Flexible ducting is one of the most widely used ventilation products on the market — and one of the most misunderstood. It's cheap, it's fast to install, and it bends around almost anything. But those upsides come with trade-offs that only tend to show up after the job is signed off, when airflow drops, joints start leaking, or a fire inspector asks for paperwork the product doesn't have. This guide breaks down where flexible ducting genuinely earns its place, and where it falls short of what semi-rigid ducting can offer instead.
Flexible ducting is a corrugated duct, typically made from PVC, foil, or polyester, designed to bend easily by hand into almost any shape. It's been the default choice for decades in domestic ventilation, extract fans, and short HVAC connections, largely because it's inexpensive and requires no special tools to fit.
Low upfront cost: Flexible duct is generally the cheapest ducting option on the market, which is why it remains popular on tight-budget domestic jobs.
Easy to install in confined spaces: It bends by hand into almost any shape, which can make it quicker to route through awkward voids on a like-for-like basis.
Lightweight: Flexible duct is easy to handle and transport, with no need for lifting equipment even on longer lengths.
That might look like a short list, and that's because the qualities that make flexible duct attractive at the point of purchase are largely the same qualities that cause problems once it's installed and in use.
Yes, it's cheap, it's lightweight, and it's quick to install. But those qualities together inherently result in a product that doesn't perform as well or last as long as alternatives such as semi-rigid or rigid ducting.
Airflow loss from corrugation: Every ridge in a corrugated duct wall creates turbulence, and turbulence means pressure loss. The fan has to work harder to move the same volume of air, and on longer runs this can mean noticeably reduced airflow at the terminal — even when the fan itself is correctly specified.
Sagging and collapse over time: Unsupported flexible duct sags, especially on longer horizontal runs, creating low points where condensation can pool. In worse cases, it can partially collapse and choke airflow altogether.
Easily punctured and crushed: The thin outer wall is simple to snag, crush, or puncture — during installation, during other trades' work on site, or just over years sitting in a loft or ceiling void. Once punctured, airtightness is gone for good, and the duct is rarely inspected again until something goes wrong.
Inconsistent, often poor fire performance: Fire rating varies significantly across the flexible duct market, and a lot of budget products only achieve a low Euroclass rating (Class E) under BS EN 13501-1:2019 — a real compliance risk under the Regulatory Reform (Fire Safety) Order 2005 and Building Regulations Approved Document B, particularly on commercial specifications.
Leaky joints: Corrugated ends can be tricky to seal cleanly compared to a smooth duct wall, and every loose joint is conditioned air leaking straight out of the system before it reaches the room.
No insulation as standard: Flexible duct typically needs insulation lagged on separately after installation, an easy step to rush or skip, which stores up condensation and thermal leakage problems for later.
Shorter practical lifespan: Between physical damage, sagging, and joint failure, flexible duct tends to need replacing far sooner than a more robust duct system — an ongoing cost that rarely gets factored into the "cheap and easy" pitch at the point of sale.
Flexible duct isn't obsolete. On very short, low-consequence runs, where budget is the dominant factor and the stakes of underperformance are low, it can still do the job.
The problem is that it's routinely specified well beyond that use case, on runs and applications where its weaknesses genuinely matter. Even on short runs, we see countless systems using flexible ducting entirely wrong as a contractor weaves and twists the ductwork around the building.
Flexible duct can still be the right choice — but only when every condition below genuinely applies. If you can't tick most of these boxes, it's might be worth looking into semi-rigid ducting instead:
The run is short (<1.5m total length according to Building Regulations) with no more than one or two bends
The duct will only be used as a final connector, such as joining semi-rigid ducting to a fan unit
The duct will remain fully accessible for inspection and replacement throughout its service life
The product has a tested Euroclass fire rating to BS EN 13501-1:2019, not just a generic "fire-resistant" claim
The installation meets Building Regulations Approved Document B and the Regulatory Reform (Fire Safety) Order 2005 for the specific building type and use (e.g., flexible ducting cannot pass through fire-resisting walls)
The duct will be correctly and fully supported along its length, not left to rest on ceiling voids or other services
Insulation will be appropriately and professionally lagged on-site to the thickness required for the application, with no gaps or compression
Airtightness at every joint has been tested or can be verified to EN 12237
The system's design airflow has enough headroom to absorb the pressure loss from corrugation
Nobody involved is planning to reuse or extend the ductwork later, since flexible duct rarely survives a second fix
For anything where airflow performance, fire safety, longevity, or indoor air quality actually count — MVHR systems, VRF/FCU connections, commercial fit-outs, or any run longer than a couple of metres — semi-rigid ducting addresses nearly every weakness flexible duct has, without giving up the installation speed that made flexible duct the default choice in the first place. The lower price tag on flexible duct and the familiarity that contractors have with the ducting rarely accounts for the call-backs, energy loss, and compliance risk it can bring with it.
Our range of semi-rigid ducting is a step above flexible ducting without the drawbacks of rigid ducting; it's cleaner, more durable, better at maintaining consistent airflow, and just as quick to install. Browse our products to see whether Elephant Duct might be the better fit for your ventilation system.