor, R-Value, and C-Factor?

What is meant by K-Factor, R-Value, and C-Factor?

K-Factor (Thermal Conductivity Factor)

The measure of heat in BTUs that pass through one square foot of a homogeneous substance, 1 inch thick, in an hour, for each degree F temperature difference. The lower the K-value, the higher the insulating value. Textbook definition: The time rate of steady heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area.

Insulation materials usually have K-Factors less than one and are reported at what is called Mean Temperature. To determine the mean temperature, measure the surface temperatures on both sides of the insulation, add them together and divide by two.

When comparing the insulating value of different types of insulation, it's important to look at K-Factor and the mean temperature. As mean temperatures rise, so does the K-Factor.

C-Factor (Thermal Conductance Factor)

C-Factor is the number of BTUs which will pass through 1 square foot of material with 1 degrees F temperature difference for a specified thickness. The C-Factor is the K-Factor divided by the thickness of the insulation. The formula is the reciprocal of the R-Factor formula. The lower the C, the better the insulator.

R-Value (Thermal Resistance Value)

The National Commercial & Industrial Insulation Standards Manual defines R-Value as a measure of the ability to retard heat flow rather than to transmit heat. "R" is the numerical reciprocal of C, thus R=1/C. Thermal resistance designates thermal resistance values: R-11 equals 11 resistance units. The higher the "R", the higher (better) the insulating value.

What is the difference between Mean Temperature and Ambient Temperature?

Temperature is a property unto itself. It is not a measurement of the amount of heat present. For example, if you pour two cups of coffee, one to the brim, and the other only halfway, the temperature will be the same in both cups, but the partially filled cup will only contain half the heat (BTUs) of the full one.

Mean Temperature is the average of the sum of a hot surface temperature and a cold surface temperature. Insulation conductivity (K-Factor) is tested at a number of mean temperatures to develop conductivity curves that simulate actual service conditions under which insulation systems are used. All conductivity figures (K,C,R) must be qualified by a mean temperature.

Ambient Temperature is the average temperature of the medium, usually air, surrounding the object under consideration.


National Insulation Association (NIA). 2016. FAQ. [ONLINE] Available at: http://www.insulation.org/techs/faq.cfm

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