Which of the following is NOT an effective tool for air sealing a home's ductwork?

Duct tape is not an effective tool for air sealing a home's ductwork primarily because it is not designed for the long-term demands of duct sealing. While duct tape may initially adhere well, it can degrade over time, particularly when exposed to temperature changes, humidity, and airflow. The adhesive can lose its stickiness, leading to potential leaks in the ductwork.

In contrast, sealant mastic is a thick, paste-like product that can be easily applied to duct seams and connections, providing a durable, long-lasting seal. It remains flexible and can withstand the changes in environment that duct systems encounter. Foam insulation, while often used for insulating, can also be employed to seal gaps around ductwork, providing both insulation and air sealing benefits by conforming to irregular shapes. Metal tape is highly effective as well; it is specifically rated for sealing ductwork, providing a strong, durable, and long-lasting seal that adheres well under different conditions.

Thus, duct tape is not suitable for air sealing ductwork because it fails to meet the necessary performance criteria required for effective sealing, especially over time.