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Install flex duct fully extended, supported at 4-foot intervals, with no kinks and every joint sealed using UL 181B-listed tape or mastic.
Flex-duct failures rarely announce themselves. A crushed liner, a sagging belly, or a joint sealed with ordinary tape quietly chokes airflow and wastes energy — and all three are installation errors you can avoid. Learning how to install flex duct properly comes down to four rules: run it fully extended, support it every 4 feet, keep bends gentle, and seal connections with UL 181B-listed materials.
Installing Flex Duct Properly Starts With Full Extension
Compressed, coiled, or looped sections add friction loss that starves the registers at the end of the run — and hidden slack is the most common flex-duct mistake. Don’t tension the liner like a guitar string; simply pull out the slack so the jacket lies smooth and taut.
Flex-duct diameter gets decided before any liner is pulled. If your run calls for 10-inch line, comparing the best 10-inch flex duct up front saves a second trip to the supply house.
What Support Spacing Keeps Flex Duct From Sagging?
The answer is 4 feet or closer.
- When the run must cross pipes or joists, shield the duct — sharp metal edges slice the liner and start kinks.
How Do You Connect And Bend Flex Duct Without Leaks?
An airtight flex-duct joint is layered: inner liner first, outer jacket second, each sealed independently.
Two more cautions.
| Installation Task | Required Practice |
|---|---|
| Duct tension | Fully extended; no coils, loops, or bunched sections |
| Support spacing | 4 feet or closer between supports |
| Maximum sag | 1/2 inch per foot of unsupported run |
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