Semi-Rigid Aluminium Flexible Ducting — Practical, Efficient & Easier to Install
Most people turn to semi-rigid ducting after encountering the limitations of their first choice. Fully flexible ducting is easy to route but can sag, kink, and create airflow resistance in longer runs. Rigid ductwork is efficient but unforgiving — one misaligned measurement and the whole run needs rethinking. Semi-rigid ducting sits neatly in the middle, and once you've used it, it's hard to go back.
What is semi-rigid flexible ducting?
Semi-rigid flexible ducting is a ventilation duct made from corrugated aluminium that holds its shape when bent but remains flexible enough to adjust by hand. Unlike insulated or combination flexible ducting, it doesn't rely on a wire helix for structure. Instead, the corrugated aluminium wall provides its own rigidity while still allowing the duct to be shaped and repositioned during installation.
The result is a duct that:
- Holds its shape once positioned — no sagging, no kinking, no collapsing under its own weight
- Bends smoothly around obstacles without specialist tools or fittings
- Offers lower airflow resistance than corrugated flexible ducting, thanks to a smoother internal profile
- Connects cleanly to standard spigots, fans, and ventilation fittings
Where is semi-rigid ducting used?
Semi-rigid aluminium ducting is a genuinely versatile product used across a wide range of installations:
- Bathroom and kitchen ventilation: It's a popular upgrade from standard flexible ducting for extractor fan runs, particularly where the duct needs to hold its shape through a loft or ceiling void without support at every point.
- Loft and roof void installations: Semi-rigid duct handles temperature variations in loft spaces well, and its ability to maintain a set shape makes routing through awkward spaces much more manageable than fully flexible alternatives.
- HVAC connection runs: In HVAC systems, semi-rigid ducting is commonly used for short connection runs between rigid ductwork and terminal fittings or diffusers, where a small amount of adjustability is needed, but a clean, low-resistance profile is important.
- Tight spaces and awkward routes: Anywhere a rigid pipe won't fit, but a flexible duct would sag, or kink, semi-rigid ducting finds its place. It can be shaped by hand to navigate around beams, joists, and other obstacles cleanly and precisely.
Choosing the Right Duct Size
- 100mm semi-rigid ducting: Standard for most domestic bathroom fans and smaller kitchen extractors.
- 125mm semi-rigid ducting: Used for higher output fans and light commercial ventilation connections
- 150mm semi-rigid ducting: Suited to larger extraction systems, commercial HVAC connections, and higher-airflow applications.
Always check your fan or unit's outlet diameter before ordering to ensure the correct fit: it's listed in our product specification. In case of any queries, reach out to us via email sales@flexibleducting.com or on 0117 330 2277.
A Few Tips for a Clean Installation
- Shape it before you fix it. A semi-rigid aluminium duct can be bent and adjusted by hand. Work out your route and shape the duct to fit before securing it in place.
- Keep bends gradual. Smooth, gentle bends maintain better airflow than sharp angles. Take your time shaping the duct, and your system will perform better as a result.
- Seal all connections with aluminium foil tape. At every joint and connection point, use proper aluminium foil tape for an airtight, long-lasting seal.
- Support longer runs. Although semi-rigid aluminium flexible ducting holds its shape far better than flexible alternatives, supporting runs over a metre in length with duct clips keeps everything aligned and tidy.
FAQs
1. Is semi-rigid ducting difficult to install?
Not at all. It bends and shapes by hand without any specialist tools, connects to standard spigots and fittings, and cuts cleanly with tin snips or heavy-duty scissors. Most installers find it easier to work with than rigid pipe — particularly in confined spaces — and neater than fully flexible ducting over longer runs.
2. What sizes does semi-rigid ducting come in?
Our range includes 100mm, 125mm, and 150mm semi-rigid aluminium ducting — the most commonly specified sizes for domestic and commercial ventilation applications. Always check your fan or unit's outlet diameter before ordering.
3. Can semi-rigid ducting be used for bathroom extractor fans?
Yes — it's an excellent choice for bathroom extractor fan installations, particularly where the duct runs through a loft or ceiling void. It holds its shape without needing support at every point, offers better airflow efficiency than corrugated flexible ducting, and connects cleanly to standard bathroom fan spigots.
4. How does semi-rigid ducting compare to rigid ductwork?
Rigid ductwork — typically made from galvanised steel or rigid PVC — offers the lowest airflow resistance and the cleanest finish for permanent installations. However, it requires precise measurements and purpose-made fittings for every bend and junction. Semi-rigid ducting is more forgiving — it can be shaped on site to navigate obstacles, making it a practical choice where rigid pipe would be difficult or time-consuming to fit.
5. Does semi-rigid ducting need insulating?
Standard semi-rigid aluminium ducting does not include thermal insulation. If your duct run passes through cold spaces — such as an unheated loft or roof void — wrapping it with duct wrap insulation or using an insulated flexible ducting product for that section will prevent heat loss and condensation. For internal runs in heated spaces, standard semi-rigid ducting performs well without additional insulation.