In order to choose the ideal materials for construction when it comes to energy efficiency, savings and comfort, it is necessary to know their thermo-physical properties; it is not enough to apply low thermal conductivity materials only.

The thermal resistance of a material represents its capacity to oppose to heat flow and is defined as the ratio of product thickness to its thermal conductivity coefficient; therefore, achieving a given thermal resistance is only a matter of thickness against lambda, λ.

Thermal conductivity is an intrinsic characteristic of materials; a parameter—measured at steady state and constant temperature—which must be provided by the manufacturer and accredited by means of trials and tests according with UNE-EN 12667 or UNE-EN 12939 standards.

A generic thermal conductivity, representative of a whole family of materials cannot be considered, let alone insulators since a different value can alter significantly the thermal resistance of the element to calculate.

Thermal conductivity must be defined clearly for each product by the value obtained from tests at the laboratory.

Every material has unique characteristics which define it; no material is identical to any other in their microscopic structure. The intrinsic properties of a material are conditioned by the nature of its components and manufacturing parameters—doses, methods, application, etc.—and any modification to the components or parameters can mean a deviation of intrinsic values.

The basic thermo-physical properties for evaluating the dynamic-thermal response capacity of a material are: density (r), specific heat (c) and thermal conductivity (k).

From these parameters, we can calculate the thermal storage capacity per volume unit (r.c) and thermal diffusivity (α=k/r.c) which measures the speed with which a material responds to a given thermal disturbance. The lower the thermal diffusivity is, the higher thermal inertia of the material.

In order to prepare an energy savings and thermal comfort program by means of designing a thermal insulation system, it is necessary to evaluate the thermal resistance, thermal storage capacity and thermal diffusivity.

Buildings with low thermal inertia react quickly to solar radiation, heating up quickly during the day but also cooling more quickly during the night; the lag between heat input and achieved temperature is short. By contrast, in dwellings with high thermal inertia, the solar radiation does not cause a quick rise of the temperature because the heat is stored and then released slowly during the night; sudden temperature drops are not produced and temperature variations are absorbed, never reaching extreme values.

The best response in the envelope of a building is achieved using low thermal diffusivity materials in the external layer of the enclosure and high thermal diffusivity in the internal one because these are the ones which store thermal energy.

The best choice is a system with the highest thermal resistance and thermal storage capacity and the lowest thermal diffusivity.