Duct Cowls: External Vent Types, Anti-Rattle Advice, and What to Buy

The builder fits your new cooker hood, cores a 125mm hole through the brick, screws a basic white plastic cowl on the outside, and everyone goes home. Two months later the first proper gale hits. The cowl's louvres clatter all night like a tin roof in a hailstorm. You can't sleep. You can't turn the extractor on without the smell of next door's bins coming back through the duct. And in April you realise a starling has moved in, which under the Wildlife and Countryside Act 1981 you can't legally evict until the chicks have fledged. All of this traces back to one £4 plastic fitting nobody thought about.

What a duct cowl is and what it's for

A duct cowl is the external termination that caps the end of a ducted extraction run. It sits on the outside face of the wall, covers the duct opening, deflects rain, blocks birds and vermin, and, if specified correctly, stops outside air reversing back down the duct when the extractor is off.

Every ducted kitchen cooker hood has one. So does every ducted bathroom extractor, utility room fan, and mechanical ventilation outlet. The cowl is almost always the last item fitted on the external wall and the first thing you see of the ventilation system from outside. It's also the part most builders pick up as an afterthought from the nearest merchant at the end of the job, which is why so many installations perform poorly.

The cowl serves four jobs at once:

• Weather protection (keeps rain and wind-driven water out of the duct)
• Non-return function (stops back-draught through the duct when the fan is off)
• Exclusion (stops birds, wasps, rodents, leaves, and debris entering the duct)
• Appearance (the visible face of the ventilation system on the outside of the house)

Approved Document F Volume 1 (the English building regulation covering ventilation in dwellings) requires all kitchen extract air to be ducted to outside. Recirculating-only hoods do not satisfy Part F where extract ventilation is required. The regulation sets the extract rate (30 l/s over a full-width cooker hood, or 60 l/s for a fan elsewhere in the kitchen) and requires the fan itself to be installed as high as practicable, maximum 400mm below ceiling. What it does not prescribe is the specific cowl type. That decision is left to the installer, which is exactly where most installations go wrong.

The four types of duct cowl

Louvred cowl

Fixed angled vanes, no moving parts. The hood shape deflects rain. Quiet at rest because nothing flaps. But it offers no back-draught prevention at all. Outside air passes in both directions freely, which in practical terms means cold draughts in winter and smells finding their way back indoors. It also lets insects and small birds through the gaps between the vanes. Suitable for passive ventilation openings and static air bricks. Not appropriate as the sole termination on a kitchen extractor run.

Gravity flap cowl

The most common residential extractor termination. A hinged flap (single large flap on better designs, or a row of small slats on cheaper ones) sits inside a cowled hood. Fan pressure lifts the flap open during extraction. Gravity drops it closed within a second or two of the fan stopping. The closed flap acts as the non-return valve.

This is the type you'll see on almost every extension built in the last twenty years. The standard Manrose cowl range (Screwfix, Toolstation, Wickes) and the Airvent wall outlets are both this type. Priced between £3.75 and £11 depending on brand and diameter.

The critical split within this category is single-flap versus multi-flap. A single large flap is quieter, closes more reliably, and is less prone to wind noise. Multi-flap (sometimes called multi-louvre gravity) designs are notorious for rattling and are the root cause of almost every complaint about kitchen extractor noise on DIY forums.

Inline spring-loaded non-return valve with plain external cowl

A dedicated spring-loaded valve fitted inside the duct, usually within 300mm of the fan or within the vertical riser, combined with a plain (non-flapped) cowl on the outside. The valve seals under spring tension, not gravity, so it closes reliably regardless of wind direction or duct orientation. The external cowl handles rain and bird exclusion only.

This is the preferred specification for exposed locations, north-facing or gable-end walls, and any installation where wind-rattle and back-draught must be eliminated. The downside is two fittings instead of one, both needing to be bought and installed. Combined cost runs £15 to £40 depending on the valve grade and cowl finish.

Low-profile or render-finish termination

Flat grille, slim louvre, or airbrick-format fitting for walls where a projecting cowl is unwanted. Rendered walls (where a cowl hood sits proud of the render and catches runoff stains), conservation areas, listed buildings, and contemporary extensions with visible architectural lines all benefit from a flush or near-flush termination.

