Flexible Ducting: Why It Fails and When It's Actually Acceptable

Your builder fits the kitchen, boxes in the units, and the extractor sounds like it's working. Six months later the windows are steamed up during every boil, the hood drips grease onto the hob, and commissioning airflow readings never appear. Behind the wall units there's six metres of corrugated plastic hose, partially kinked around a joist, sloping the wrong way. The hood is rated at 500 cubic metres per hour. It's delivering less than half that. Ripping the kitchen apart to replace the duct costs more than the hood did. Understanding flexible ducting, and why Building Regs now limit it severely, is the difference between a kitchen that extracts and a kitchen that performs theatre.

What it is and what it's for

Flexible ducting is corrugated, bendable hose used to carry extracted air from a cooker hood or fan to an outside vent. The corrugations are what make it flexible: the ribbed wall lets the hose bend around joists, dodge pipework, and reach awkward wall positions without needing elbow fittings. For installers it's fast. For performance it's a compromise, and often a bad one.

There are four distinct product classes sold under the "flexible ducting" label. They look similar on the shelf and perform very differently in a wall cavity:

1. PVC corrugated flexible. White or grey plastic hose with a wire helix reinforcement. The cheapest product. Fire-retardant to BS 476 Parts 6, 7 and 20 but still plastic. Sold in 1m, 3m, 6m and 45m rolls.
2. Aluminium laminated flexible. A corrugated aluminium foil hose bonded to a fabric scrim. Silver. Better fire resistance than PVC because the metal won't melt or support flame. The "ducting kit" hose supplied in the box with many cooker hoods.
3. Insulated flexible. A corrugated aluminium inner liner wrapped in 25mm of mineral wool insulation with a foil outer. Used where the duct runs through cold or unheated spaces such as loft voids, where warm moist kitchen air would otherwise condense inside the duct.
4. Semi-rigid aluminium. A different product class, supplied compressed and stretched on site. Shallow bellows rather than deep corrugations. Holds its shape once set, doesn't sag, bends without elbows. Significantly better airflow than corrugated flexible, though still not as good as true rigid pipe.

All four types serve the same nominal job: moving air from inside the kitchen to the outside wall. What separates them is how much of that airflow survives the journey.

The governing regulation is Approved Document F (Ventilation), the England Building Regulations document covering ventilation in dwellings. Part F sets minimum extract rates for kitchen ventilation: 30 litres per second for a cooker hood directly over the hob extracting to outside, or 60 l/s for a fan located elsewhere in the kitchen. BSRIA BG 43/2013 (Flexible Ductwork: specification, procurement, installation and maintenance) sits underneath Part F and sets the practical limits on how much flexible duct you can include in a run and still meet those rates.

Why flexible ducting is tempting and why it fails

Flexible ducting looks like an installer's best friend. It's cheap. A metre of 100mm PVC flexible costs £1.10 to £1.50, against roughly £9 for a metre of 100mm rigid round plastic. It bends around obstacles without fittings. The installer doesn't need to plan the run precisely because the hose will conform to whatever shape is left over once the units are in.

That same flexibility is the problem.

The corrugated bore creates turbulence. Every rib in the internal wall is a small obstacle. Air hitting the ridges loses velocity and generates eddies. The peer-reviewed engineering position is consistent: at the same diameter and flow rate, corrugated flexible duct has roughly double the pressure drop of smooth rigid duct, even when installed perfectly taut. In practical terms, that's the equivalent of making your hood's motor work twice as hard for the same delivered airflow. Over a long run, the hood can't compensate, and delivered flow collapses. The 25 cubic metres per hour per metre loss figure sometimes quoted for 100mm flexible at domestic extraction velocities is directional rather than a published manufacturer specification, but it's consistent with the measured pressure drop data. Treat it as illustrative: flexible costs you airflow per metre, steadily, for the entire length.

Compression is catastrophic. This is the part no consumer guide explains. Engineering test data (OSTI research on corrugated flex duct) shows that at just 4% compression, a 6 inch flex duct moves 70 cubic feet per minute against 110 cfm for the rigid equivalent at the same static pressure. That's a 36% airflow loss from barely visible compression. At 10% compression, airflow approximately halves. At 15% compression, the pressure drop rises to four to eight times the fully-extended value. Flexible duct hidden behind wall units is almost always partially compressed. The fitter couldn't see it. You can't see it. The commissioning test reveals it.

