From always, the man has tried to defend from the cold; first covering with animal skins and inhabiting in the caves, using later the fire and constructing houses increasingly efficient to the cold. A little time ago, a nice house of bricks and a solid ceiling they were satisfying this exigency, but the exponential demographic current increase and the corresponding increase of the number of buildings, they have created a pollution source not compatible with the environment and the economy.

A response adapted to this problematic is to add, to the structural part and to the coating of a building, an insulating thermal who avoids the energetic dispersion, increasing the comfort of the housing and diminishing the costs of a significant way.

The thermal isolation is, without more ado, the system of more effective energetic and economic saving that exists nowadays, since the costs of investment recover in a few years of life of the product. A KWh saved thanks to the use of the suitable thermal insulation, costs more than a KWh consumed by the most efficient boiler, since the life of the insulating materials is longer than the life of the facilities.

** INSULATING MEANS PREVENTING THE PASSAGE OF ENERGY BETWEEN TWO BODIES OR TWO ENVIRONMENTS WITH A DIFFERENT TEMPERATURE**. In terms of thermal isolation it wants to say to manage the behavior of the thermal flow in the environment where the man habitually lives. Consequently, the insulating ideal material should have a characteristic: to be opposed to the passage of this heat flow.

The heat passage towards the exterior, in winter, and vice versa, in summer, will be minor how much minor is the Thermal Conductivity (lambda) of the material. Every material is characterized by an own value of thermal conductivity. The lower the value of lambda is (expressed in W/mK to 10ºC) the more effective the material will be as thermal insulation.

Moreover, the thickness of the material plays an important role. To view the importance of the thickness, we introduce a new concept, which is the thermal resistance. The relation among the thickness and the thermal conductivity of the insulating material is the * Thermal Resistance (R)*, expressed in m

^{2}K/W

*(01).*For the same thermal conductivity, the more thickness the product has, the more thermal resistance the product has. And for the same thickness, the less thermal conductivity the product has the more thermal resistance the product has.

The thermal global resistance of a building is obtained adding the thermal resistances of the materials that compose the structure, including the internal and external coatings. In a wall, for example, we find material of structure as bricks, wood, cement, etc... they have values of thermal conductivity extremely high and, therefore, very small thermal resistances. So, the element that it contributes more to the thermal global resistance of a structure is the thermal insulation and the thickness of this one. The thermal insulation manages to diminish, with a high percentage, the quantity of heat flow that it crosses a structure. For example, only 5cm of an insulating thermal PUR obtains an isolation equivalent to 60cm of brick or 2m of concrete.

The thermal total resistance of a structure (the sum of the thermal resistances of all the materials that compose the above mentioned structure) permits of the calculation of the * Thermal Transmittance (U-values) *of the structure. The U-value can be expressed as the inverse of the total resistance, expressed in W/m

^{2}K, as it is needed in the in force laws [

*U = 1 / R*

*]*.

The Thermal Transmittance is the heat quantity that, for unit of time (h:hours), passes across the surface unit (m^{2}), when the difference of temperature between both faces of the structure is 1 Kelvin (K) *(02)*

The thermal Resistance and, therefore, the thermal transmittance bear in mind the effects of change of the air on the faces and have the following value, where alpha (α) is the coefficient of adducing of the internal and external air *(03)*