Sealed sheet metal wins for main trunks and anywhere pressure runs high, flexible duct earns its keep only on short final connections to diffusers, and ductboard suits quiet low-pressure branches where noise matters more than airtightness. Fabric duct and specialty metals like stainless or aluminum solve narrower problems: exposed architectural runs, corrosive air, or grease-laden exhaust. Pick by pressure class and exposure first, budget second.
TL;DR:
- Rigid sheet metal trunks, especially spiral and round ducts, offer the best long-term durability, energy efficiency, and ease of cleaning for main HVAC pathways.
- Flexible duct should only be used for short connections, ideally not exceeding six to eight feet, to minimize friction and static pressure issues.
- Ductboard provides quiet airflow but is vulnerable to moisture damage and mould, making it unsuitable for humid or unconditioned spaces.
- For corrosive or grease-heavy environments, stainless steel and aluminum outperform galvanized steel despite their higher upfront costs.
- Proper installation quality, sealing, and support are often more critical than material choice in ensuring HVAC duct system performance and longevity.
Table of Contents
- Understanding hvac ductwork types by shape, material and pressure class
- Sheet metal ducts: the performance standard for trunks and branches
- Flexible ductwork types: correct use versus common mistakes
- Ductboard and fibreglass board: quiet air movement with a catch
- Fabric and specialty metal ducts: niche solutions for specific problems
- Comparing duct types across the factors that actually matter
- How to choose the right duct type for your project
- Maintenance and knowing when to repair versus replace
- What fabrication reveals about duct decisions
- What actually decides a good duct job
- Custom duct fabrication built and installed in Greater Montreal
- Sources
- FAQ
Understanding hvac ductwork types by shape, material and pressure class
Every duct on the market sorts into three overlapping categories: shape, material, and pressure class. Shape covers round, rectangular, oval, and spiral profiles. Material determines corrosion resistance, weight, and fire performance. Pressure class, set by the fan and system design, dictates how much leakage and structural rigidity a duct run can tolerate before it fails or wastes energy.
The materials you will encounter across residential and commercial ductwork installations include:
- Galvanized steel, the default for sheet metal trunks and branches
- Aluminum, lighter and more corrosion resistant, common in coastal or humid sites
- Stainless steel, reserved for corrosive, high-moisture, or grease-exposure environments
- Fibreglass duct board, which insulates and conducts air in one panel
- Polymer-coated fabric, used for exposed distribution runs
- Flexible plastic or foil duct, for short connections only
Quebec building projects fall under the National Building Code, which points installers toward SMACNA and ASHRAE design manuals for sizing, leakage class, and installation practice. Provincial adoption follows the CNB with local amendments, so a contractor working in Laval or the West Island still answers to the same core material and fire-safety rules.
Sheet metal ducts: the performance standard for trunks and branches
Galvanized steel remains the workhorse for sheet metal ductwork, with aluminum and stainless reserved for environments where corrosion or hygiene demands justify the extra cost. Shape changes performance more than most homeowners expect.
- Round and spiral duct have fewer seams, which cuts leakage points and lowers friction against airflow
- Rectangular duct fits tight joist bays and dropped ceilings where round profiles will not clear
- Round duct typically runs 15 to 25 percent lower friction loss than rectangular duct carrying the same airflow
- Properly sealed sheet metal resists sagging, crushing, and pest intrusion far better than any flexible product
Statistic to note: Controlled comparative testing found sheet metal systems used roughly 5 to 17 percent less operating energy than comparable flexible-duct installations, with the gap widening as installation quality drops. A well-fabricated spiral run can last 25 years or more with only periodic seam inspection.
Flexible ductwork types: correct use versus common mistakes
Flex duct is a wire helix wrapped in plastic or foil, usually jacketed with fibreglass insulation, and it is built for one job: a short, easy connection between rigid trunk and diffuser. Problems start when installers use it as a substitute for rigid runs across a whole basement or attic.
- Keep runs short. Every extra foot of flex adds friction that a rigid duct would not.
- Avoid sag. A flex run left to droop between joists creates a low spot where air stalls and total external static pressure (TESP) climbs.
- Eliminate kinks. A sharp bend can choke airflow as badly as a partially closed damper.
