Reasons for potentially keeping the existing rigid include cost of replacing existing ductwork, the decreased airflow/furnace efficiency and mainly health concerns (with a family already prone to severe allergies with dust being a major cause, I am concerned about the amount of dust that would collect in the dips and valleys of flex duct and their more fragile nature not being condusive to cleaning).
Are there insulated wraps that can be placed around rigid ductwork? Would they help some? Would something *like* a mass loaded vinyl be effective? (Although I anticipate much more costly than insulated flex duct, if it would help to alleviate the health issues than the cost would be worth it)