roof insulation and roof renovation

[dhaulagiri]

Hi,

I'm starting a lot of work soon and looking for advice.

In a week, we completely redo the cover: we put in particular an HPV film, because there was none.
The house has several separate roof volumes:
> one where the frame is visible: the garage;
> one with roof space on a concrete slab;
> one with a floor fitted out under crawlers;

In a few months, we re-insulate the lost roof spaces. I already bought the insulation: cellulose wadding to spray.
First, I do not know if I should put a vapor barrier underneath, that is to say on the concrete slab. The advice of different suppliers was not of much help to me, as it was not well argued or contradictory.
Second, is there a possibility, even a small one, that condensation may occur under the roofing film and wet the cotton wool located about 1m below?

Thank you in advance for your advice!

[dedeleco]

Condensation occurs if the water vapor reaches a place cold enough (dew point) to condense.
It can be a porous wall with water vapor (is calculated with tables also given on econology posts) or by holes or leaks of warm indoor air with more or less water vapor, which sneaks towards the cold and condenses;
so you have to look carefully at the exact configuration of porosity or leaks and air circulation.
The porous or leaking wall requires a steamer on the hot side.

Vacherie is that even at very low porosity or leakage, the liquid water of condensation slowly accumulates in a closed environment tight with liquid water over days and months, even over the whole winter without ever leaving it again, if bad luck, which destroys thermal insulation.
We must therefore be very careful and think carefully about this serious difficulty, which is often not very visible.