Sagging creates water traps. A flexible duct that droops between supports collects condensation in the low spots. Warm moist kitchen air rises, cools as it travels, and drops water into the corrugation troughs. That water sits, restricts flow further, breeds mould, and eventually drips back down the duct and out of the hood. Homeowners describe this as "the hood is leaking." The hood isn't leaking. The duct is returning six months of accumulated condensate.

Grease cannot be removed. Fat particles in extracted air coat the corrugations. The ribbed interior is effectively impossible to brush or swab. Over two or three years of cooking the buildup becomes both a restriction and a fire risk. Rigid smooth-bore duct can be rodded and cleaned. Flexible cannot.

Powerful fans can collapse the bore. High-output hoods (750 cubic metres per hour and above) create enough suction on the intake side to partially collapse flexible hose, especially PVC, reducing the effective diameter toward zero in severe cases. This is rare but known, and it defeats any attempt to oversize the duct to compensate for the turbulence problem.

Click to enlarge

When flexible is actually acceptable

Flexible ducting has a legitimate use case: as a short final connection, not as the main run.

For standard intermittent cooker hood extraction, BSRIA BG 43 sets the ceiling at 1.5 metres of flexible duct on an axial fan installation, or up to 6m on a centrifugal fan at flow rates up to 30 l/s. Approved Document F references these limits directly. The same standard permits a maximum of 2 bends in the flexible section. The duct must be pulled taut to at least 90% of its maximum stretched length to achieve its rated internal diameter. The run must slope slightly toward the outside wall so any condensation drains outwards rather than back into the hood.

Mechanical Ventilation with Heat Recovery (MVHR) systems, a completely different product class used in highly insulated new-builds and retrofits for continuous whole-house ventilation, apply a stricter rule: a maximum of 300mm of flexible per connection and no more than 1.5m of flexible across the whole installation. If your kitchen is served by an MVHR system, the 300mm limit is what applies.

The two numbers, 1.5m and 300mm, cause confusion on site. The practical position is:

• Standard cooker hood through an outside wall: up to 1.5m of flexible permitted, with performance penalties.
• Cooker hood integrated with an MVHR system: 300mm maximum per connection.
• Conservative best practice in either case: keep flexible to 300mm of bridging between the hood and the rigid run, and between the rigid run and the wall terminal.

A 90 degree bend in flexible duct generates the equivalent resistance of several metres of straight rigid pipe. Never use flexible duct to form a bend. Use a rigid elbow fitting at every change of direction.

The Part F commissioning test

Since 15 June 2022, Approved Document F requires commissioning airflow tests on every mechanical ventilation fan installed in new work. The installer must measure the actual delivered airflow at the extract point and record it on a commissioning sheet. That sheet must be left with the homeowner and provided to Building Control as part of sign-off.

This is the trap that full-run flexible installations now fall into routinely. At installation, with a powerful hood on boost and a short duct, the airflow might be adequate. Commissioning catches the real-world condition: cumulative pressure drop across a full flexible run, minor compressions behind cabinets, any bends, and the inevitable slight sag. The measured flow at the vent terminal often comes in below the 30 l/s minimum for a hood-over-hob or 60 l/s for a ceiling or wall fan. Building Control can require remediation. Remediation after the kitchen is fitted means ripping the kitchen apart.

Comparing the four flexible product classes

The fire safety gradient matters near the hob. PVC corrugated duct is technically fire-retardant under BS 476, but "retardant" means resists ignition under test conditions, not "equivalent to metal in a grease-laden kitchen extraction run". For a duct that carries combustible fat vapour directly away from flames, aluminium or rigid metal is the correct choice. PVC flexible is more appropriate for bathroom or utility room extraction where the air is damp but not greasy.

Semi-rigid aluminium is the product most kitchen fitters don't know about and should. It threads through joists and around obstacles like flexible duct, but once set it holds its shape and presents a near-smooth internal bore. Manrose, Domus and Blauberg all produce it in 100mm, 125mm and 150mm. Fans4less and similar specialists stock it from around £4 per 1.5m section. Availability varies: builders' merchants often don't stock it, Toolstation and Screwfix carry it intermittently, and online ventilation specialists are the reliable source.

Click to enlarge

How to work with it on site

Flexible ducting handling is simple but unforgiving.

Pulling taut. Extend the hose to 90% of its maximum stretched length before fixing. A 3m hose sold compressed to 1m should be stretched to 2.7m in use. Compression to fit the available route destroys airflow. If the route is shorter than the taut length, cut the hose, don't compress it.

