Flexible ducts sagging between supports in an attic reduce performance because:

Flexible duct is rated on the assumption that it is pulled taut. When it sags between widely spaced supports, two things happen to the airflow. The path lengthens — a duct dipping between hangers travels farther than the straight-line distance, and every foot of flex duct costs pressure. And the inner liner corrugates and folds at each sag, so the air is repeatedly forced to change direction, generating turbulence that costs far more pressure than a smooth run of the same length. The result is measured as effective length: a sagging run can behave like several times its actual length, which is why a register at the end of it barely delivers. Correct installation is duct pulled tight and supported at close intervals with wide straps that do not pinch it.

Sags lengthen the path and corrugate the inner liner, generating turbulence at every dip. Together these raise the run's effective length dramatically and cut the airflow reaching the register.

Flexible duct must be stretched taut and supported at manufacturer-specified intervals with straps of adequate width. Compression, sagging and excess length substantially increase friction loss, reducing airflow to the served register.

Slack Costs Feet. Every dip adds effective length, and every corrugation adds turbulence. Pull it tight or the register goes quiet.

Check support spacing and strap width. Narrow straps that pinch the duct cause the same restriction as a sag and are just as commonly found.

• •Attributing sag-related losses to seam leakage
• •Overlooking narrow straps that pinch the duct
• •Assuming excess duct length is harmless if it is insulated