Before start, it is worth pausing on some general definitions:
THERMAL ISOLATION:
It is the resistance that we oppose the passage of cold or heat with one or a few materials inserted in an enclosure, in order to obtain the interior comfort of the buildings.
COEFFICIENT OF THERMAL CONDUCTIVITY ():
It is the amount of heat or cold that passes through 1 m2, of a body with plane-parallel faces, with a thickness of 1 mt, for 1 hour, when between the two faces there is a difference of 1 ° C
THERMAL RESISTANCE OF THE MATERIAL (R):
It is the quotient between the thickness of the material itself and the coefficient of thermal conductivity. The higher this value, the greater the insulation capacity of the material it represents.
RT (m2 K / W) = E (m) / (W / m K)
K (W / m2 K) = 1 / RT (m2 K / W)
THERMAL CONDUCTIVITY OF THE MATERIAL (K, U, C):
It is the amount of heat or cold that is changed, through 1 m2 of enclosure for one hour, when between the exterior and interior there is a temperature difference of 1 K. Therefore, the higher this value is, the lower it will be the insulation capacity of the material.
The only difference between the K value (or C value) and the U value, is that the U value designates the sum of the thermal transmission of all the materials involved in a composite element section (for example: finish + insulator + concrete + insulator + finish), while the K value designates the thermal transmission of a particular material.
Heat is known to tend to flow from a high to a low temperature, through one or any combination of any of the three methods of temperature transfer, such as: conduction, convection, and radiation. The range at which heat will flow through a material will depend not only on the nature of the material, but also on the difference in temperature between the hot and cold sides. The comparison of the effectiveness of the insulation should be made on a basis that excludes the influence of variable factors such as the thickness of the material and the difference in temperatures.
Put simply, the K value (also known as the U value or the C value) represents the number of watts that will be lost per square meter at a given temperature difference in degrees centigrade. As a simple example, if the interior of a room is at 20 ° C, and the outside of it is at 10 ° C, that gives us a difference of 10 ° C. Assuming a roof with R-2 insulation, power will be lost in a range of 5 watts for every square meter of the roof.
Some countries that do not use the international unit system define the value R using the units R = ft2 Fh / BTU.
The conversion between both systems is as follows:
1 ft2 Fh / BTU = 0.1761 m2 K / W or 1 m2 K / W = 5.67446 ft2 Fh / BTU
The thermal insulation capacity of a material is defined by its coefficient of thermal conductivity (), which in the case of EPS products varies, as well as mechanical properties, with the apparent density.
The best thermal insulators on the planet, that is, those with the lowest coefficients of thermal conductivity, are in order of importance the Airgel with a value of 0.014 (W / m K), followed by isocyanurate and phenolic foams, with values of 0.017 and 0.020 (W / m K), respectively. They are closely followed by polyurethane and polystyrene with values of 0.02 and 0.03 respectively. Cellulose, fiberglass and mineral wool hardly have a value of 0.05. It is for this reason that it is said that the products and materials of expanded polystyrene (EPS) have an excellent capacity for thermal insulation against heat and cold.
In fact, many of its applications are directly related to this property: for example when it is used as an insulating material for the different closings of buildings or in the field of packaging and packaging of fresh and perishable food such as fish boxes.
This good thermal insulation capacity is due to the structure of the material itself, which essentially consists of air entrapped within a cellular structure made up of polystyrene.
Approximately 98% of the volume of the material is air and only 2% solid matter (polystyrene). It is known to all that air at rest is an excellent thermal insulator.