Rytons, Timloc, and Verplas all offer flat-faced or low-profile options in terracotta, brown, beige, and black finishes. Some include a gravity flap, some are louvred only. Check the product page before buying if you need back-draught prevention.

Click to enlarge

The wind-rattle problem

If you read any UK DIY forum on kitchen extractors, the single most common complaint is wind-rattle. Cheap multi-flap cowls (the type with a row of small plastic slats) resonate in any meaningful wind. On exposed walls they can be loud enough to wake a household. In severe storms the slats eventually fatigue at the hinge and break off, leaving the duct open to the weather.

The mechanism is simple. Each small flap weighs almost nothing. Wind pressure lifts them, gravity drops them back, and they slap against the cowl body. Multiply by a dozen slats and you get a continuous rattle that tracks wind direction. The louder your wind, the louder your kitchen vent.

Three fixes exist if you already have a rattling cowl. In order of durability:

Fit foam draught-excluder tape to the inside of the cowl body where the flaps land. This cushions the impact and eliminates most of the noise. Works for a year or two before the foam perishes.

Add small weights (wheel-balancing weights, a washer taped to each flap) to damp the movement. This makes the flaps less responsive to wind pressure but also slightly restricts extraction airflow. A compromise.

Replace the cowl with a single-flap gravity design or fit an inline spring-loaded non-return valve inside the duct and swap the external cowl for a plain louvred hood. The inline NRV approach is the definitive solution and the one professional installers use on exposed sites.

Sizes: match the duct diameter

Cowl size must match duct diameter exactly. Reducing adapters cut airflow and defeat the point of buying a decent extractor.

• 100mm (4 inch) for bathroom, utility room, and WC extraction. Rarely adequate for a cooker hood because the flow restriction is too high for typical cooker hood fans, which need 125mm to deliver their rated extract rate.
• 125mm (5 inch) for standard kitchen cooker hoods. This is the size most domestic extractors are designed for and what you'll spec on a kitchen extension 95% of the time.
• 150mm (6 inch) for high-volume extraction: large island hoods, commercial-grade domestic extractors, and venting rates above 600 cubic metres per hour.

Check the cooker hood specification before buying the cowl. The flange on the back of the hood tells you the duct diameter. Match the cowl to that, not to what the merchant has in stock.

Materials and finishes

White PVC and polypropylene

The default. Every merchant stocks white plastic cowls between £4 and £11. Fits white uPVC windows and painted white render. Will yellow under UV over five to ten years, especially on south-facing walls. Affordable, easy to replace when it degrades.

Brown and beige PVC

Standard stock at Manrose and Airvent. Brown suits most UK brick elevations (red brick, multi-stock, weathered stock). Beige suits buff sand brick and rendered sandstone finishes. Same price range as white.

Terracotta

Verplas offers five-colour ranges including a proper terracotta (RAL 2001) for period brick matching. Timloc's external cowl range includes terracotta and brown. Useful on Victorian and Edwardian brick where white plastic would stand out badly.

Black

AirTech-UK, Blauberg kit, and a handful of specialist suppliers offer black cowls at around £9 to £25. Matches anthracite window frames, dark render, black uPVC cladding, and contemporary extension aesthetics. Popular on the current generation of new-build and refurb kitchens.

304 grade stainless steel

Verplas SS103 at around £38, Just Fans HU-SS/100 at around £107. Does not corrode, does not yellow, does not stain. The right choice for coastal locations (salt-laden air strips plastic within years), highly exposed elevations, and premium installations where the cowl is visible and needs to age gracefully. Large price step from plastic.

Render-matched finishes

For conservation areas or rendered walls where any visible cowl is a problem, specialist suppliers will powder-coat an aluminium grille to a specified RAL colour. Custom work, usually four to six week lead time, typically £40 to £80 depending on finish. Worth the money on a heritage property.

How to install a duct cowl

The cowl is usually the last step in a ducting run. By the time you're fitting it, the core hole should already be drilled, the duct run should be installed to the room side, and the fan should be ready to commission.

