Thermal conductivity is the property of a material that indicates its ability to conduct heat. Heat transfer by conduction allows heat to flow within a material in the direction of decreasing temperature without any motion of the material as a whole. Conduction only takes place when a temperature gradient exists in a solid or stationary fluid medium. As higher temperature equates to higher molecular energy or more molecular movement, energy is transferred from the more energetic to the less energetic molecules when neighboring molecules collide.
Units and Measures
Thermal conductivity can be quantified as the amount of heat transmitted through one unit thickness of a material in a direction normal to a surface that material of unit area due to a unit temperature difference under steady state conditions and when the heat transfer is dependent only on the temperature difference.
Thermal conductance is the exact same measure, but taken across a material of a given thickness.
Generally thermal conductivity is measured using steady-state or the transient techniques. In steady-state techniques, temperature remains constant where the transient techniques perform a measurement during the process of heating up.
Following are the units of thermal conductance:
- SI Units: Watt per meter kelvin (W/m•K) ?????????
- Imperial Units: British thermal unit per hour feet degree Fahrenheit (Btu /h•ft•°F)
Example Values
Following are a range of thermal conductivity values for common building materials, given in W/m•K:
| MATERIAL | W/m•K |
|---|---|
| Air | 0.024 |
| Clear Glass | 1.31 |
| Concrete | 1.7 |
| Fibreglass | 0.04 |
| Timber | 0.17 |
| Steel | 43.0 |
| Aluminium | 250.0 |
Table 1: Thermal conductivity values for common building materials.
References
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