The Relationship Between Thermal Conductivity and Moisture

Understanding how moisture affects thermal conductivity of a material is a key factor in how quickly it can transfer heat. It is an intrinsic property that varies significantly based on the type, structure and state of the material. It is a function of temperature and is also affected by the presence of moisture. A high thermal conductivity can be desirable in certain applications such as conductive heating and cooling. Conversely, low thermal conductivity is often desirable in applications where heat must be prevented from moving to surrounding components like in an insulator.

The relationship between a materials thermal conductivity and its temperature is illustrated by the equation below. There are four factors that affect the rate at which a material can conduct heat: the thermal conductivity constant k, A (cross sectional area), DT (difference in temperature between one side of the material and another) and d (the thickness of the material).

Metals generally have very high thermal conductivities because of the delocalized electron movement within their metallic bonding. This is why they can heat up more rapidly than non metallic materials such as plastics and wood. However, within this group of metals there is a great deal of variation. Non metallic solids conduct heat through vibration of the atoms or molecules that comprise their molecular structure. This means that they tend to have higher thermal conductivities than gases.

But the role of moisture in this equation is important. Moisture in a material can cause the thermal properties of that material to change dramatically. When a material has an excessive amount of moisture, its conductivity will increase. This can be a serious issue in some applications such as when buried pipes are being used for heating or cooling. When the thermal conductivity of soil is too high, it can cause those buried pipes to overheat and potentially combust.

The effect of moisture on a materials thermal conductivity can be measured with a simple laboratory system. It consists of a guarded hot plate that can be heated up to a set temperature and a specimen that is placed in the center of the heat source. A sensor measures the temperature rise of the sample over time to determine its specific thermal conductivity.

This data is compared to the same test performed under medium humidity conditions to determine a moisture related variation in a samples thermal conductivity. This variation can then be calculated and expressed as a coefficient (). The coefficient is referred to as a thermal insulance and is typically reported in watts per meter-kelvin. For more information on thermal conductivity, contact a Custom Materials, Inc. Our experts can provide more information about this important thermal property and help you choose the right prototyping materials for your project.