1. Core drill the external wall. Use a 107mm core drill bit for 100mm duct, 133mm for 125mm duct, 160mm for 150mm duct. Slope the core downward toward outside, 3 to 5mm fall per 300mm of wall thickness. This ensures any condensation inside the duct runs outward, not back into the kitchen.

2. Feed the duct through the core and set the external collar. The cowl collar should sit flush with the outer face of the brick or render. Mark the fixing hole positions on the wall.

3. Drill and plug the fixing holes. Use a 6mm masonry bit and standard nylon rawlplugs unless the manufacturer specifies otherwise. Four fixings at the corners of the face plate is standard.

4. Seal the annular gap around the duct. On the inside, pack expanding foam between the duct and the core hole. On the outside, pack mortar or exterior sealant tight against the collar. Without this step, wind-driven rain tracks into the cavity.

5. Fit the cowl and apply perimeter mastic. Push the cowl body onto the duct or collar, secure with the supplied screws, and then run a continuous bead of exterior-grade silicone sealant around the full perimeter of the face plate where it meets the wall. Colour-match the sealant to the wall finish if possible. This perimeter seal is the primary weatherproofing, not the cowl itself. Skipping it is the most common installation fault and the reason most duct leaks trace back to the external face, not the duct body.

6. Check flap operation before signing off. Turn the extractor on at its lowest setting. The gravity flap should open fully and stay open under fan pressure. Turn the extractor off. The flap should fall shut within one to two seconds. If it doesn't, the cowl is defective, the duct is restricting airflow, or the flap is fouling against the body.

Positioning: the 2m rule and Part F

A widely repeated installer convention holds that the external terminal should be positioned at least 2 metres away from any openable window, and not directly below an openable window at any distance. The stated reason is to prevent extracted air (odours, moisture, cooking grease particulate) being drawn back into the building through a nearby opening.

The 2 metre figure is cited by manufacturers, trade guides, and training material across the UK ventilation industry. It is commonly understood to derive from Approved Document F Volume 1, but the specific 2 metre distance could not be independently verified against the current ADF1 text during research for this guide. Treat it as practitioner convention rather than a regulatory absolute.

In practice, the rule is sensible regardless of its regulatory status. Extracted kitchen air contains warm moist grease-laden particulate that you do not want blowing straight back through a bedroom window. Site the cowl as far as practicable from the nearest openable window, and above window head height where possible. If the only available wall position puts the cowl close to a window, consider routing the duct through the eaves or soffit to terminate at roof level instead.

Bird and vermin exclusion

An uncovered duct is a nesting site. Starlings, sparrows, and house martins are cavity nesters. They will colonise any accessible cavity of roughly the right size, and a 125mm duct is ideal. Once a nest is established during the breeding season (broadly February to August), you cannot legally remove it. The Wildlife and Countryside Act 1981 protects active nests regardless of where they are. Your only option is to wait until the chicks have fledged and the adult birds have departed, then fit exclusion before the following spring.

Prevention is straightforward. A gravity flap cowl in the closed position physically blocks the duct entrance. A louvred cowl does not, because the gaps between the vanes are large enough for a sparrow to squeeze through. Once a year (typically late autumn, after the nesting season) inspect the cowl from outside and confirm the flap closes fully and freely. If it's stuck open, clean it, replace it, or fit an inline non-return valve behind it.

Cost and where to buy

Duct cowls are small-ticket items. The full price range across the UK market is £3.75 to £110. Most residential installations sit at the lower end, between £4 and £12 for a standard gravity flap cowl in a stock colour.

Screwfix, Toolstation, and Wickes all carry the standard white and brown Manrose and Airvent cowls in 100mm and 125mm. Prices are virtually identical across all three. A Manrose 100mm white cowl is £4.98 at Screwfix and £4 at Wickes. A 125mm white cowl is £8.99 at Screwfix. Toolstation's Airvent equivalent gravity flap outlets are slightly cheaper at £3.59 to £3.75.

