There are three main types of ductwork:
Flexible – Round tubes with steel wire reinforcement and covered with plastic or aluminum. Lightweight and easy to install but shortest lifespan. Prone to tears, pest infestations, and moisture damage.
Fiberglass / fiberboard – Insulated by fiberglass to prevent condensation and muffle the HVAC system sound. Fiberglass can deteriorate and release fibers into the air over time, causing health issues.
Sheet metal – Rigid in shape, made from galvanized steel or aluminum, lightweight, easy to install, and mold-resistant.