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Topic of your question:: Insulation I had a company carry out a total renovation of the frame and roof of a Norman house. The roofer used a "thin insulation" as a roof underlay. You can therefore see my problem: what insulation can be implemented, knowing all the disadvantages of this "thin insulation", (which is not one, it is only a complement, and again...), namely its very high impermeability to water vapor? (I would like to avoid filing all the blanket...). My idea was: to leave a sufficient air gap between the "thin insulation" and extruded polystyrene, laid in two crossed layers in order to limit the thermal bridges, then put a film by vapor over the layers of insulation to make everything totally waterproof. It goes without saying that a VMC would be implemented in parallel. What do you think?

I am sorry I can not endorse your idea, moreover clever. An under-roof screen must necessarily be permeable to water vapor when it is associated with insulation (see DTU 40.1-40.2).

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