The Basic Properties of Heat-insulating Material
In architecture, the material which prevents the outflow of indoor heat is normally called adiabator. On the other hand, the material which prevents the inflow of outdoor heat is called heat insulator. These two are collectively called heat-insulating materials.
This kind of material is mainly used for walls, ceilings, thermal equipment and thermal pipelines. It is sometimes used to preserve heat in winter construction. Generally, it may also be used a lot for refrigerating chamber and equipment.
It is known that heat current flows from high temperature to low temperature. In order to keep the room warm in winter, constant indoor heat supply must be provided to compensate for the heat loss due to the temperature difference. This problem can be partly addressed by using the heat-insulating material. For example, a six-storey residential building of four units in Beijing, China, uses the frame structure of mineral cotton composite plate, and this reduces the heat loss by 40% compared with masonry-concrete structure.
According to the statistics, a good insulation building can cut down the fuel consumption by 25% to 50%. And to achieve this, one should consider the following questions:
- What kind of structure is easy to lose heat, and vice versa;
- How the composition of the material relates to its heat-insulating property;
- What are the factors that influence the material’s heat-insulating property, and
- How to select the proper material.
Exterior protected construction is made of different building materials, the thermal conductivity and specific heat of which are the important parameters to the designation of the wall, roof, floor, and to the heat engineering calculation. The material with low thermal conductivity and high specific heat can improve the heat-insulating property of the exterior protected construction and keep the indoor temperature stable.
The basic requirement for selecting heat-insulating material is that the thermal conductivity is well below 0.23 W/ (m*K), the apparent density below 600kg/m3, and the compression strength above 0.3MPa. Moreover, the material’s hygroscopicity, temperature stability, and corrosion resistance all need taking into account according to the characteristics of the project.
The following is an introduction of the basic properties of the heat-insulating material.
1. Thermal Conductivity
Thermal conductivity is the measurement of the heat transfer ability of the material itself. It is influenced by the material constitution, porosity, temperature of the surroundings, and the direction of the heat current.
The Material Constitution
The thermal conductivity can be influenced by the material’s chemical composition and molecular structure. Material with simple chemical composition and molecular structure has higher thermal conductivity than the complex.
The thermal conductivity of solid matter is higher than that of air. Therefore, the higher the porosity is, the lower the thermal conductivity will be. In this aspect, not only the porosity matters, but also the size, distribution, shape, and connectivity of the pores.
Materials in damp conditions have higher thermal conductivity. What should be noted is that if water is frozen, its thermal conductivity will become higher. This is because the thermal conductivity of water is 20 times higher than that of air while that of ice is 80 times higher than air. Therefore, special attention should be taken to guard the heat insulating material against damp.
If the temperature becomes higher, the thermal conductivity increases accordingly. As the temperature rises, the thermal motion of the molecular solids becomes more active; the heat conduction of the air in the pore gets boosted, and the radiation effect of the pore wall is strengthened.
The Direction of Heat Current
In case of the material being anisotropic (like the fibrous material wood), when the heat current flows parallel to the fibers, there will be no strong resistance; However, when the heat current flows against the fibers, strong resistance will be incurred.
2. Temperature Stability
Temperature stability is the ability of the material to retain its original property when exposed to heat. It is generally expressed by the ultimate temperature, exceeding the point of which the material will lose its heat-insulating function.
The heat insulating material is usually measured by its compression strength and flexural strength. As the material is highly porous, its strength is weak. Thus it is better not to allow the heat insulating material to carry more weight.