Guide · VELUX Window Installation
Roof window ventilation flaps and filters
The bar you grip to open a roof window is hollow, and inside it there is a flap. Pull the handle to its first position and the flap lifts, opening a slot that runs the width of the sash while the window itself stays shut and locked. Behind that slot sits a removable air filter.
It is the least discussed part of a roof window and one of the most useful, because it is the only way a window at high level provides continuous ventilation without being open to rain, wind or anybody who fancies a look at the loft. This page covers how the flap works, how much air it actually moves, what the filter does, and where the arrangement fits against Building Regulations. The wider installation is on the VELUX window installation page.
What sits inside the top bar
Three components in a shallow box. There is the flap itself, a hinged or sliding panel operated by the handle mechanism. There is an air channel behind it, formed in the sash profile, that leads from the outside face through to a slot on the room side. And there is a filter, a rectangle of open-cell foam or fine mesh sitting in a carrier that clips out.
The path is deliberately not a straight line. Air entering from outside turns upwards, crosses the top of the channel and turns back down into the room, so water running down the outer face of the glass cannot follow it. That geometry is why the flap can be left open in weather that would have the window itself shut.
Ventilating with the window locked shut
This is the point of the whole arrangement. The handle has a position between fully closed and open in which the sash is still latched into the frame and the flap is lifted. Nobody can get in, nothing can blow open, and air moves continuously.
For a loft bedroom that means the room ventilates overnight without an open sash, which matters on a ground-accessible slope and matters more when the forecast turns. For a room that is empty during the day it means air keeps moving while the house is locked up, which is exactly the condition in which stale air and condensation build.
The flap is the setting to leave a house on, and it is the one most owners never discover.
How much air the flap actually passes
Ventilators are rated by equivalent area, expressed in square millimetres, which is the area of a sharp-edged hole that would pass the same amount of air under the same pressure. It is not the same as the visible size of the slot, because the turns in the air path cost flow.
A roof window flap on a typical domestic unit delivers a useful fraction of what a habitable room needs, and the figure is published by the maker for each size. Larger sashes give more, because the slot runs the width of the sash. What that means practically is that the flap is counted alongside the other background ventilators in the room rather than being assumed to do the whole job on its own.
Where the flap counts towards the Regulations
Approved Document F sets background ventilation for dwellings. Where a window is being replaced in an existing home, the guidance figures for a background ventilator are 8,000 square millimetres of equivalent area for a habitable room and 4,000 for a kitchen, utility room or bathroom.
Those are the numbers a Building Control body will look for, and the roof window flap contributes its published figure towards them. Where the flap alone falls short, the balance comes from ventilators elsewhere in the room. Establishing this at specification stage rather than at inspection is straightforward, and the notification for the rooflight work is handled as part of the job.
What the filter is actually stopping
The filter is a coarse one and it is not there to clean the air chemically. It stops what would otherwise blow straight through a 20 millimetre slot: insects, spiders, wind-blown seed, leaf fragments, road dust and, in the Essex arable belt, a great deal of pollen and chaff at harvest.
Anyone who has lived under a roof window without a filter, or with one that was thrown away during decorating, knows what the difference looks like on the sill in August. The filter also breaks the airflow up, which reduces the draught sensation of cold air arriving in a narrow jet.
Getting the filter out, and how often
Open the window fully and rotate the sash so the top bar comes to a workable height. The filter carrier clips out of the bar without tools, usually with a squeeze at each end.
Once a year is enough in an ordinary suburban house. Twice is better on a roof near an unsurfaced lane, near open farmland, or over a kitchen. It washes in warm water, and it goes back only when it is genuinely dry, because a damp foam filter sitting in a sash is the beginning of a smell that nobody traces for months. Replacement filters are ordered against the window size code, which is explained on roof window sizes and how they are coded.
| Situation | Filter interval | Why |
|---|---|---|
| Suburban bedroom | Annually | Dust and insects only |
| Kitchen or utility | Twice a year | Grease carried in the air |
| Open farmland nearby | Twice a year | Pollen, chaff, harvest dust |
| Bathroom or shower room | Twice a year | Constant moisture in the channel |
| Beside an unsurfaced lane | Twice a year | Fine grit driven onto the slope |
Draughts in January, and closing the flap properly
A background ventilator is a hole, and in a cold snap with a north-easterly running up the Blackwater it will be felt. The flap closes fully at the locked handle position, and closed means closed: there is a compressible seal along the flap that is designed to shut the channel off.
Where a room feels draughty with the flap apparently closed, the usual causes are a filter carrier not fully seated, a seal that has been painted, or a handle mechanism out of adjustment so the flap is not being pulled all the way home. All three are quick to sort. What is not a fix is stuffing the channel, because it blocks the drainage of the air path as well as the airflow.