Verplas (specialist ducting manufacturer, trade supply) offers the 5-colour polypropylene range and the stainless steel options. The Verplas SS103 stainless 100mm cowl with non-return flap is around £38 via PlumbHub and similar online merchants. The Just Fans HU-SS/100 premium stainless cowl is £107, a notable step up aimed at exposed coastal or architectural installations.

Blauberg UK sells a black 125mm cowled wall shutter vent kit (model CHK-125-3-VKBL) at around £22, which includes the ducting kit as well as the cowl. Useful on contemporary extensions where black anthracite is the theme.

Brand summary:

• Manrose for standard white and brown plastic cowls at Screwfix, Toolstation, Wickes
• Airvent for budget gravity flap outlets at Toolstation
• Verplas for 5-colour polypropylene and stainless steel options (specialist merchants)
• Rytons for louvred and low-profile wall vents including render-finish options
• Timloc for terracotta and brown external cowls suited to period properties
• Blauberg UK for black-finish kits and wind-baffle specialist products
• Just Fans for premium 304 grade stainless steel cowls

If you're ordering ducting kit from a single supplier, the Verplas and Blauberg system approach (cowl, duct, reducers, clamps all matched) is worth the small premium over mixing brands. Dimensional tolerances vary between manufacturers and the tightest seals come from matched-brand components.

Alternatives and when to use them

The duct cowl is the standard termination for wall-mounted extraction. The two alternatives worth naming are roof terminals and soffit terminals.

Roof terminals are used when the duct rises vertically from a cooker hood and exits through the roof tiling. A dedicated roof vent tile (or stainless roof cowl on a flat roof) replaces the wall cowl. This is the right approach for island hoods where there's no external wall behind the unit, and for any layout where the wall-mounted route would put the external cowl in an aesthetically or functionally bad position.

Soffit terminals exit through the soffit (the underside of the roof overhang) rather than through the wall face. Useful when the wall elevation cannot accommodate a cowl (for example on a heritage frontage) and the eaves give a concealed exit point. The downside is a longer, more complex duct run and potentially more horizontal sections where grease can accumulate.

Both alternatives require different terminal products and are covered in detail on their own knowledge pages.

Common mistakes

Choosing the cowl as an afterthought. The external termination is specified last and bought cheap, which is why standard multi-flap plastic cowls dominate domestic installations. For any exposed wall, specify a single-flap gravity cowl or inline non-return valve at design stage, not at fit-out.

Not sealing the perimeter with mastic. The cowl face plate alone is not a weather seal. Water tracks behind it through the gap between the face plate and the wall surface, entering the cavity and often appearing inside as staining or damp below the duct position. Every cowl installation needs a continuous silicone bead around the full perimeter.

Using a 100mm cowl on a 125mm cooker hood. Reducing adapters restrict airflow. If the cooker hood specifies 125mm ducting, the cowl must be 125mm. A standard cooker hood won't hit its rated extract rate through a 100mm restriction, which matters if a Part F commissioning test is required.

Relying on wire mesh for bird exclusion. Mesh clogs with grease within weeks. Use a flapped cowl instead. The moving flap prevents build-up because it shifts every time the fan cycles.

Not checking flap operation after installation. A stuck or partially closed flap reduces extraction rate and defeats the non-return function. Test before signing off the fan install. Test again after the first year.

Fitting the cowl directly below a window. Extracted grease particulate will stain the window frame and sill within months, and wind-driven back-draught can carry cooking smells into the room above. Site the cowl above window head height or at least 2 metres horizontally from any openable window.

Where you'll need this

• Extractor and ventilation - cooker hood selection and ducting route decisions determine the cowl diameter and position
• Walls and blockwork - core hole positions for ducting must be planned before the wall is plastered and decorated

Duct cowls are used on any ducted extraction run across any extension, renovation, or refurbishment project. The cowl specification is a small decision with a long tail: the wrong choice on a £10 fitting can generate years of noise complaints, smell complaints, and back-draught complaints in an otherwise well-built kitchen. Getting it right at first-fix stage costs no more than getting it wrong, and avoids the awkward conversation about replacing the termination after the render has been painted.