Flex duct
Sag allowed per span
Straps ≤ 4 ft apart, ≥ 1.5 in wide
Flex duct vs rigid duct
Flex duct vs rigid duct is not a contest of materials so much as a contest of installations. Sheet metal holds its bore wherever you hang it; flex holds its bore only when it is stretched, strapped and routed the way the standard says. Get flex right and it is a fine branch duct. Get it wrong and it eats the static pressure the whole design was built on.
Updated 2026-10-18
Our pick — rigid for trunks and long runs; flex for short, straight, strapped branch runouts and final connections
Rigid duct
Sag allowed per span
Holds its bore between hangers
How to read those two dials
There is no single published friction figure that compares the two fairly, because flex friction depends on how it was installed. So the dials show the thing that does differ by material: how far the duct may droop between supports. The flexible-duct installation standard allows no more than 0.5 inch of sag per foot between supports spaced no more than 4 feet apart. Multiply them and a correctly hung flex run may still dip 2 inches mid-span. A sheet-metal run has no sag allowance at all. It keeps its section between hangers, and a dip in it is damage, not tolerance.
Sag matters because the inner liner is the airflow path. DOE's field guide names flex that is not properly supported as a typical cause of excessive static pressure.
Sources: DOE Building America Solution Center, Support Intervals for Flex Ducts (citing the ADC Flexible Duct Performance and Installation Standard, 5th ed., §4.6) (read 2026-10-18)
Friction: why flex costs pressure
Rigid duct has a smooth wall and fittings with fixed shapes, so its losses are what the design says they are. Flex has a corrugated inner liner over a wire helix. Pulled taut it behaves predictably; pushed together, the corrugations bunch up and the resistance climbs. That is why the guidance is that flex “should be stretched taut when installed correctly”.
Bends are the other half. A proper-radius 90° bend in flex adds 15 feet of equivalent length at 700 feet per minute, and a sharp one, tighter than one duct diameter, costs far more. The friction-rate method needs that equivalent length to be right, which is why installation quality is a sizing input. Run the numbers in the duct size calculator, and see the duct sizing guide for the allowance to make when a run is flex.
Sources: DOE Building America Solution Center, No Kinks or Sharp Bends in Flex Duct Installation (read 2026-10-18)
The comparison
| Factor | Flex duct | Rigid duct |
|---|---|---|
| Inner surface | Corrugated liner on a wire helix | Smooth sheet metal |
| Friction | Depends on stretch, sag and bends | Set by the design and fittings |
| Bends | Proper radius: 15 eq. ft per 90° | Fabricated elbows of known shape |
| Support | Straps ≤ 4 ft, ≥ 1.5 in wide | Hangers per the adopted mechanical code |
| Sag | ≤ 0.5 in per ft between supports | None allowed |
| Insulation | Usually factory-jacketed | Wrapped on site in unconditioned space |
| Labor | Fast to route around obstacles | Measured, cut and fitted |
| Common defects | Kinks, compression, excess coiling | Unsealed seams and joints |
| Best place | Short branch runouts, final connections | Trunks, long runs, returns |
Installation rules for flex
- Supports at the manufacturer's interval, never more than 4 feet apart.
- No more than 0.5 inch of sag per foot between supports.
- Straps or saddles at least 1.5 inches wide, so the support does not pinch the liner.
- A support at every connection to rigid metal duct, and one between a metal connection and a bend.
- Stretched taut; trimmed to length rather than coiled.
- A metal elbow at the boot where the turn would otherwise kink the flex.
- No kinks, no bends tighter than one duct diameter, no compression into a cavity smaller than the duct.
Duct construction and installation sit in IRC Chapter 16 of the International Residential Code, alongside the manufacturer's installation instructions. Your jurisdiction's adopted edition governs; check it before a rough-in inspection.
Sources: DOE Building America Solution Center, Support Intervals for Flex Ducts (citing the ADC Flexible Duct Performance and Installation Standard, 5th ed., §4.6) (read 2026-10-18) · DOE Building America Solution Center, No Kinks or Sharp Bends in Flex Duct Installation (read 2026-10-18) · EPA ENERGY STAR, National Rater Field Checklist v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 6.1 and Footnote 24 (read 2026-09-04) · ICC, 2024 International Residential Code, Chapter 16 Duct Systems (read 2026-10-18)
Where each one belongs
The common good design uses both. The trunk is rigid, sheet metal or duct board, sized per ACCA Manual D, because it carries the most air and the longest distance. Branch runouts can be rigid round or short, straight flex. The last few feet to a boot are where flex earns its place, absorbing misalignment and vibration. Returns deserve rigid duct or generous flex, because the return side is where static pressure most often goes wrong.
Sources: EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 5.1 (read 2026-09-04)
A long flex run is a design decision, not a shortcut
If flex has to run long, size it up and hang it to the standard. Then measure: the static pressure calculator will show whether the installed system is inside the blower's rating. Weak airflow at the end of a sagging run looks exactly like the complaints in the AC not cooling guide.
Leakage is about joints, not material
Both kinds leak at the same places: seams, take-offs, collars and boots. The federal new-homes checklist tests the whole system the same way regardless of what it is made of, and its total-leakage limit is area-based with a floor of 40 CFM25 at rough-in and 80 CFM25 at final, per system. Seal the flex liner to every collar, seal metal at every seam, and seal every boot to the drywall or subfloor.
Sources: EPA ENERGY STAR, National Rater Field Checklist (Rev. 14), Item 6.4 (read 2026-10-18)
Related
Duct size calculator
Sizing & LoadRound and rectangular duct sizes from CFM and friction rate, with equivalent diameter.
CFM · in. w.c./100 ft
How to size ducts
Sizing & LoadCFM per ton, friction rate, and the duct calculator method step by step.
10 min
Static pressure explained
Sizing & LoadWhat total external static pressure is, how to read it, and what a high number costs.
9 min
BTU calculator for heating and cooling
Sizing & LoadRoom-by-room heating and cooling load in BTU/h from area, climate zone and envelope.
BTU/h · Manual J
More head-to-heads in the HVAC equipment comparisons index.