Flex Duct — flexible HVAC duct for branch air runs

A flex duct is a flexible insulated air duct that carries conditioned air through HVAC systems where rigid duct is hard to route.

Flex Duct diagram — labeled parts, dimensions, and installation context

What It Is

Flex duct is a bendable HVAC duct made from a wire helix covered by an inner liner and insulation jacket. It is convenient for branch runs, but performance drops quickly if it is kinked, crushed, or too long.

Types

Most residential flex duct is insulated and sized by inner diameter. Variations mainly involve insulation thickness, jacket quality, and pressure rating.

Where It Is Used

Flex duct is commonly used in attics, crawlspaces, ceiling cavities, and short branch runs between a trunk line and a supply register. It is less ideal for long runs with high friction loss.

How to Identify One

It looks like a round silver or gray insulated tube that can curve between supports. Sharp bends, sagging spans, or crushed sections are signs of a poor installation.

Replacement

Replace flex duct when the liner tears, the insulation gets wet, the run is badly crushed, or the layout causes chronic airflow problems. A new run should be pulled tight and supported correctly.

Flex Duct — FAQ

What does a flex duct do?

It carries conditioned air through HVAC systems where rigid duct is hard to route. In a home, that helps the surrounding system work the way it should.

How can I tell if a flex duct needs attention?

Hot or cold rooms, weak airflow, torn jackets, and visible kinks are common warning signs. Condensation and wet insulation can also mean the vapor barrier has failed.

Can I replace a flex duct myself?

Short accessible replacements are possible for a careful DIYer who can seal and support the duct properly. Larger systems are harder because sizing and balancing matter.

How much does flex duct repair or replacement cost?

Flex duct material is relatively affordable. Labor rises when attic access is poor or multiple runs need resizing or rerouting.