Roof window ventilation flaps and filters

What the flap on the handle bar actually is

On a hinged pitched-roof unit the control bar runs across the top of the sash. Behind it, moulded into the head of the frame, is a ventilation slot that runs most of the window width. The flap covers that slot. In the closed position it presses against a seal and the window is airtight. Pull it to the first detent and the flap lifts a few millimetres, opening a continuous slot along the head while the locking mechanism stays engaged.

The important part is that the sash is still latched. The window cannot be pushed open from outside, rain cannot drive in past the flashing, and the unit reads as closed to anybody looking at the roof. That is why the position is worth using: it is ventilation you can leave running when the house is empty, which an open sash is not.

Fakro, Keylite, Roto and VELUX all build a version of this, and the geometry differs between them. Some use a sliding vent rather than a hinged flap, some put the slot in a separate strip above the sash, and a few of the flat-roof and fixed products have no vent at all. If you are matching a new unit to an existing one, the vent arrangement is one of the things worth checking, because a room that has been ventilating through a flap for fifteen years will notice if the replacement has none.

Why background air movement matters with the sash shut

A converted loft produces a surprising amount of water vapour and has very few routes out of it. Two sleeping adults put out something in the region of a litre of moisture overnight. That vapour rises, and a loft room is where it collects. If the only ventilation route is a sash somebody opens for ten minutes in the morning, the room spends the other twenty-three hours holding what it made.

Background ventilation is a slow, continuous exchange rather than a purge. It does not clear a room quickly and is not meant to. What it does is stop humidity climbing steadily through the night, which is what puts water on the coldest surface in the room. On a roof window, the coldest surface is usually the lower edge of the glass or the frame rebate, and that is where the first wet marks appear.

The flap position exists so that background ventilation happens without anybody deciding to make it happen. That is the entire design intent. A control that has to be operated daily will not be operated daily.

How much air moves through the slot

Manufacturers publish an equivalent area for the vent, given in square millimetres, and this is the number Building Control cares about rather than any description of the flap. Typical figures for a standard roof window vent land somewhere between 2,500 and 10,000 square millimetres depending on the model and the size, with larger windows generally offering more slot length and therefore more area.

Equivalent area is not the same as the physical hole. It is a measured airflow performance expressed as the area of a sharp-edged orifice that would flow the same amount at the same pressure. A slot with a filter and a right-angle turn in it flows less than its raw dimensions suggest, which is why the tested figure is always smaller than the geometry implies.

If a room is being assessed for background ventilation and the roof window is expected to contribute, ask for the equivalent area of the exact model code rather than the range. It varies by size within the same product family.

The filter behind the slot, and what it catches

Lift the flap and you will see a strip of open-cell foam or fine synthetic mesh sitting in a channel. That is the filter. Its job is to catch airborne dust, pollen and insects before they enter the room, and on a roof window it is doing that in a fairly hostile position: high on a slope, exposed to whatever the wind carries, with no eaves above it to shelter the opening.

It is a coarse filter, not a fine one. It is not doing anything for fine particulates or for smoke, and nobody should specify a roof window vent as an air-quality measure. What it does is stop the slot filling with the material that would otherwise blow into it, and stop flying insects using a 4mm gap as a doorway in July.

What collects on it depends on where the house is. Under trees you get pollen and fine leaf litter, and the filter darkens within a season. On an exposed slope near a road it collects road dust and it goes grey. Either way, the filter is a consumable. It is meant to load up, and a loaded filter is doing its job right up to the point where it stops passing air.

Taking the filter out and cleaning it

On most units the filter pulls straight out of its channel once the flap is fully open, either by hand or with a thin blade lifted under one end. It is held by friction rather than by fixings. Work from a stable position: this is a job at the top of a window, often at the top of a stair, and it is worth having somebody hold the ladder rather than reaching sideways off one.

Cleaning is a vacuum with a brush head, working along the length rather than across it, on the outward face first. If it is heavily loaded, warm water with a little washing-up liquid and a gentle squeeze will shift the rest. Do not wring it and do not scrub it. The foam tears easily once it has aged, and a torn filter no longer seals against the channel, which lets everything past the edges.

Put it back completely dry. A damp filter in a slot is exactly the condition you were trying to avoid, and it will hold that moisture against the frame head for days.

When to replace the filter rather than clean it

Foam filters harden and crumble with age. After roughly eight to ten years of daylight and thermal cycling, the cell structure breaks down and the strip loses its spring, which is when you find crumbs of it on the sash. At that point cleaning is pointless. The strip is a stock item from the manufacturer for current models and generally very cheap.

Replace it earlier if the foam has torn, if it stays discoloured after cleaning, or if closing the flap no longer produces the resistance it used to. A filter that no longer fills the channel is drawing air around itself, which means the slot is effectively unfiltered.

