Water damage ceiling structure in timber frame construction

Hi,

Does the small hole for the blowing opening in the chipboard actually have to be sealed (if yes, why?) or can it be left open? It would no longer be visible if the drywall is closed.

The hole must be sealed. Even if it disappears behind the drywall, it remains an open connection between two protected layers in the ceiling structure. Air exchange through the not tightly sealed chipboard can promote condensation, odors, and ultimately mold growth. Especially without a vapor barrier, any further leak is far from advisable. Visibility doesn’t matter, the function of the layer is the decisive criterion.

The leak detection service could most easily look into the ceiling from below through the hole with a camera or probe until they see the pipes. Punching a 1.2 cm hole into the insulation wool should also be uncritical???

That makes technical sense if it is clear beforehand that the insulation wool does not replace a load-bearing or airtight layer. A small hole for the purpose of leak detection is acceptable as long as it is properly sealed again afterwards. When drilling and searching, one should not forget to restore the tightness of the layers afterwards as well.

Could you cut open the underlay membrane and glue it back and saw out a piece of batten and reattach it after the repair or is that not permitted?

Technically, that is possible; craftsmen work similarly in such cases. But in the end, the underlay membrane is part of the weather protection and must be restored really airtight afterwards.

Good luck!