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To technically evaluate the thermal insulators it is useful to know the coefficients λ (lambda) and K of the materials in order to apply the resulting calculations. The 'lambda' value indicates' the thermal conductivity, the 'kappa' value the insulating capacity. Let's see them better.
The λ of thermal insulators is the amount of heat that passes through a 1 square meter with a thickness of 1 linear meter in the time of 1 hour, given a temperature difference between the two faces of the material of 1 ° centigrade.
The K of thermal insulators is the amount of heat that passes through a wall of the surface of 1 square meter in the time of 1 hour, with a difference in temperature between the two faces of the wall of 1 ° C.
More simply, the λ represents the amount of heat that a material disperses under the same conditions: the higher this value is, the lower the insulating capacity of the material. Here is for example the λ value of some of the most used thermal insulators:
- glass and rock wool λ 0.038
- expanded polystyrene λ 0.035
- polyurethane λ 0.030
- wood fiber λ 0.050
- cork λ from 0.30 to 0.100 depending on whether it is in granules or panels
- cellulose fiber λ 0.032
But the useful value for establishing the effective insulating capacity of a material is not the λ, but the K that derives from it. In fact, K expresses the insulating capacity (linked to the λ of the material in the optimal density) calculated as a function of the thickness chosen for the insulating layer.
To quickly calculate the K value of an insulator, divide the λ of the material by the chosen thickness expressed in meters: K = λ / thickness in meters. Example: if the λ of an insulator is equal to 0.030 and the chosen thickness is 10 cm, the K (i.e. the insulating capacity) will be: 0.030 / 0.10 = 0.30. For a solution to be in line with the rules on energy saving, the K value must not exceed 0.50.
Another important factor in the evaluation of thermal insulators is the phase shift time. This definition refers to the interval that passes between reaching the maximum temperature on the outer surface of the insulation (for example a panel) and reaching the maximum temperature on its inner side.
More simply, the phase shift indicates the time taken by heat (thermal energy) to pass from one part of a layer to another. That the wall of a building or a roof has a high phase shift time (the higher it is, the better it is) is important especially in summer because it will allow the heat due to solar radiation to be kept outside. Here is roughly the phase shift value of some thermal insulators:
- wood fiber and natural cork 15.1
- cellulose fiber 8.7
- sheep's wool 7.0
- rock wool 6.7
- polystyrene 6.3
- mineral wool 5.9
You may also be interested in our article: "Natural thermal insulation"