What Does the VELUX Ventilation Flap Do?

A VELUX roof window gives you two ways to let air in, and the flap is the smaller of the two. The main opening is the sash itself: on a centre-pivot window it turns on a pivot part-way up the frame, and on a top-hung window it swings out from a hinge at the head. Either way, opening the sash moves a large pane of glass and lets in whatever the weather is doing outside. The ventilation flap sits apart from that, built into the bar that also carries the operating handle at the base of the sash on most manual ranges. A small lever within that bar lifts a narrow flap behind an insect screen, opening a slot the width of the window without moving the glass at all.

Because it sits low, in the same bar as the handle, rather than as a glazing bar crossing the middle of the pane, the flap does nothing to the view of open sky above it. A centre-pivot or top-hung VELUX keeps a clean line up through the glass whether the flap is open or closed, which is part of why we plan the handle position and reveal together on the sightline sketch for a loft room rather than treating them as separate decisions.

A room that relies on a roof window as its only source of fresh air needs some ventilation that does not depend on the window being fully open, and that is the job the flap does. Current Building Regulations expect habitable rooms to have background ventilation that keeps working in ordinary use, not only when someone remembers to open a window, and a flap built into the handle bar gives a loft bedroom or landing exactly that: air moving gently through the room around the clock, in a way that does not need to be switched on or thought about.

That matters more in a new-town loft conversion across Bretton, Orton or Werrington than it might in an older house with a chimney and looser floorboards, since a well-sealed modern roof holds moisture in rather than letting it drift away on its own. A flap that stays open through the day and night keeps a slow exchange of air going, which is also part of what keeps condensation off the glass; our guide to why a VELUX window gets condensation goes into how ventilation and heating work together on that.

The lever that controls the flap moves through a short range rather than a single click, so it can sit anywhere between fully closed and fully open. Closed, it seals against the frame and behaves like any other well-fitted window: no draught, no whistle in a strong wind. Opened even slightly, it lets a background trickle of air through behind the screen, and opened fully it gives noticeably more airflow without exposing the room to rain the way a fully open sash would in a shower.

• Closed for a hard frost, a high pollen count or a very windy night.
• Part open for ordinary day-to-day use through most of the year.
• Fully open on a warm, still day when the main sash is also going to be used.

The flap is background ventilation, not a way to clear a steamed-up bathroom or a smoky kitchen quickly. For that, the main sash still needs to open fully so a real volume of air can move through in a short time. Leaving the flap open through winter is sensible; relying on it instead of opening the window after a shower or a pan left on too long is not, and no flap is built to do that job on its own.

It is also worth knowing that not every VELUX range ventilates the same way. Flat roof windows move air differently from a pitched centre-pivot or top-hung window, which our guide to VELUX flat roof windows against other flat rooflights sets out, and an electric INTEGRA window can pair the flap with a rain sensor and automatic control rather than a manual lever, covered on our page about INTEGRA electric and solar opening. Either way, the principle is the same: a small, controllable opening doing a different job from the main sash.

The flap and its insect screen sit in the path of whatever the room's air carries, so dust settles there the same way it does on a windowsill. A light wipe as part of ordinary cleaning keeps the slot moving freely and the screen clear, and our guide to VELUX window maintenance covers this alongside the hinges and seals that also want occasional attention. A flap that feels stiff or will not sit fully closed is usually a cleaning job rather than a sign of anything more, though it is worth pointing out at a service visit if it does not settle after a clean.

The flap also plays into how a window holds its heat. When it is shut, it forms part of the seal that VELUX's own frame design is built around; our guide to VELUX ThermoTechnology explains how the frame and glazing work together to hold warmth, and a flap that seals properly when closed is part of that picture rather than separate from it.

Whether the flap suits the room it serves is one of the smaller questions we work through on a VELUX survey, alongside the opening type, glazing and where the frame sits in the roof. Our VELUX window installation page sets out how that survey runs from first visit to finished job, and our guide to GGL or GPL, centre-pivot or top-hung covers how the two main manual ranges differ, flap included, by handle position and headroom.

Does an open flap let rain track through in a shower?

In ordinary rain, no: the flap sits behind an insect screen and is designed to stay open through normal weather. In wind-driven rain or a heavy downpour it is worth closing, the same as you would with any small vent.

Can I leave the flap open all winter?

Yes. Background ventilation is exactly what it is there for, and leaving it part open through the colder months is normal use rather than something to correct, though closing it on a sharp frost is a matter of comfort rather than necessity.

Does every VELUX window have this flap?

Most manual centre-pivot and top-hung ranges do. Flat roof windows and some electric models ventilate differently, so it is worth checking against the specific range rather than assuming every roof window works the same way.

Will a VELUX INTEGRA window still have a flap I control by hand?

Often it works alongside automatic control rather than replacing it entirely; our guide to INTEGRA electric and solar opening covers how the sensors and remote control fit around background ventilation on those models.