Thermal Measurement

Understanding Thermal Conductivity and Thermal Resistance

According to Wikipedia, thermal conductivity, k (also denoted as λ or κ), is the property of a material’s ability to conduct heat. In other words the ability of heat to pass through a material, is one of its intrinsic properties. This property can be measured using special instruments. Thermal conductivity measurement assumes that the heat transfer through a material is the same throughout its thickness.

According to American Society for Testing and Materials (ASTM) standard C168, thermal conductivity λ, is the time rate of steady state heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area (at a given temperature).

That is, λ = (W/m2)/(K/m) = W/m .°K. or mW/m.°K.

Thermal conductivity testing is usually done in one of two apparatus/specimen geometries: flat-slab specimens with parallel heat flux lines, or cylindrical specimens with radial heat flux lines.

On the other hand thermal resistance is a measure of a material’s ability to prevent heat from flowing through it and is written R_Value = m².°K/W

The R-Value is the thermal resistance of a material, at a given thickness.

R-Values are written for example, as “R 1.2” (without the quotation marks).

For insulation purposes the higher the R-Value the more effective the insulation product at a given thickness.

The R-Value (thermal resistance) of a complex system – for example, multilayer thermal insulation – is equal to the sum of the R-Values of its parts.

That is R_{(sum of parts)} = R_{part 1}+R_{part 2}+R_{part 3} and so on.

When measuring the thermal conductivity of a material, such as expanded polystyrene (called EPS), the temperature at which the measurement was carried out, must be taken into account. For example, the thermal conductivity, for one manufacturer of VH grade of EPS  at zero degrees centigrade (0°C) is quoted as 0.032 W/m.°K, whereas the thermal conductivity at 15°C is 0.034 and at 25°C it is 0.035. In other words the higher the temperature at which the measurement was carried out, the greater the conductivity which also means the less apparent thermal resistance. i.e. a lower R-Value. As a result of this, it is important when comparing the thermal properties of two different materials, to check at which temperature the measurements were made at.

To compare thermal performance properties, the measurements ought to be carried out at the same temperature, which for New Zealand is 15°C and Australia is 23°C.

To compare R-Values, the material thickness must be the same, as well as the testing having been carried out at the same temperature.

For information about the thermal test equipment used by BEAL, click here.