Semi-rigid ducting sits proudly in a relatively newly-established space between flexible and rigid ductwork, offering the bending flexibility needed for tight installation runs paired with airflow performance and durability that's far closer to rigid duct than standard flexible PVC hose.
That being said, its benefits haven't exactly made headline news; many installers neglect to specify semi-rigid ducting because they just don't understand how best to use it, or they would rather choose the cheaper, albeit worse, flexible ducting.
If you're specifying or installing ventilation for MVHR, VRF, FCU, or general HVAC systems, understanding how semi-rigid ducting is actually built (and where it fits) will help you avoid rework, failed inspections, or underperforming airflow further down the line. By the end, you'll understand why we offer semi-rigid for the majority of ductwork installations.
Elephant Duct's semi-rigid ducting consists of a reinforced, multi-layer polyolefin structure built around an embedded steel wire helix, though other brands' aluminium semi-rigid variants also exist on the market. The steel helix gives Elephant Duct its shape-holding strength, while the polyolefin layers form an airtight, puncture-resistant skin around it.
Some semi-rigid products, and all Elephant Duct products, also come pre-insulated, with a closed-cell foam layer built into the duct wall rather than requiring lagging to be fitted on-site after installation. Insulation on flexible ducts is available but typically flimsy and offers weaker thermal insulation than our brand or thermal sleeves. Rigid aluminium ducts can be lagged (fitted) with a thermal sleeve on-site, but these take time to install and can leave gaps exposed around ductwork joints, leading to temperature exchange.
We offer a product that bends by hand around obstacles, joists, and structural elements — no crimping tools or rigid elbow fittings required for minor direction changes — while holding a constant cross-section that keeps airflow consistent through the bend. That's what makes semi-rigid ducting different.
Flexible ducting (fully corrugated, often PVC or foil-based) is the easiest to install in confined spaces but has the highest resistance to airflow. Lazy contractors sometimes abuse this ducting's flexibility to wrap it around corners, in between obstacles, and into all sorts of shapes. The corrugations create turbulence, which increase pressure loss and can reduce the effective performance of fans and HVAC units. Unreinforced flexible duct is also prone to ripping and sagging over time, which building regulations increasingly restrict.
Rigid ducting (spiral-wound or rectangular sheet metal) offers strong airflow performance and durability but is inflexible by nature. Every direction change needs a manufactured bend or fitting, coordination has to be pin-point accurate on site, and it typically needs lagging fitted after installation to control condensation — all of which adds cost and time, especially on retrofit projects where routes rarely match the drawings.
Semi-rigid ducting bridges the two. Its steel-helix-reinforced construction holds a constant cross-section through bends — minimising the turbulence and pressure loss associated with corrugated flexible duct — while its ability to bend on-site by hand reduces the number of fittings and the installation time needed compared with rigid systems. Pre-insulated variants also remove the separate lagging step altogether.
Fire performance varies significantly between semi-rigid products, so it's worth checking the Euroclass rating before specifying a brand. Elephant Duct semi-rigid ducting is tested to BS EN 13501-1:2019, achieving Class B-s2, d0 — a high rating for limited flame spread, moderate smoke production, and no flaming droplets.
Many lower-cost products on the market only achieve Class E, which is a meaningfully lower level of fire performance. In the UK, fire-safety responsibilities sit under the Regulatory Reform (Fire Safety) Order 2005, with Building Regulations Approved Document B setting the relevant performance expectations for ductwork.
Airtightness matters just as much for real-world performance: leaky ducting undermines a correctly designed HVAC system regardless of how efficient the unit itself is. Look for products tested to EN 12237 and EN 12599, the standards covering ductwork airtightness and ventilation performance respectively.
As with any legislation-heavy topic, always verify current figures and requirements against up-to-date Building Regulations guidance, as standards are periodically revised.
Beyond fire and airtightness, day-to-day performance comes down to how well the duct holds its cross-section and resists pressure loss. Well-built semi-rigid ducting can maintain air velocities of up to 20m/s and pressures in the range of 2,000–10,000Pa, depending on diameter — figures that matter on higher-pressure commercial installations, not just domestic extract runs.
Bend radius is also worth checking against the manufacturer's data sheet. Quality steel-helix semi-rigid duct typically achieves a bending radius of around 1.2–1.8 times its own diameter without kinking or restricting airflow, which is tighter than most installers assume.
Hygiene is another differentiator that's easy to overlook. Elephant Duct semi-rigid ducting is manufactured with an antimicrobial polyolefin resin film built into the internal surface, inhibiting microbial growth at the source rather than relying solely on periodic cleaning — relevant for sick building syndrome concerns and indoor air quality objectives more broadly.
As a general rule of thumb: choose semi-rigid ducting where you need airflow performance and durability closer to rigid duct but where a fully rigid system would be impractical or costly to install — MVHR runs, VRF/FCU connections, and confined retrofit routes are the classic examples.
Choose fully rigid duct only where project-specific requirements make it unavoidable. Avoid unreinforced flexible duct wherever building regulations or long-term reliability are a concern.
If in doubt, checking the ventilation system manufacturer's installation guidance — and the duct manufacturer's own fire, airtightness, and performance data — is the fastest way to confirm which duct type is appropriate for a specific application.