What is a ventilation duct?
A ventilation duct is a pipe, channel or shaft that carries air into or out of a building, so that damp or stale air is removed and fresh air comes in. The Dutch term ‘ventilatiekoker’ most often refers to a small duct that ventilates the crawl space under a ground floor, but it also covers the ducts of a ventilation system.
Crawl space and underfloor vents
Many Dutch homes have a shallow crawl space under a suspended ground floor. Its ventilation duct runs in an S-shape through the cavity wall: on the outside it starts just above ground level behind a grille, on the inside it ends in the crawl space below any floor insulation. In the UK the same product is known as a periscope vent, fitted behind an air brick under suspended timber or beam-and-block floors. These ducts are usually polypropylene, sometimes stainless steel, and often telescopic. They let ground moisture evaporate and help prevent mould, odours and rot in timber floors.
Vertical extract ducts
Vertical ducts and shafts remove air from kitchens, bathrooms and toilets. With natural ventilation, the temperature difference and wind across the roof create the draw; with mechanical ventilation, a fan does the work. Such ducts often run through a shared shaft serving several homes.
Tips and points of attention
- Never block the grille: not even in winter, or the underfloor ventilation stops working.
- Vermin-proof: use grilles that keep mice out and check them for damage.
- Clean occasionally: ducts and grilles collect dirt. Some extract ducts have an access hatch for cleaning.
- One is not enough: the free airflow per vent is small, so a house needs several.
- Seal around it: fit the duct airtight and watertight, with a cavity tray or mortar guard so falling mortar does not create a cold bridge.
- Fire separation: where a duct passes through a fire-separating wall or floor between homes, the separation must not be weakened. Have penetration seals or fire dampers specified by a competent installer.
- No flue gases: never use a ventilation duct as a flue for a stove or boiler.
Related terms
- Crawl space
- Balanced ventilation
- Natural ventilation
- Mechanical ventilation
- Utility riser shaft
- Fire spread
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.