Flexible ducts are often used in tight spaces or with an unconventional design, but they can be more difficult to seal than sheet metal ducts. This can lead to air leaks and decreased efficiency. In addition, the material itself is less durable than sheet metal, and improper installation can cause further issues. The friction in a standard duct is usually consistent, but this is not the case with flexible ducts.
The speed of friction in flexible ducts depends on the design of the ducts, and sources of inefficiency due to friction can usually be seen with the naked eye. Factors such as the extension of the duct, the duct support system, and the curves of the duct network all affect friction. The key to optimal performance with flexible ducts is proper installation. If not installed correctly, they can end up folded, bundled, undersized or flabby.