Identify the source of damp first
Persistently damp joints can result from shower steam, cold surfaces, a leak or several factors together. Replacing sealant and repainting do not solve the moisture source. An inspection should distinguish inadequate airflow from plumbing or waterproofing faults.
Extraction needs replacement air
Air removed by the extractor must be replaced. Check the supply route and system performance with the bathroom door closed. Extraction should not discharge moist air into an enclosed ceiling void. Inspect existing shafts and ducts before deciding how to connect.
Select for the actual duct
Consider room size, duct length and bends, grille resistance and manufacturer requirements. Noise, cleaning access and practical controls also matter. A timer or humidity control can improve daily use when it is properly matched to the system.
Check the finished installation
Check actual airflow and performance after a shower, not just motor noise. Preserve access to the fan and duct. Clean grilles and blades periodically: dust can impair even correctly selected equipment.