Flexible Ducting

Frequently asked questions

What is flexible ducting used for?

Flexible ducting is used to connect ventilation equipment to duct runs, terminals and rigid sections where a straight, bendable length of duct is needed. Typical applications include connecting extract fans and MVHR units to rigid spigots, running cooker hoods to external wall vents, linking tumble dryers to wall kits, short runs in loft and ceiling voids, and any situation where routing around pipework, joists or steelwork rules out rigid duct. The concertina design lets it compress, extend and bend around tight corners. That is useful during install, but runs should be pulled taut once in position to minimise pressure loss.

What types of flexible ducting do you stock?

We stock three main types. Aluminium flexible ducting is the standard choice: full aluminium construction, suitable for temperatures up to around 250°C, and the right answer for most extract and supply applications. Combi flexible ducting has a PVC outer and an aluminium inner around a wire helix. It is lighter, cheaper and fine for lower-temperature extract use such as bathroom fans and cooker hoods (non-greasy). Insulated flexible ducting has an outer jacket with glass wool insulation around the inner duct, used where condensation risk is present (MVHR supply in cold lofts, extract runs through unheated voids) or where acoustic attenuation is needed.

What sizes of flexible ducting are available?

We stock flexible ducting in diameters from 82mm through 100mm, 125mm, 150mm, 160mm, 180mm, 200mm, 229mm, 250mm, 300mm, 315mm and 355mm up to 406mm. The most common domestic sizes are 100mm / 102mm for bathroom fans and short extract runs, 125mm for cooker hoods and MVHR, and 150mm for larger kitchen extracts and higher-flow MVHR installs. 200mm and above are typically used on light commercial extract and supply. Aluminium and insulated flexible ducting come in 10m lengths and combi in 6m lengths, and bulk quantities can be ordered for larger projects.

What's the difference between flexible and rigid ducting?

Rigid ducting (PVC or galvanised steel) is smooth-bored, which gives significantly lower pressure drop and better airflow over a given length compared with flexible ducting. Flexible ducting is easier and quicker to install but the corrugated inner wall adds friction. A run of flexible ducting can have 3 to 10 times the pressure loss of the equivalent rigid duct. Best practice is to use rigid ducting for the main run and flexible ducting only for short connections and tight bends. If you're installing MVHR or a whole-house extract system, rigid ducting gives better performance and longer service life.

When should I use insulated flexible ducting?

Use insulated flexible ducting wherever there's a risk of condensation on the duct surface, or where you need to reduce fan noise transmitting down the duct. The main applications are MVHR supply and extract runs passing through unheated lofts or external walls (warm moist air in a cold duct causes condensation that drips back into the room), kitchen extract runs through cold voids, and acoustic attenuation on noisy extract fans. Uninsulated ducting in cold voids will cause condensation problems and is a common cause of MVHR install failures. If in doubt, use insulated.

Can flexible ducting be used outside?

No. Flexible ducting is not UV-stable or weatherproof and should never be used as an external duct run. If the duct has to exit the building, transition to rigid ducting or an insulated spiral duct before the external wall, and terminate with an appropriate external cowl, louvre or wall terminal. Flexible duct exposed to sunlight and weather degrades quickly and will split, lose shape and allow water ingress back into the system.

How do I join flexible ducting to fans, rigid duct or terminals?

Flexible ducting is pushed over the spigot of the fan, rigid duct or terminal and secured with a worm-drive hose clamp or a duct tape wrap. For air-tight MVHR and building-regs-compliant extract, use a proper clamp plus a bead of duct sealant around the joint, then foil tape over the top. Pull the flexible duct taut (do not leave slack or sag) before clamping, because sagging runs trap condensate and wreck airflow. Support the duct with saddle clips or banding straps every 600mm on horizontal runs.

Does flexible ducting affect airflow and fan performance?

Yes, significantly. The corrugated inner wall and any bends, kinks or slack in the run add pressure loss, which reduces the airflow delivered by the fan. As a rule of thumb, every 1m of flexible duct is equivalent to roughly 3–5m of smooth rigid duct, and every 90° bend adds the equivalent of another 1m of rigid duct. Keep flexible runs short, pull them taut, and avoid tight bends. If an extract fan or MVHR unit isn't delivering its rated airflow, excess flexible ducting is almost always the cause.

New to ducting? Read our guide: What is Ducting? Types, Sizes & Materials Explained.