- Support it properly. Straps every four feet, pulled taut, prevent compression that squeezes the internal diameter.
- Inspect annually. Foil jackets tear, insulation slumps, and connections at the collar loosen over a heating season.
Pro Tip: If a contractor proposes flex duct for a run longer than about six to eight feet, ask why. Field guidance from installers consistently limits flex to short connector runs, not primary distribution.
Ductboard and fibreglass board: quiet air movement with a catch
Fibreglass ductboard does two jobs at once: it forms the duct wall and insulates it, which is why installers like it for low-pressure branches where a quiet, budget-friendly option matters.
- Built-in insulation eliminates a separate wrap step, trimming labour cost
- The fibrous panel dampens airborne noise better than bare sheet metal
- Fabrication is fast for straight runs and simple transitions
Statistic to note: Industry guidance on fibreglass ductboard flags a real trade-off: the exposed fibre surface is prone to erosion over time, and moisture intrusion can trigger mould growth that is difficult to clean or remediate once established. That makes ductboard a poor fit for humid basements, laundry rooms, or any space with a history of moisture problems.
Fabric and specialty metal ducts: niche solutions for specific problems
Fabric (textile) duct hangs in exposed spaces like gymnasiums or warehouses, where its porous or perforated surface distributes air evenly across a long run without visible diffusers. It is washable, lightweight, and often chosen for architectural visibility rather than raw efficiency.
- Fabric duct suits exposed distribution where even air spread matters more than airtight sealing
- Aluminum earns its place in humid or coastal buildings where galvanized steel corrodes faster
- Stainless steel is the standard for pool enclosures, commercial kitchens with grease-laden exhaust, and other corrosive environments
Fabrication and material upcharges for stainless or aluminum run higher than galvanized steel, but in the wrong environment, galvanized ductwork simply fails sooner.
Comparing duct types across the factors that actually matter
Here’s how the main options stack up on the dimensions that decide a real project, not a spec sheet:
| Duct type | Best for | Durability | Airflow performance | IAQ / cleanability | Moisture resistance | Relative cost | Maintenance |
|---|---|---|---|---|---|---|---|
| Round/spiral sheet metal | Trunks, long exposed runs | High, 25+ years | Lowest leakage, lowest friction | Smooth surface, easy to clean | Excellent | Moderate to high | Low |
| Rectangular sheet metal | Tight joist bays, ceiling cavities | High | More seams, higher friction than round | Good, flat surfaces | Excellent | Moderate | Low |
| Flexible duct | Short diffuser connections | Low to moderate | Degrades fast with sag or kinks | Fair, hard to inspect internally | Moderate | Low | Moderate to high |
| Fibreglass ductboard | Quiet low-pressure branches | Moderate | Good when dry and sealed | Poor if damp, fibre erosion risk | Poor | Low to moderate | High if damp |
| Fabric/textile duct | Exposed architectural runs | Moderate | Good even air distribution | Washable | Moderate | Moderate to high | Moderate |
| Stainless/aluminum | Corrosive, grease, or pool environments | Very high | Comparable to galvanized | Excellent | Excellent | High | Low |
Statistic to note: The Airflow Research and National Comfort Institute testing that produced the 5 to 17 percent energy figure also found installation quality mattered almost as much as material choice: a carefully installed flex run can approach metal performance, while a sloppy one falls well short.
The practical rule that comes out of this table: metal trunks for the backbone, short flex tails for the last connection, and ductboard only where noise control outweighs the moisture risk.
How to choose the right duct type for your project
Work through this checklist before signing off on any ductwork proposal, whether it’s a home renovation or a commercial retrofit.
- Confirm the pressure class. Low-pressure residential systems tolerate more duct types than medium or high-pressure commercial systems, which generally demand sealed sheet metal.
- Measure exposure. Attics, crawl spaces, and unconditioned basements punish flex and ductboard harder than conditioned interior runs.
- Weigh indoor air quality priorities. Anyone with asthma or allergies in the household should lean toward smooth, cleanable metal over fibrous ductboard.
- Set a real budget. Sheet metal costs more upfront but saves on energy and lasts longer; ductboard costs less but carries higher long-term maintenance risk.
- Check code requirements. Confirm your contractor is working from current CAN/ULC-S110 test standards and local CNB provisions.