Sloping. Install with a continuous fall toward the outside wall, typically 10 to 15mm per metre. This lets condensation drain outward rather than puddling in the hose or draining back into the hood. In practice this means the hood end should be slightly higher than the wall vent.

Clipping and support. Flexible ducting must be supported every 450mm to 600mm to prevent sagging. Use proprietary plastic duct clips or galvanised banding. Don't use wire ties pulled tight enough to pinch the duct.

Connecting to rigid pipe. Use a proper jubilee clip or duct clamp over the joint. The flexible hose slips over the rigid pipe spigot and is clamped down. Tape alone is not a seal, it's a dust filter. Aluminium foil tape can supplement a clip but should never replace one.

Cutting. Aluminium laminated and semi-rigid aluminium are cut with sharp scissors or tin snips. PVC corrugated is cut with a craft knife or fine-tooth hacksaw. The wire helix inside most flexible duct needs to be cut separately and bent back. Leave no sharp edges protruding into the airflow.

Loft and roof void runs. If the duct crosses cold space, use insulated flexible. Uninsulated ducting in a loft produces dripping condensation inside the duct during winter, and the water eventually finds its way back to the hood or the ceiling. The 25mm mineral wool on insulated flexible handles this. Wrapping uninsulated flexible with separate insulation is an acceptable retrofit but messier than buying the insulated product.

How much do you need

For a typical kitchen extension cooker hood installation, your duct budget is straightforward: the distance from the hood spigot to the outside wall termination, plus allowances for slope and slack.

Work out the run length by measuring on your layout drawing: horizontal along the top of wall units, up through any soffit, and out to the external wall vent position. Add 15% for vertical transitions and the slight fall toward the vent. Total stretched length is your target.

The breakdown, once you've committed to the flexible-only-at-transitions approach:

• 300mm of flexible at the hood end, to allow alignment with the rigid run.
• The bulk of the run in rigid round or flat channel duct.
• 300mm of flexible at the wall terminal end, to make the final connection.

A worked example: kitchen sink wall to external wall is 4m. Rigid section: approximately 3.4m. Flexible: 600mm total. Budget: 4 lengths of 1m rigid round plastic at around £9 each, one 1m flexible hose (cut in half for the two ends) at around £3.29, plus a wall vent kit (£15 to £25) and a roof cowl if discharging through a soffit. Total duct materials: £55 to £75 before clips and clamps. Against a full-flexible alternative at £6 to £10 for the whole hose, the difference is roughly £50. Against the cost of failing commissioning and reopening the kitchen, the difference is marginal.

For a full-flex installation that meets the 1.5m BSRIA limit, you need 1.5m of aluminium laminated or semi-rigid (not PVC) hose, plus the wall vent kit, plus rigid pipe for anything beyond 1.5m. Don't stretch the limit. A run of 2m in flexible is a Part F failure waiting to happen.

Cost and where to buy

Pricing is simple and stable. The UK market is dominated by four brands: Manrose (Screwfix, Wickes, Toolstation), Domus (specialist merchants and kitchen suppliers), Verplas (Toolstation, Howdens), and Envirovent (MVHR and commercial specialists). Own-brand equivalents at Wickes and Toolstation are often Manrose rebadged.

Current retail pricing (2025 to 2026) for 100mm diameter:

• PVC corrugated flexible: £1.10 to £1.50 per metre, sold in 1m (£3.29), 3m (£4.40 to £5.09), and 45m bulk rolls (£54 at Wickes for Manrose).
• Aluminium laminated flexible: £2.80 to £3.50 per metre. Manrose 10m x 100mm at Screwfix: £29.99.
• Insulated flexible: £33 to £58 per 5m to 10m roll. Blauberg 100mm insulated 5m is typical.
• Semi-rigid aluminium: approximately £4 per 1.5m section in 100mm, 125mm and 150mm (Fans4less and specialist stock).
• Pre-made kits: Verplas cooker hood flexi kit 125mm 1m at Toolstation £12.89; Howdens 150mm x 1.5m kit £12.95; Cata 2.5m kit with wall vent £30.57.

For 125mm, add roughly 15% to 100mm prices. For 150mm, add roughly 30%.

Where buying: Screwfix and Toolstation are priced near-identically for single lengths and keep stock of PVC and aluminium laminated. Wickes stocks the bulk 45m roll at better per-metre pricing if you're doing multiple kitchens. Howdens sells the all-in-one cooker hood kit as a default convenience product. Specialist ventilation suppliers (Fans4less, BPC Ventilation, Ventilation Megastore) are the reliable source for semi-rigid aluminium and MVHR-grade insulated flexible.

