Definition of Thermal Resistance
Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. The thermal resistance for conduction in a plane wall is defined as:
Since the concept of thermal resistance can be used in a variety of engineering branches, we define:
- Absolute thermal resistance, Rt, which has units of [K/W]. Absolute thermal resistance is a property of a particular component, which has defined geometry (thickness – L, area – A and shape). For example, a characteristic of a defined heat exchanger. Only a temperature difference is needed to solve for heat transferred.
- Specific thermal resistance or specific thermal resistivity, Rλ, which has units of [(K·m)/W]. Specific thermal is a material constant. Material thickness and a temperature difference is required to solve for heat transferred.
- R-value. R-value (thermal insulance factor) is a measure of thermal resistance. The higher the R-value, the greater the insulating effectiveness. Thermal insulance has the units [(m2.K)/W] in SI units or [(ft2·°F·hr)/Btu] in imperial units. It is the thermal resistance of unit area of a material. The R-value depends on the type of insulation, its thickness, and its density. An area and a temperature difference is required to solve for heat transferred.