Flex Duct vs Rigid Metal Duct
Flex is fast and cheap; rigid metal moves air with less resistance. The real difference on a job is friction rate and install quality. Here is how each type affects pressure loss, cost, and the CFM you actually deliver.
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In This Guide
The Two Duct Types
Flexible duct is a wire-helix inner liner wrapped in fiberglass insulation and a reinforced vapor-barrier jacket. It is UL 181 listed (Class 1), sold in R-4.2, R-6, and R-8 insulation values, and it ships in compressed boxes that a single tech can run in minutes. That convenience is exactly why it is on most residential jobs today.
Rigid metal duct is galvanized sheet metal, either rectangular trunk or round/spiral pipe. The interior is smooth, so it moves air with much less resistance and holds its shape for decades. The trade-off is labor: it has to be fabricated, hung, and sealed joint by joint.
The core idea: a duct only delivers the airflow its installedfriction allows. Rigid metal starts with a big head start because its interior is smooth. Flex can match it on paper, but only when it is pulled tight and supported correctly — two things that fail constantly in the field.
Friction Rate and Pressure Loss
Duct design is built around friction rate — the pressure the system loses per 100 feet of duct. In ACCA Manual D you back it out of the blower's available static pressure:
Friction Rate
- FR = friction rate (iwc per 100 ft)
- ASP = available static pressure (iwc)
- TEL = total equivalent length of the longest run (ft)
The catch is that a friction chart for smooth metal does not apply to flex. Fully extended flex duct carries roughly 2 to 3 times the friction of galvanized pipe at the same diameter and airflow, because the corrugated inner wall trips the airstream. Most manufacturers publish a separate flex chart or an effective-diameter correction. If you size flex off a metal chart, the run will be undersized and starved.
Field rule: keep residential total external static pressure at or below the equipment rating — commonly 0.50 iwc for a PSC-blower system. High static almost always traces back to restrictive flex: too long, too many turns, compressed, or undersized.
The Compression Problem
A flex chart assumes the inner core is pulled fully taut. Reality is messier. When flex sags between supports or gets left long and relaxed, the helix bunches and the effective diameter shrinks. The numbers are brutal:
- Just 4% longitudinal compression can cut capacity by roughly a quarter.
- 15% compression — a common amount of sag — can more than double the pressure loss of that section.
- A single sharp kink or a hard turn against a joist behaves like a partially closed damper.
Why callbacks happen
A system that passed at startup but "can't keep up" two seasons later is frequently a flex run that relaxed, sagged off a strap, or got crushed by stored attic junk. The equipment is fine; the installed friction rate quietly doubled.
Worked Example: Friction Rate + Sizing
Problem
A 3-ton system needs ~1,200 CFM (about 400 CFM per ton). The blower is rated for 0.50 iwc. After deducting coil, filter, and grille losses you have 0.20 iwc of available static pressure. The longest supply run has a total equivalent length of 200 ft. Size a 150 CFM branch.
Step 1: Find available static pressure
ASP = 0.50 − (0.15 coil + 0.10 filter + 0.05 grilles) = 0.20 iwc
Step 2: Calculate friction rate
FR = (0.20 / 200) × 100 = 0.10 iwc/100 ft
Step 3: Size the branch at 0.10 friction, 150 CFM
From a smooth-metal friction chart, 150 CFM needs about a 6" roundmetal duct. On a flex chart at the same friction rate, that same 150 CFM needs a 7" flex duct — one size up to cover the extra friction.
Step 4: Sanity-check velocity
V = (CFM × 144) / Area. A 7" duct = 38.5 in² → V ≈ (150 × 144)/38.5 ≈ 560 FPM
Comfortably under the ~600 FPM branch target — quiet and low-loss.
Takeaway
Same job, same friction rate: flex almost always lands one nominal size larger than metal. Upsize the flex, or the branch will be starved even when the paperwork looks fine.
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Side-by-Side Comparison
Neither type wins on every metric. Here is how they stack up on the factors that actually matter in the field:
| Factor | Flex Duct | Rigid Metal |
|---|---|---|
| Friction / pressure loss | ~2–3x higher | Lowest (smooth bore) |
| Material cost | Low | Higher |
| Labor / install time | Fast | Slow, skilled |
| Insulation | Built-in (R-4.2 / R-6 / R-8) | Must be wrapped separately |
| Sensitivity to install quality | Very high | Low |
| Durability / lifespan | Tears, sags, crushes | Decades |
| Cleanability | Poor (ribbed) | Good |
| Best role | Short branch runouts | Trunks & long runs |
The pattern most quality shops land on: rigid metal trunks with short flex connections to the boots. You get the low-loss backbone of metal and the fast, vibration-isolating final connection of flex.
Installing Flex So It Performs
If flex is going in, the difference between a starved system and a rated one is entirely workmanship. Follow this sequence:
1. Cut to length. Route first, then cut. Never store slack in a joist bay — extra length is extra friction.
2. Pull the core fully taut. Stretch the inner liner tight before you fasten anything. This is the single biggest lever on delivered CFM.
3. Use long-radius turns. Keep the centerline radius at least one duct diameter. No kinks, no hard bends against framing.
4. Support every 4–5 ft. Use 1.5"+ saddle straps — never wire or zip ties that pinch the core. Limit sag to a half inch per foot of support spacing.
5. Seal in two stages. Band the inner core to the metal collar with a nylon draw band, add mastic or UL 181 tape, then pull the outer vapor jacket over and band it separately. The jacket must not be the airtight seal.
6. Verify. Measure total external static pressure against the equipment rating and confirm CFM at the registers before you leave.
Don't skip the leakage check. A perfectly sized run still fails if the joints leak. Seal every collar with mastic and pressure-test conditioned-space and attic ductwork — leakage undoes the CFM you just calculated.
When to Use Which
Reach for rigid metal
- Supply and return trunks
- Long runs where friction budget is tight
- Exposed duct (garages, shops, commercial)
- High-velocity or high-static systems
- Anywhere durability and cleanability matter
Flex makes sense for
- Short branch runouts to boots (keep under ~6–8 ft)
- Final connection to registers (isolates vibration/noise)
- Tight retrofits where rigid won't fit
- Cost- and speed-sensitive residential work
Whichever you run, size it off the correct chart for that material, keep velocities in range (roughly 700–900 FPM in residential trunks, ~600 FPM in branches), and hold total external static pressure to the equipment rating. Get those three right and the system delivers what Manual J asked for.
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