FAQ Thermal Conductivity.
Thermal conductivity (λ) is the rate at which heat can pass through a material. In metric units this is expressed in terms of W/(m・K).
Materials which have a low thermal conductivity rate resist the flow of heat and are known as thermal insulators.
Which factors influence thermal conductivity?
Aside from material composition and structure, the single greatest influencing factor on thermal conductivity is the average temperature of a material. Normally a material will experience a reduction in thermal conductivity as it cools and an increase in thermal conductivity as it warms.
Other factors include the compression of the material, age and the moisture content.
Is the thermal conductivity the same as the U-Value or R-Value?
Thermal conductivity is different to the U and R values but is closely related.
U and R values are measures of insulation performance used in the structural insulation industry. The U-Value is the thermal conductivity of a material divided by its thickness whilst the R-Value is the reciprocal of this (1 divided by the U-Value).
The thermal conductivity is the preferred measure for pipe and duct insulation.
Is a lower thermal conductivity value always better?
The heat loss through insulation is primarily defined by a combination of the insulation thickness and the thermal conductivity so a lower thermal conductivity is usually desirable.
There are however other variables, such as the surface emissivity or the quality of installation, which can have just as great an impact on the insulation thickness required. It is not usually a good idea to select based on low thermal conductivity alone.
Will the thermal conductivity change over time?
Due to the high thermal conductivity rate of water, thermal conductivity of all insulation will increase dramatically if moisture enters the cell structure.
On cold pipes the partial water pressure gradient forces small amounts of moisture into the insulation over time. The higher the inherent moisture resistance the slower the increase in thermal conductivity.