Apparently everyone agrees with his:
the inertia should be seen as a buffer tank that smooths the temperature variations providing additional comfort.
There, in mid-season, it can be important to have the capacity to passively store this solar energy and benefit from it in the evening and until the morning (without having to restart the heating).
But if the heat that I store in my walls with good inertia I restore it at night, my temperature when I do not heat will be higher than temperature with walls with bad inertia (since the heat will be distributed outside). So when I start heating again I will consume less if my non-heating temperature is higher so I will make money.
So 4th question: how with the value of inertia I can know my non-heating temperature at night when my temperature when I stop heating is 20 ° C and outside 9 ° C?
You seem to have replaced the brick with thick polystyrene and therefore the heat takes 9 days, which seems high to me, because the defects (cladding and others) will short-circuit it.
I did not replace the brick with polystyrene I added polystyrene in external insulation but I found a great inertia after insulation because I found on a table a density of polystyrene of 1380kg / m3 and I am not too wary of where my improbable value, I will take the value of the density in the table post above.
I just changed my value and I find inertia after 80h work (the difference is probably due to the wooden cladding)
In any case, thank you all for your answers. It allows me to see a little more clearly.