Alternatives

Rigid round plastic duct is the default correct choice for the bulk of any extraction run. Manrose and Domus produce it in 100mm, 125mm and 150mm at £9 to £12 per metre. Elbows, T-pieces, reducers and wall vent adapters are all stocked alongside. The installation takes longer because every change of direction needs a fitting, but the result is a smooth bore that can be cleaned, passes commissioning, and lasts the life of the house.

Rigid flat channel (typically 110 x 54mm or 204 x 60mm) is the alternative where vertical space above units is limited. Same smooth-bore performance as round rigid, but the flattened profile hides above wall cabinets without a bulky soffit. Verplas is the dominant brand. Approximately £5.80 per metre. Flat channel cannot directly connect to a round hood spigot, so a round-to-flat adapter is used at each end.

Semi-rigid aluminium is the compromise product, covered in the comparison table above. Not as good as rigid, significantly better than corrugated flexible, and the only product class that can route around obstacles while still presenting an acceptable internal bore. Preferable to corrugated flexible in almost every scenario where flexible would otherwise be used.

Recirculating hood (no ducting at all) is the fallback where an external vent is genuinely impossible. The hood filters the air through activated charcoal and returns it to the kitchen. Building Regs Part F still requires a compliant kitchen ventilation strategy, which usually means an intermittent fan elsewhere in the kitchen running alongside the recirculating hood. Charcoal filters need replacement every 3 to 6 months of active use. Recirculation is a compromise, not an equivalent to proper extraction, but it beats a badly installed flexible run that fails commissioning.

Where you'll need this

Flexible ducting decisions sit at the intersection of kitchen design and first-fix, with consequences that surface at commissioning and beyond:

• Extractor and ventilation - duct route, hood sizing, and the decision between ducted and recirculating extraction
• First fix electrics - cooker hood power supply, fan isolator positions, and coordinated routing with the duct run before walls and ceilings close up

These decisions apply across any extension or renovation project where a kitchen, utility room, or bathroom needs mechanical extraction. The same rules hold regardless of project type: flexible is for short transitions, rigid is for the main run, and the Part F commissioning test is the final judge. Specify correctly at design stage, verify the route before units are fitted, and you avoid the single most common post-completion extraction failure mode in UK kitchens.

Common mistakes

Running flexible for the entire distance. The single most frequent malpractice pattern. Installer buys a 3m or 6m flexible hose, routes it from hood to wall vent, boards over. The corrugated bore halves the rated airflow before any compression or bends are counted. Under Part F commissioning this will often fail measured flow.

Compressing flexible duct behind wall units. Even 10% compression halves airflow at the same static pressure. 15% compression raises pressure drop by a factor of four to eight. Units fitted hard against a wall with flexible duct pinched between are a guaranteed extraction failure, visible or not.

Forming bends with flexible. A 90 degree bend in flexible ducting is equivalent to several metres of extra straight rigid pipe in pressure drop terms. Every direction change should use a rigid elbow. Flexible is for straight, taut bridging only.

Sloping the wrong way, or not at all. Flat or reverse-sloped flexible duct pools condensation in the corrugation troughs. Water accumulates, further restricts flow, and eventually drips back through the hood. The slope is always toward the outside vent.

Using PVC near the hob. PVC flexible is technically fire-retardant under BS 476 but it's still plastic carrying grease-laden hot air. For hood runs, aluminium laminated or metal rigid is the correct specification. PVC is more appropriate for bathroom and utility room extraction.

Uninsulated flexible through a cold loft. Warm moist kitchen air condenses inside uninsulated duct in cold spaces. The water collects, drips, and finds a way back into the living space. Insulated flexible or separately wrapped rigid pipe is required for any run crossing an unheated loft or roof void.

Trusting the kit that came with the hood. The flexible hose and wall vent supplied in the box with many cooker hoods is designed for marketing convenience, not performance. It's typically 100mm PVC corrugated, adequate for a hood with a 200 cubic metre per hour rating over a 500mm run, inadequate for anything larger or longer. For any hood over 400 m3/h or any run over 1.5m, buy the duct separately and specify correctly.

Skipping the commissioning certificate. Since June 2022 this is a Building Regulations requirement, not an optional extra. If your installer cannot measure airflow at the vent terminal and produce a signed certificate, the extraction system isn't legally commissioned and Building Control can refuse sign-off. Ask for it before final payment.