Rain, wind and whether the flap lets water in
Under normal conditions, no. The reversed path and the external hood over the slot mean water on the glass runs past the opening rather than into it, and the channel drains outwards.
Two conditions change that. Driving rain arriving horizontally at the slot, which on an exposed slope facing the prevailing south-west can push moisture into the channel, and a filter that is soaked and holding water in place. In an exposed position on the coastal side of the county, closing the flap during a storm is sensible, and reopening it afterwards restores the ventilation. Any water that does enter reaches the channel, not the room, provided the drainage slots are clear.
Noise through an open slot
A slot in a sash is also a path for sound. On a quiet suburban slope it is inconsequential. Under a flight path, beside the A12 or on a main road through one of the market towns, the difference between flap open and flap closed is audible.
The filter itself damps some of it, and a foam filter damps more than a mesh one. Where road noise is a live concern, the specification that actually works is an acoustic laminated inner pane in the glazing, with the flap used at night when the road is quiet. Glazing choices are covered on energy efficient rooflights.
Bathrooms, kitchens and the flap against a fan
A flap is background ventilation. An extract fan is purge ventilation. They do different work and a room with a lot of moisture usually wants both.
The fan removes the burst of moisture from a shower or a hob quickly and dumps it outside. The flap keeps a slow air change running for the hours afterwards, which is what actually stops moisture settling into the fabric of the room. A bathroom under an Essex loft slope, with a warm humid air mass and a cold roof plane above it, is the exact condition in which continuous background air movement earns its place.
Working with an opening lower down the room
Warm air rises, so an opening at the highest point of a room does the most work, and the flap sits at the top of a window that is already high in the roof. On its own it will still move air, driven by the temperature difference across it.
Give it a partner and it becomes far more effective: a window or trickle vent low in a wall provides make-up air, and the two together set up a continuous slow flow through the room. In a loft room with a gable window and a roof window, opening the gable vent and the roof flap together shifts more air than either does alone, with no fan and no electricity.
Condensation, and the part the flap plays
Surface condensation on the room side of roof glass happens when moist air meets a cold surface. Warmer glazing helps. Moving the moist air out before it reaches the glass helps more, and that is what continuous background ventilation is for.
Where a loft room mists up every winter morning, the sequence to check is: is the flap open at all, is the filter clogged, and is there make-up air anywhere else in the room. Those three account for a large share of complaints about condensation on roof windows, and none of them involves changing the unit.
Electric units, and ventilating without touching anything
On a motorised roof window the flap function is generally replaced by the window’s ability to open a few millimetres on command. A programmed micro-opening for a set period, or triggered by a humidity or carbon dioxide sensor, gives the same background ventilation with better control and closes automatically when the rain sensor is wet.
That arrangement suits windows too high to reach, which is often the point of motorising them in the first place. The wiring and control side is covered on electric rooflight installation, and the cable is worth running at first fix whether or not the actuator is fitted straight away.
How this differs from a trickle vent in a wall window
A trickle vent sits in the head of a wall window frame and passes air horizontally through a short straight route. A roof window flap sits in the sash, on a sloping plane, with a reversed path and a filter in it, and it is opened by the same handle that opens the window.
Two consequences follow. The roof flap is at the highest point of the room, so it works with the temperature difference rather than against it, and it moves air even on a still day when a wall vent does very little. And it is adjustable in a way a wall vent generally is not, because the flap has a fully closed position that a trickle vent’s small slider does not really match. The trade is reach: a wall vent is always within arm’s length, and a roof flap on a high slope is not.
Older windows already in Essex roofs
Roof windows fitted through the eighties and nineties across the loft conversions of the commuter towns have flaps too, though the mechanisms differ and the filters were a simpler affair. Many have lost the filter entirely, and some have flaps that no longer close against a perished seal.
Filters and seals for older generations are often still available against the original size code. Where they are not, and where the glazing is misting between the panes as well, the sensible moment to bring the ventilation up to current standards is at replacement, since the opening is already there and the new sash arrives with the flap, the filter and the published equivalent area on the data sheet.
What to settle before the window is ordered
Three questions. What equivalent area does this room need, and what does the flap on the chosen size deliver towards it. Does the room have any other source of make-up air, or is the roof window the only ventilation in it. And can somebody actually reach the handle to change the setting, because a flap on a window three metres up is a flap that will be left in whatever position it was in on handover.
Answer those at specification and the ventilation works from the first week. Leave them and the usual result is a room that is stuffy, a filter nobody has ever seen, and a window that only gets opened when it is already too warm.
Keep reading
More on velux window installation
Roof window escape openings and the rules
Read the guide →Roof window blinds and when to specify them
Read the guide →Roof window flashing kits by roof covering
Read the guide →Roof window sizes and how they are coded
Read the guide →Roof window cill height and the view out
Read the guide →
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