Cowl — Outdoor Pipe & Duct Cowls
A cowl is the outdoor cap fitted to the outside end of a ventilation duct or exhaust pipe. It protects the ducting from rain, dust, and debris while letting air pass freely, and stops birds, rodents, and insects entering your ventilation system from outside. Every inline fan, kitchen exhaust, or bathroom extractor that vents through an external wall or roof needs a matching outer pipe cowl to complete the installation.
Astberg cowls are available in stainless steel SS304, powder-coated steel, and PVC, in diameters from 100mm (4") to 315mm (12"), with built-in bird mesh on every model to keep pests out without restricting airflow. Steel cowls suit exposed rooftop and coastal installations where corrosion resistance matters; PVC cowls are a lightweight, cost-effective option for standard residential terraces and balconies.
Not sure which cowl fits your ducting? Match the cowl diameter to your existing pipe or fan outlet size, and use the Duct Fan Size Calculator on this site if you're speccing the ventilation system from scratch.
How to Choose the Right Cowl
Four things determine which cowl is right for your installation:
1. Material
Stainless steel SS304 cowls resist rust and corrosion, making them the best choice for coastal areas, industrial sites, or anywhere exposed to heavy weather. Powder-coated steel adds a painted finish for a neater look on visible facades, in a range of colours to match the building exterior. PVC cowls are lighter, cheaper, and resistant to rust by nature, suiting standard residential use where appearance and budget matter more than industrial-grade durability.
2. Diameter / Pipe Size
The cowl must match the outer diameter of your duct or fan outlet exactly. Common sizes are 100mm (4"), 150mm (6"), 200mm (8"), 250mm (10"), and 315mm (12"). Measure your existing pipe before ordering, or check the outlet size listed on your fan's product page.
3. Mesh Protection
Look for a cowl with built-in bird/insect mesh. This is essential on any duct that terminates at an external wall or roof — without it, birds and rodents can nest inside the ducting and block airflow entirely.
4. Mounting Location
Roof and high-exposure installations benefit from steel cowls for weather resistance, while wall-mounted cowls at lower levels (terraces, balconies) can use PVC or powder-coated steel depending on the look you want.
FAQs
What is a cowl in ventilation?
A cowl is the external cap fitted over the outdoor end of a ventilation duct or exhaust pipe. It shields the ducting from rain and weather, prevents birds and pests from entering, and lets exhaust air escape freely. It's the finishing component on any duct run that exits a building.
What's the difference between a cowl and a grill?
A cowl is typically a rounded or hooded cap designed to sit over the end of a round duct pipe, directing airflow outward while blocking rain from entering. A grill (or vent grill) is usually a flatter, slatted cover, often square or rectangular, mounted flush to a wall. Both use mesh for pest protection, but cowls are better suited to pipe-end terminations while grills suit flush wall openings.
Should I choose a steel or PVC cowl?
Choose a stainless steel SS304 or powder-coated steel cowl for rooftop, coastal, or industrial installations where corrosion resistance and durability matter most. Choose a PVC cowl for standard residential terraces and balconies where cost and light weight are the priority and exposure to harsh weather is limited.
What size cowl do I need?
Match the cowl's diameter exactly to your duct or fan outlet size. Common sizes are 100mm (4"), 150mm (6"), 200mm (8"), 250mm (10"), and 315mm (12"). Measure the outer diameter of your existing pipe, or check the outlet spec on your inline or exhaust fan before ordering.
Does every cowl come with bird mesh?
Not always, so check the product spec before buying. Astberg cowls include built-in bird mesh as standard, which keeps birds, rodents, and larger debris out of the ducting without restricting airflow.
