Circular Weather Louvres

Circular weather louvres are external wall vents that terminate round ductwork at an outside wall while keeping weather, debris and birds out of the system. We stock cast aluminium circular louvres with bird mesh in 7 standard diameters to match the most common UK duct sizes, with 35° fixed blades that shed rainwater and block direct line-of-sight into the duct. Suitable for both external and internal applications including bathroom and kitchen extract terminations, MVHR intake and exhaust, commercial kitchen extraction, and plant room ventilation. All sizes in stock with next-day delivery available.

Grilles & Louvres / Ducting / MVHR Units / Fire Dampers

What is a circular weather louvre used for?

A circular weather louvre is fitted to the outside of a wall where round ductwork terminates, providing a weatherproof finish that lets air pass through while keeping rain, leaves and wildlife out. The angled blades stop driving rain from entering the duct, and the rear-fitted bird mesh prevents insects, birds and rodents getting into the system. Typical uses include bathroom and kitchen extract fan terminations, MVHR supply and exhaust points, dryer vents, commercial kitchen extract, and plant room intake or exhaust. They're the standard external fitting for any round duct penetrating an external wall.

What sizes of circular weather louvre do you stock?

We stock 7 standard diameters that match the most common UK round duct sizes: 100mm, 125mm, 150mm, 160mm, 200mm, 250mm and 315mm. The stated size is the spigot diameter, which matches the duct it connects to. The outer face diameter is larger to accommodate the mounting flange and blade frame. 100mm is the standard size for bathroom extract fans, 125mm and 150mm are common on kitchen extract and MVHR, and 200mm upwards is used on commercial and plant room applications. If you need a non-standard diameter or rectangular louvre, get in touch and we can source to order.

Do the louvres come with a bird mesh?

Yes. Every circular weather louvre we stock is fitted with a galvanised bird mesh at the rear of the frame as standard. The mesh prevents birds, rodents, insects and leaves from entering the duct — a requirement under Building Regulations Part F for any external ventilation opening. If you need a stainless steel mesh for coastal or high-humidity environments, or a finer insect mesh for food production facilities, call us on 0161 818 2050 and we can specify an alternative.

What is the difference between a weather louvre and a cowl?

Both terminate external ductwork, but they work differently. A weather louvre is a flat, wall-mounted grille with angled blades — the air passes straight through the wall and exits across a large louvred face. A cowl is a hooded or capped terminal that projects out from the wall and deflects air downward or to the side. Louvres are used where you have a clean wall termination and want a flush, architectural finish. Cowls are used where you need stronger weather protection, wind deflection, or where the duct exits a roof rather than a wall. For most standard bathroom, kitchen and MVHR terminations on a vertical wall, a circular weather louvre is the right choice.

What is the free area of a circular weather louvre?

Our circular louvres offer approximately 50% free area — roughly half the face is open to airflow, with the rest taken up by the fixed blades and bird mesh. That's typical for weather louvres and reflects the balance between airflow and the need to block rain ingress. For sizing, your ventilation designer should allow for this when calculating the fan's static pressure — a 100mm louvre has less effective open area than a 100mm open duct, so system pressure drop needs to account for the louvre and mesh. For high-volume extract applications, consider stepping up the louvre diameter to reduce velocity and noise at the terminal.

Can circular weather louvres be used internally?

Yes. While they're designed primarily for external wall termination, the same louvre can be used internally as a transfer grille or duct terminal where you want a clean round finish with blade protection over the opening. Common internal uses include plant room air transfer between compartments, riser cupboard ventilation and extract terminations in service voids. For fire-rated walls or floors, the louvre alone does not provide a fire rating — you'll need a fire damper behind the louvre to maintain compartmentation under Building Regulations Part B.

What material are your circular weather louvres?

Our standard circular weather louvres are cast aluminium with a mill finish and a galvanised steel bird mesh. Aluminium is the correct material for external use — it's corrosion-resistant, lightweight and will not rust in UK weather. The mill finish is the natural aluminium appearance and is suitable for most industrial and commercial applications. If you need a painted finish (RAL colour, white, black or brown) to match architectural requirements, we can source powder-coated versions to order with a typical lead time of 2–3 weeks.

How to choose the right circular weather louvre

Selecting the correct circular weather louvre is critical to maintaining efficient airflow and protecting ductwork. Consider the diameter to match your duct size, the material for durability in outdoor conditions, and the location, wall-mounted or roof applications. 

For projects requiring a variety of options or sizes, circular louvres complement the wider range of weather louvres available at eFans, providing flexibility for any commercial, industrial, or residential installation.

Benefits of circular weather louvres

Circular weather louvres offer reliable protection against rain, wind, and debris while ensuring smooth airflow through ventilation systems. Their robust construction reduces maintenance needs and prolongs system life, particularly in exposed or high-wind environments. 

These louvres also enhance energy efficiency by reducing air leakage, helping maintain building comfort and reducing operating costs.

What materials are best for circular weather louvres in coastal areas?

For coastal or corrosive environments, aluminium or stainless steel louvres are ideal thanks to their rust and corrosion resistance. A powder-coated finish adds further protection, ensuring long-term durability in harsh weather conditions.

How do circular louvres affect system airflow?

Circular louvres are designed to minimise pressure drop while shielding ductwork from the elements. When correctly sized, they maintain efficient airflow and consistent ventilation performance. Undersized units can restrict air movement and reduce system efficiency.

Can circular louvres be used for both supply and extract applications?

Yes. Circular weather louvres are suitable for supply and extract systems, protecting openings from rain and debris while promoting smooth, balanced airflow. They’re commonly used across commercial, industrial, and residential installations.

Are circular louvres suitable for high-velocity duct systems?

Yes, provided the model is rated for the system’s airflow velocity. High-velocity systems may require reinforced blades or larger diameters to maintain performance and reduce noise or mechanical stress.