We just purchased an 100 plus year old house with forced air heat and a
1970's era furnace to use as a rental property.

My brother in law insists that the existing non-insulated ductwork
cannot be used with Air Conditioning as it will cause moisture
problems. We already have tenants scheduled to move in...they have
allergies and are very upset about this new revelation.

My HVAC guy disagrees and claims that the existing duct is fine.

Which one is correct and what should I do?

Thanks

Dean in Cincy