Roof window ventilation flaps and filters

Where the flap sits, and the path the air takes

On a centre pivot or top hung roof window the flap is in the top rail of the sash, usually operated by the same bar you grip to open the window, or by a small slider next to it.

The air route is not a straight hole through the sash. Outside air enters through a slot on the external face, turns and travels through a chamber inside the top rail, passes the filter, and leaves through a slot on the room side. That dog leg is deliberate. It slows the air, it stops rain being driven straight in, and it makes the incoming air give up some of its water before it enters the room. It also means the flap has depth, which is why it can be blocked internally without anything looking wrong from below.

Equivalent area, and the number that appears on a building notice

Ventilation openings are specified by equivalent area, measured in square millimetres, and it is not the same as the physical size of the slot. Equivalent area is the size of a sharp edged hole that would pass the same amount of air at the same pressure, which accounts for the resistance of the dog leg, the filter and the mesh.

A roof window flap typically declares somewhere in the region of 2,000 to 6,000 square millimetres of equivalent area depending on the size of the unit and the range. The figures are published per size code and they are what a Building Control officer will want against the room, since the whole room has to reach the required total. A larger window generally offers a larger flap, which is another reason the size code decides more than it looks like it does.

Background ventilation and purge ventilation are two separate duties

These get conflated and they are not the same thing at all.

Background ventilation runs continuously, twenty four hours a day, at a low rate. Its job is the slow removal of the moisture and stale air a household produces overnight, and it is met by trickle openings such as the flap in the roof window. Purge ventilation is a burst: the window opened properly for a few minutes to clear a bathroom after a shower, a kitchen after cooking, or a bedroom that has gone stuffy. Its job is rapid dilution.

A room needs both. A window that opens beautifully and has its flap taped shut fails the first duty completely, and the record of that failure appears as condensation on the coldest surface in the room, which is the glass overhead.

The one twentieth rule, and how a roof window contributes to it

Purge ventilation to a habitable room is commonly satisfied by an openable area of at least one twentieth of the floor area of that room. For a 12 square metre loft bedroom that is 0.6 square metres of openable area.

The openable area of a roof window is not its frame size. On a top hung sash opened to its normal stop, the clear aperture is a good proportion of the frame. On a centre pivot, the sash sits across the middle of the opening, so the openable area is the two slots above and below it added together. That distinction affects the purge calculation exactly as it affects the escape calculation, which is covered on the escape openings page.

Ventilating with the window locked shut

The practical value of the flap is that it works while the window is closed and locked. Nobody leaves a roof window ajar in February, and nobody leaves one ajar when the house is empty, so a ventilation strategy that depends on somebody opening something is a strategy that lapses.

Most ranges also have an intermediate position: a locking catch that holds the sash slightly off its seal, giving considerably more air than the flap alone while the window is still secured against being pushed open from outside. On a bedroom in a warm week that is the setting people actually use, and it is worth being shown where it is at handover, because it is not obvious.

The filter, what it collects, and why it exists

Sitting in the air path inside the top rail is a filter, usually a strip of open cell foam or a fine synthetic mat, held in a slot behind a clip on cover. It is there to stop the incoming air bringing dust, soot, pollen and insects into the room.

It works well and it loads steadily. Over a year it goes from white or pale grey to a uniform dark colour, and as it loads the equivalent area falls, so the ventilation the window is credited with quietly stops being delivered. A filter left for ten years is often a solid felted mat with no measurable airflow through it at all, and the owner has no idea it is there.

The insect mesh, and what happens when it blinds over

Behind the external slot there is normally a fine mesh screen, and it is the part that causes the most complaints because it is the hardest to reach. In a Gloucestershire loft it collects seed fluff in June, aphids in July, and a mat of small flies and cobweb by September.

A blinded mesh has exactly the same effect as a blinded filter, and it can be worse, because it also traps water in the chamber. Clearing it means a soft brush and a vacuum nozzle at the external slot with the sash rotated, which is the other reason the annual clean is done with the sash in the cleaning position rather than from inside with the window shut.

Wasps, and the chamber they will move into

An open ventilation chamber in the top of a sunny south facing sash, warm and sheltered and out of the wind, is an attractive place for a wasp queen in spring. It happens, and it is far more common where the external mesh has torn or been removed.

The cure is to keep the mesh intact. Where a mesh is damaged it should be replaced rather than left, because the flap is then a direct route into the room as well as a route for insects into the wall of the sash. It is a small part and it is cheap; the annoyance of not replacing it is not.

The flap on a centre pivot against the flap on a top hung sash

Both designs carry a flap in the top rail, but they behave differently once the window is also being opened.

A centre pivot sash, opened a little, produces two slots: one at the top pushing air out and one at the bottom drawing air in. That is genuinely efficient at moving air, because it sets up a circulation across the sash itself. A top hung sash opened a little produces one wide mouth at the bottom, which is a stronger single stream but less of a circulation.

