Listing the parameters of insulation, the thermal conductivity of the material is always put first. It depends on how much air is contained in the substance. After all, the air environment serves as an excellent natural heat insulator. Note that the ability to conduct heat decreases with increasing rarefaction of the medium. So it is best to keep the heat from the vacuum layer. The work of thermos is based on this principle. But during the construction of a vacuum to create a problem, therefore, limited to ordinary air. For example, the low thermal conductivity of polystyrene, especially extruded, due to the fact that this very air in it is more than enough.
In order to clearly understand what heat conductivity is, take a piece of material of a meter thickness and an area of one square meter. Moreover, one side of it is heated, and the other is left cold. The difference of these temperatures should be tenfold. Measuring the amount of heat, which in one second passes to the cold side, we obtain the coefficient of thermal conductivity.
Why is it that expanded polystyrene is able to retain well both heat and cold? It turns out that the whole thing is in its structure. Structurally, this material consists of a set of sealed polyhedral cells having a size of from 2 to 8 millimeters. Inside they have air - it is 98 percent and serves as a great heat insulator. Polystyrene accounts for 2% of the volume. And by weight, polystyrene is 100%, since the air, relatively speaking, has no mass.
It should be noted that the thermal conductivity of extruded polystyrene foam remains constant over time. This distinguishes this material from other foams, the cells of which are filled not with air, but with other gas. After all, this gas has the ability to gradually evaporate, and the air remains inside the sealed polystyrene foam cells.
When buying foam plastic, we usually ask the seller what the density value of the material is. After all, we are accustomed to that density and the ability to conduct heat are inextricably linked with each other. There are even tables of this dependence, with which you can choose the appropriate brand of insulation.
|Polystyrene foam density kg / m 3||Thermal conductivity W. / MKV|
However, at the present time, an improved insulation has been invented introduced graphite additives. Thanks to them, the thermal conductivity coefficient of polystyrene foam of various densities remains unchanged. Its value is from 0.03 to 0.033 watts per meter per Kelvin. So now, acquiring a modern improved Epps, there is no need to check its density.
Marking of polystyrene foam whose thermal conductivity does not depend on density:
|Brand of polystyrene foam||Thermal conductivity of W. / MKV|
|EPS 50||0. 031 - 0. 032|
|EPS 70||0. 033 - 0. 032|
|EPS 80||0. 031|
|EPS 100||0. 030 - 0. 033|
|EPS 120||0. 031|
|EPS 150||0. 030 - 0. 031|
|EPS 200||0. 031|
Compare the thermal conductivity of mineral wool and polystyrene foam. In the latter, this figure is less and ranges from 0.028 to 0.034 watts per meter per Kelvin. The thermal insulation properties of EPPS without graphite additives decrease with increasing density. For example, extruded polystyrene, whose thermal conductivity is 0.03 watts per meter per Kelvin, has a density of 45 kilograms per cubic meter.
Comparing these indicators with various insulants, we can conclude in favor of EPSs. A two-centimeter layer of this material holds the heat in the same way as a mineral wool with a layer of 3.8 centimeters, a conventional foam layer with a layer of 3 centimeters, a wooden board with a thickness of 20 centimeters. A brick will have to lay out a wall 37 centimeters thick, and foam concrete - 27 centimeters. Impressive, isn't it?