Ask a fabricator these exact questions: What material and gauge are you specifying? What leakage class are you targeting, and will you test for it? How many access panels will the run include for future cleaning? What’s the warranty on seams and joints?
Pro Tip: A proposal with no mention of duct sealing or leakage testing is a red flag, no matter how competitive the price looks. Long, unsupported flex runs standing in for rigid trunk are another.
Maintenance and knowing when to repair versus replace
A seasonal walk-through catches most problems before they become expensive. Look for disconnected joints, torn flex jackets, sagging sections, and any staining on ductboard that suggests moisture has gotten in.
- Reseal loose joints with mastic rather than cloth tape, which fails within a few years
- Shorten or replace any flex run that has sagged below its support line
- Replace ductboard sections showing mould staining or fibre shedding rather than attempting to clean them
- Inspect stainless or aluminum runs in corrosive environments annually for pitting
Full replacement usually beats patchwork repair once a system is leaking at multiple joints, when ductboard shows mould contamination, or when a renovation changes the layout enough that the old duct sizing no longer matches the new load. At that point, patching one section only delays the next failure.
What fabrication reveals about duct decisions
Shop experience with plenum design shows sizing and layout decisions get made on paper, then get tested against real joist bays, service access, and support spacing on site. Material choice often comes down to exposure: galvanized steel versus stainless is rarely a coin flip once moisture or chlorides enter the picture. Local fabrication and installation matter because tolerances that look fine on a drawing sometimes need a field adjustment the day of the install.
What actually decides a good duct job
Homeowners overthink material and underthink installation quality. A mediocre material sealed and supported correctly beats a premium material installed carelessly, every time. For most houses, that means sheet metal trunks, short flex tails, and ductboard used sparingly. For commercial or industrial spaces, lean harder toward metal and specialty alloys. Bring the checklist above to your next contractor conversation, and don’t hesitate to ask pointed questions. Questions from readers comparing options are always welcome.
— Ash
Custom duct fabrication built and installed in Greater Montreal
This local alternative offers in-shop fabrication and installation, ensuring sizing, seams, and support spacing get resolved on-site instead of guessed at from a catalogue drawing.
That matters most on the jobs where duct types compared above actually diverge: a spiral trunk that needs to clear an existing joist bay, a stainless run for a commercial kitchen exhaust, or a rectangular section built to fit a dropped ceiling with no room to spare. Manara Corp. handles custom HVAC duct manufacturing alongside sheet bending and rolling, profile rolling, and welding in steel, stainless, and aluminum, serving contractors, architects, and homeowners across the island, Laval, the South Shore, and the West Island. No job is too small or too large, and every price is quoted after a site review of your actual space, load, and access constraints. Contact Manara Corp. to get a fabrication quote started.
Sources
- OBC 9.33.6.2 – Materials in Air Duct | CodeIndex
- Airflow Research flex duct vs sheet metal HVAC duct efficiency study
- Ductwork types field guide: sheet metal, flex, and ductboard – Anvilfield
FAQ
What are the three main types of ductwork?
The three most common types are sheet metal (round, spiral, and rectangular), flexible duct, and fibreglass ductboard. Fabric duct and specialty metals like stainless steel round out the options for niche applications like exposed architectural runs or corrosive environments.
What is the two-foot rule for ductwork?
It refers to a common installation guideline limiting how far a flexible duct can sag or deviate from a straight line before airflow suffers noticeably. It is a field rule of thumb rather than a fixed code figure, and it exists because sag and kinks raise total external static pressure fast.
What is the best type of duct for an air conditioning system?
Sealed round or spiral sheet metal performs best for main trunks in most air conditioning systems, since it has the lowest leakage and friction of any common option. Flexible duct is appropriate only for the short final connection to a diffuser, not as the primary distribution line.
What are the different types of ductwork used in HVAC systems?
HVAC systems use round, spiral, and rectangular sheet metal for trunks and branches; flexible duct for short connector runs; fibreglass ductboard for quiet low-pressure branches; and fabric or specialty metal duct for exposed or corrosive-environment applications. The right choice depends on pressure class, exposure, and budget, and Manara Corp. fabricates most of these types for local installation across Greater Montreal.
