2.1.1. Equivalent Energy

The calculation method is based on psychrometry. The particles contained in the air, both indoors and outdoors, move on their own at different speeds, which implies that they contain different amounts of energy. Therefore, those air masses contain an amount of energy produced by the movement of those particles. For example, as can be seen on a psychrometric chart, if the temperature rises at a constant level of humidity, the temperature of those particles rises too, as the energy contained in them does. In the same way, if temperature decreases, the amount of energy, known as enthalpy, also decreases. In addition, for constant temperature, if humidity rises, enthalpy also rises.

The state function that allows tracking the marks left by the energy variations at a constant pressure is the enthalpy [65]. It is only possible to determine its variations after a thermodynamic process; this is the reason why it is expressed as the variation of the amount of energy that is expelled to the environment or absorbed by a system during one of those processes. Therefore, its value is expressed in terms of exchanged energy [66].

This way, by means of psychrometry and characterizing these air masses by their temperature (t) and their humidity level (ϕ), it is possible to determine the amount of energy contained in them. Knowing the amount of energy contained in the outdoor air mass (he) and the amount of energy contained in the indoor air mass (hi), the amount of energy that was contributed by the building just with its presence can be determined (Δh).

In other research proposals, the indexes which assess the energy efficiency of a construction are usually determined by the indoor and the outdoor temperature, but they do not depend on the humidity levels [67].

Contrary to the shape factor, by this method, it is possible to calculate the amount of energy that can be isolated by a building, taking into account multiple factors such as its morphology, the composition of its envelope, its orientation, the presence or absence of openings or its indoor compartmentalization. The proposed value does not consist in valuing the sustainability grade of a building, the aim of other researches [68], but on establishing the capacity of an envelope to modify the outdoor conditions provided by nature. The higher the difference between indoor conditions and outdoor conditions is, the higher this parameter is.
