What flex duct means
Flex duct is round flexible duct made of a wire coil inside plastic, wrapped in insulation and a vapor jacket. It is common for branch runs in attics.
Also called flexible duct.
Where it matters on a HVAC job
Flex duct works when it is pulled tight and supported; kinked, sagging or crushed runs choke airflow. On an attic inspection, photos of bad runs make the case for repair better than any explanation.
What the model code says
- Tapes and mastics used on flexible ducts must comply with UL 181B and be marked for it. (IRC 2021, Chapter 16)
Check which code edition your building department enforces for flex duct before you quote.
What it costs
For flex duct, the closest koira.ai price is the ductwork replacement cost guide, near $1,250 for a typical job and $450 to $2,200 for most. The guide shows its sources and what moves the price.
Work it out
- Cost guide: Ductwork replacement cost
- Licensing: HVAC license rules by state
- Tools, templates and guides: Free HVAC resources
Sources: 2021 International Residential Code, Chapter 16, Duct Systems (ICC). Checked October 9, 2026.