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Some beams on my mother's roof are arched so that the roof sinks in places. We doubled the insulation from the inside with glass wool under the vapor barrier already present under the tiles. We were told that this operation would eventually accelerate the bending because of the absorption by the wood moisture that would be created between the two insulators. We are proposing to remove the vapor barrier and "tighten" the beams to block their deformation. Is it really useful and what exactly is this last operation.

What you describe here seems very disturbing to me. If the roof collapses in places, you must proceed to repair work as soon as possible. Take the advice of a carpenter to determine if your home is still habitable without risk.
The insulation that has been achieved is unrelated to the problem of deformation of the beams.
"Retendre" a beam means, in business jargon, straighten, but it requires heavy work to be done in the rules of art. It is often necessary to extract the beam from the masonry to return it if it is still in good condition because wiring or shoring will not be enough if the roof structure is collapsed.

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