Technical Foundation
What Is a Flexible Duct?
Flexible duct construction is built around a wire helix core wrapped in a polymer or metalized film, with the insulated versions adding a fiberglass insulation layer and outer vapor barrier jacket. Insulated flex duct is rated by R-value (commonly R-4.2, R-6, or R-8) and is specified anywhere the duct runs through unconditioned space — an attic, crawlspace, or exterior wall cavity — where a bare duct would lose or gain heat and cause condensation on the duct surface. Non-insulated flex duct is used for short runs entirely within conditioned space, or for dryer venting and other non-HVAC air movement where insulation isn't relevant.
Diameter is sized to the required airflow (CFM) at an acceptable velocity and pressure drop — an undersized duct increases static pressure and reduces system efficiency, while an oversized duct wastes material and space without improving performance. Flex duct also carries a maximum recommended length and bend radius before pressure losses become excessive; installers should avoid excessive sag, kinks, or tight bends that restrict airflow well below what the diameter alone would suggest. Dryer venting specifically should use rigid or semi-rigid metal duct where local code requires it, since standard vinyl flex duct is a fire code violation in many jurisdictions for dryer exhaust.
Prefer a quick quote?
Get Flexible Ducts Pricing in Your Inbox
Skip the search — tell us the duct diameter, insulation R-value and application and we will come back with pricing.
| Spec | What It Means | Why It Matters |
|---|---|---|
| Diameter | 3" to 20", commonly 6"-12" for HVAC | Sized to required CFM airflow at acceptable velocity and pressure drop |
| Insulation R-Value | R-4.2, R-6, R-8 | Required in unconditioned spaces to prevent heat loss/gain and condensation |
| Core Construction | Wire helix + polymer/metalized film | Determines flexibility, crush resistance and maximum operating temperature |
| Application | HVAC supply/return vs. dryer venting | Dryer venting often requires rigid/semi-rigid metal duct by code, not vinyl flex |
| Max Length/Bend Radius | Per manufacturer spec, avoid sag/kinks | Exceeding recommended length or bend radius causes excessive pressure loss |
Shop Flexible Ducts
Browse Flexible Ducts Sub-Categories
Organised by insulation type and material. Pick a sub-category, or describe the diameter and airflow requirement and let ChatMRO match the duct.
How it works
From Half-Remembered Part Number to PO, in Three Steps.
Ask
Type an MPN, paste a spec sheet, or describe the part in plain English. ChatMRO understands engineer-speak.
Compare
Side-by-side specs, cross-references, and equivalents from multiple sources — without the clutter of endless browser tabs.
Quote & Buy
Request a quote or add to cart. Volume discounts, VAT-aware pricing, and consolidated procurement — all in one place.
Applications
Where Flexible Ducts Get Used
Flexible ducts are specified according to whether the run passes through conditioned or unconditioned space, required airflow, and the specific application.
Residential HVAC Installation
Insulated flex duct connecting trunk lines to supply registers through attic and crawlspace runs.
Commercial HVAC & Retrofit
Flexible ducting used for final connections between rigid ductwork and diffusers in commercial buildings.
Clothes Dryer Venting
Semi-rigid metal or code-compliant flex duct venting dryer exhaust to the exterior.
Spot Cooling & Ventilation
Non-insulated flex duct directing temporary or portable cooling and exhaust airflow in industrial settings.
Fume & Dust Collection
Flexible metal ducting connecting dust collection equipment to machine tools and workstations.
Modular & Temporary Structures
Flexible ducting used in modular buildings and temporary structures where rigid ductwork isn't practical.
FAQ
Flexible Ducts Questions, Answered
When do I need insulated vs. non-insulated flex duct?
Insulated flex duct is required anywhere the duct runs through unconditioned space — attics, crawlspaces, exterior walls — to prevent heat loss or gain and to stop condensation from forming on the duct surface. Non-insulated duct is fine for short runs entirely within conditioned space or for non-HVAC air movement like spot ventilation.
What R-value insulated duct do I need?
R-value requirements are typically set by local energy code and climate zone — R-6 and R-8 are common minimums in colder climates, while R-4.2 may be acceptable in milder regions. Check your local building code's mechanical/energy provisions rather than assuming one R-value fits all installations.
Can I use standard flex duct for dryer venting?
Many jurisdictions prohibit standard vinyl or fabric flex duct for dryer exhaust due to fire code concerns — lint accumulation in a flexible, ridged duct interior is a documented fire hazard. Semi-rigid or rigid metal duct is required in most codes for dryer venting; check local code before installing flex duct on a dryer exhaust run.
How do I size the diameter for my airflow requirement?
Duct diameter should be sized using the required CFM and a target air velocity/pressure drop from ACCA Manual D or the HVAC system design — undersizing increases static pressure and reduces system efficiency, while oversizing wastes material without improving performance. This is a design calculation, not a rule of thumb based on the trunk line size alone.
Why is my flex duct airflow lower than expected even though it's the right diameter?
Excessive length, tight bends, sag between supports, or kinks all add pressure loss beyond what diameter alone accounts for. Flex duct should be pulled reasonably taut, supported at proper intervals, and kept within the manufacturer's maximum recommended length and bend radius to avoid airflow losses well below the duct's rated capacity.
Keep browsing
Related Categories
Explore More
Find Your Next Part Now.
Describe a part number, spec or what it does.