In my attic, I have a thin insulation placed under the rafters and on the rafters a film "rain screen". But between my rafters I have nothing and I would like to know what I could add to improve the acoustic and thermal comfort?
I would try to tell you... completely redo your insulation. You bring the demonstration that this type of product is neither thermally nor acoustically efficient. It is not possible to add conventional insulation under a thin "insulator" coupled to a roofing film, all of which form a watertight barrier. The use of a thin reflective insulation as under-roofing screen is absolutely prohibited, as clearly indicated in the CSTB and G20 opinion on the subject. I therefore advise you to remove this thin "insulation" and the film and to lay a conventional insulation or to preserve the products within the framework of a global insulation, integrating a layer of fibrous insulation.
I have already mentioned the system developed by KDB Isolation called global insulation which combines a thin product highly permeable to water vapor under-roofing, a layer of fibrous insulation polyester wool and a waterproof film on the inside. which perfectly meets the insulation criteria for BBC homes that meet current standards.
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