Introduction: What are the important indicators to measure the insulation effect of thermal insulation materials? Will the "indicators" change due to external environmental influences?

First, what is the index to judge the insulation effect?

The second law of thermodynamics explains that where there is a temperature difference, there must be a transfer of heat. From the conventional form of insulation, the speed of heat transfer has a certain relationship with the thermal resistance of the insulation material itself.

However, on the one hand, the thermal resistance test is difficult, and on the other hand, the thermal resistance is generally proportional to the thickness of the material, that is, the greater the thickness of the material, the greater the thermal resistance. Therefore, when measuring the insulation effect of thermal insulation materials, it is usually based on the thermal conductivity, which is an absolute value and an inherent property of the material itself, not because of the thickness of the material, shape and other factors to change. Today, with the relatively mature development of the insulation material industry, the test standards, methods, equipment and other aspects of thermal conductivity have also been relatively perfect, which can provide a reference basis for people to choose insulation materials.

Next, we will elaborate on the theme of "Main factors affecting thermal conductivity".

2. Factors affecting thermal conductivity

Heat transfer includes three parts: heat conduction, heat radiation and heat convection. The level of thermal conductivity is directly related to the three parts of heat transfer.

Insulation materials have a significant feature, is loose porous. Taking rubber and plastic insulation materials commonly used in HVAC, drainage pipes and other daily scenes as an example, its closed cell structure can effectively reduce or block the formation of heat convection, so that the material has a better insulation effect.

For another example, the thermal insulation properties of aerogel are also based on its porous nanostructure, resulting in a long and narrow heat transfer path and a slow heat transfer rate.

The thermal conductivity of thermal insulation materials is related to three general directions of heat transfer. Although the thermal conductivity of the insulation material is not affected by the thickness and shape of the material itself, it will also fluctuate due to changes in the external environment and conditions. It will also show different thermal conductivity with other thermal insulation materials due to its own material properties.

First of all, the more intuitive point is that different insulation materials, thermal conductivity is also very different. For example, the thermal conductivity of inorganic substances and organic compounds is slightly lower, but the thermal conductivity of foam is even lower.

Secondly, the density of the material itself also has an important effect on the thermal conductivity. Under normal circumstances, the higher the density of the insulation material, the more dense the material, the higher the thermal conductivity. However, this is not set in stone. Previously, we discussed the relationship between thermal conductivity and density of rock wool insulation materials: the density of rock wool and thermal conductivity is not a simple linear relationship, when the density of rock wool is too high, the gas space will be reduced, resulting in increased thermal radiation and convective heat loss, and finally the thermal conductivity will increase, and the thermal insulation effect will also be reduced.

Moreover, humidity is also one of the factors affecting the thermal conductivity of the material. When the ambient humidity is too high, or the insulation material is damp, the thermal conductivity may increase, affecting the insulation effect. Therefore, we generally recommend that the preservation of thermal insulation materials should be placed in a dry and ventilated place.

Temperature is also one of the key factors affecting the thermal conductivity of thermal insulation materials, and the thermal conductivity is basically increased according to the increase in temperature.

The above are several important factors affecting the thermal conductivity of thermal insulation materials. The thermal insulation material may be affected by the superposition of a variety of factors. In practical application, we also need to choose reasonably according to the scene and environment of use, and use and store reasonably according to the properties of thermal insulation materials to ensure the thermal insulation effect of materials.