Rigid Ducting: Rigid ducting, including our rigid twin wall ducts or PVC BT duct, is generally used for shorter distances or when added power is necessary. This ducting is most effective useful for straight runs. ​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​... https://thesocialdelight.com/story3660592/fire-rated-ductwork-fundamentals-explained