With the window closed and only the flap running, the two are effectively the same. The difference only appears once the sash moves, and it is one of several points that separate the designs on the centre-pivot and top-hung comparison page.

Rain, wind, and the flap left open in a gale

The dog leg air path is designed so that rain driven against the external slot cannot get through in a straight line, and in normal weather a flap left open does not admit water.

Severe driven rain from the prevailing south west, hitting a slope square on, is a different matter, and a small amount of water can reach the internal slot on an exposed elevation. The Forest of Dean and the higher wolds villages see the worst of that. Where a window faces the weather on an exposed site, closing the flap during a named storm is sensible, and reopening it afterwards is the part people forget. The same wind that drives rain in also increases the flow through the flap considerably, so an exposed window on a windy night can feel draughty at a setting that is comfortable in still air.

Humidity sensors, electric openers and automatic control

A vent that has to be reached is a vent that gets used until the novelty wears off. Above a stairwell, over a landing, or high on a steep Forest of Dean slope, the flap and the opening handle are both out of reach from standing height.

The answers in order of cost are a telescopic rod, which works and is tedious; a rod operated flap linkage; and an electric unit with a remote. The electric version can be driven by a humidity sensor, so the window opens when the room passes a set relative humidity and closes again when it drops, and by a rain sensor that shuts it the moment water lands on the glass. On a bathroom or a kitchen that is the arrangement that actually gets used, because it happens without anybody deciding to do it. Powered options are on the electric rooflight installation page.

Night cooling in a loft conversion, and the stack that does the work

The flap is a background device and it will not cool a room. Purge will, and a room in the roof has the best geometry in the house for it.

Warm air collects at the highest point of a building, which in a loft conversion is directly under the roof window. Open the roof window and open a window at a lower level, and the temperature difference drives air out of the top and pulls cooler air in at the bottom. That stack effect moves a great deal more air than an open window on its own, and it works best in the two or three hours after sunset when the outside air has dropped and the room has not.

An insect screen in the same channels as the blind is what makes leaving it open overnight realistic in the vale, where the still summer air over the river brings a good deal of insect life up to a lit window.

Trickle vents elsewhere, and why the roof window is not the whole answer

The ventilation provision for a room is a total, and the roof window flap is one contributor to it. On a loft conversion where the roof windows are the only openings, they carry the whole background requirement, and the sizes have to add up to the figure the room needs.

On a room that also has a gable window or a dormer, the provision is shared. What matters is that nothing in the chain has been quietly closed off, and in older houses that is exactly what has happened: replacement windows fitted without trickle vents, an airbrick blocked behind a fitted cupboard, a chimney capped during a refit. Each removed a path air used to take, the house got tighter, and the moisture generated inside now has fewer ways out.

Bathrooms, and the flap that is not a substitute for extraction

A shower room or bathroom in the roof needs mechanical extraction at the rate guidance sets for the room, and the roof window flap does not replace it. What the flap does is provide the background air change between showers, which is when the residual moisture in the plaster, the towels and the grout is drying back into the room.

A roof window in a bathroom is genuinely useful for purge as well, since a fully opened sash clears a steamy room faster than most extract fans. The specification worth having is both: an extract fan with an overrun, and a window with a working flap, on a unit finished in polyurethane rather than bare timber so the moisture does not get into the sash. Wet room considerations are on the bathroom rooflight page.

Kitchens, grease, and the filter that loads twice as fast

Where a roof window sits over a kitchen, the filter has a harder life. Cooking aerosol carries fine grease into the air, it rises, and some of it condenses in the coolest part of the room, which is the top of the sash. Grease and dust together form a sticky film that foam does not wash out easily.

Two consequences. The filter wants doing twice a year rather than once in that position, and it wants replacing more often than the manufacturer’s general guidance suggests. And a recirculating cooker hood makes it worse, since it returns everything except grease and smell to the room, including all of the water vapour. Whether a hood ducts or recirculates is worth establishing before anything else is blamed.

The Severn Vale, river mist, and the filter that loads faster still

Gloucester, Tewkesbury and the vale villages sit low, beside a tidal river, on ground that holds mist through the night for much of the winter. External relative humidity on a still winter night is frequently at or near saturation, and the air the flap is admitting is already carrying as much water as it can hold.

That does not make background ventilation pointless, because the air still warms as it enters and its capacity rises with it, so it can pick up moisture from the room and carry it away. It does mean the flap will not perform miracles on the coldest, stillest, dampest nights, and a border of condensation on the glass on a hard frosty morning in the vale is normal rather than a defect. It also means damper air passing through the filter for months on end, which is why vale lofts load their filters faster than hillside ones.