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I have to redo the insulation, a roof under crawling throughout the house (living room with mezzanine). Currently 6 to 7 cm of glass wool between rafters.
Can I keep the existing ceiling and insulate on either side by recreating a false ceiling between the beams or do I have to remove the existing ceiling to have continuous insulation?
In the first case, is there a risk of condensation? (height of the rafters: 15 cm, height of the visible beams of the interior: 21 cm).

The insulation must always be continuous to avoid thermal bridging. It is always desirable to start on a healthy basis by depositing the old insulation.
However, if your glass wool is in perfect condition, nothing prevents you to extend it over the entire surface and then cover with a second layer cross, lacerating the vapor barrier insulation already installed, and always placing the vapor barrier (or membrane) of the new insulation into the house. The choice of the thickness of this layer will achieve a technical performance according to the requirements of the regulations. The airtight mambrane between the insulation and the decorative facing will complete the insulation.

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