For units older than about fifteen years, the exact filter strip may no longer be made. Generic open-cell foam of the right thickness can be cut to length as a substitute, and this is one of the few things on a roof window a homeowner can sensibly do without a trade visit.

Insect mesh, and what it will not stop

The mesh behind the flap keeps out flies and most flying insects at the size range that would otherwise find the slot. It does not keep out thunder flies, which are small enough to pass through almost any mesh fine enough to still flow air, and it does nothing at all about anything entering through an open sash.

If insects are the actual concern, the answer is a separate insect screen that fits over the whole aperture and rolls down from a cassette at the head, rather than anything happening at the vent. Those are available for most current roof window ranges and fit against the internal face. The vent filter and the insect screen are different products solving different problems, and one is not a substitute for the other.

Where the flap sits in Part F compliance

Approved Document F sets background ventilation requirements for habitable rooms, expressed as equivalent area per room or per dwelling depending on which route the design takes. A roof window vent can count towards that provision, and in a loft conversion it is frequently the only route available, because there are no walls to put trickle vents in.

Two things decide whether it counts. The vent has to be positioned high in the room, which a roof window vent inherently is, and it has to be controllable by the occupant without tools, which the flap is. Where a loft conversion is being signed off, the model codes and their equivalent areas go into the paperwork, and Building Control will look at the total against the room area.

The practical consequence is that specifying a fixed rooflight in a new loft bedroom can create a ventilation shortfall that has to be made up elsewhere. It is worth settling before the order rather than after.

What the flap will not fix

A ventilation flap is not a solution to a room with a serious moisture problem. It moves a modest volume of air continuously, and where the source is a shower with no extract, an unvented tumble dryer, or a room being dried out after works, the flap will be overwhelmed and the humidity will still climb.

It is also not a substitute for mechanical extract in a bathroom or kitchen, where the regulations expect a fan with a rated flow rate and the flap simply provides make-up air. Where somebody tells you a roof window vent alone will handle a shower room, that is not correct.

Nor does it help with water forming between the panes of a sealed unit, which is a failed unit rather than a ventilation matter and is covered on our page about misting inside the glass. The flap only affects the air in the room.

Draughts, whistling and the winter complaint

The most common complaint about a ventilation flap is that somebody has closed it in November and cannot work out why the room is stuffy by February. The second most common is the opposite: cold air falling off the glass onto a bed directly below the window.

That falling air is not usually the vent. It is convection off the inner glass surface, and it happens whether the flap is open or shut. The vent contributes a small stream of outside air along the head, which then mixes and drops. Where a bed sits directly under a window, the answer is usually a blind and a rethink of the layout rather than closing the vent permanently.

Whistling is different and is worth investigating. A vent that whistles in wind is generally partly blocked, so the air is being forced through a reduced gap. Clean the filter and the noise normally goes.

Older windows with a different vent arrangement

Roof windows from the 1980s and 1990s often had no filter at all, just a slot with a flap, and some had no vent whatsoever. Others used a rotating barrel vent at one end of the head rather than a slot along it, with far less open area than a modern flap.

Where we are replacing an old unit, the ventilation gain is often more noticeable to the household than the thermal gain, particularly in a bedroom. A current unit with a full-width filtered slot moves considerably more air than a 1990s barrel vent, and it does it without the draught that came with the old arrangement.

It is worth knowing that some conservation-pattern and flush units, chosen for appearance on visible slopes, offer less vent area than the standard product. If ventilation matters in that room, check the figure before committing to the flush option.

Fixed rooflights and units with no vent

Fixed units have no ventilation at all. That is not a fault, it is the product, and on a landing or over a stair it is frequently the right answer because the ventilation is happening elsewhere in the house.

Where a fixed unit has been put into a habitable room and the room now feels closed in, there is no retrofit vent for the unit itself. The options are a separate wall vent, a mechanical extract, or replacing the unit with an opening one. Replacing is not as dramatic as it sounds where the aperture and the flashing kit are the same family, though the structural opening usually needs checking.

We will say plainly which of those is appropriate. Sometimes the answer is that the room needs an extract fan and the roof window is not the problem.

Electric and solar units, and automatic ventilation

Electric and solar-powered roof windows change how ventilation is handled, because the sash itself can be programmed to open on a timer or on a humidity reading rather than relying on somebody using the flap. The mechanical vent usually still exists on the frame, but it stops being the main route.

A rain sensor closes the sash when it detects water, which makes automatic opening practical on a roof. Without one, an unattended open sash is a risk nobody sensible takes. The sensor sits on the outer face of the sash and needs an occasional wipe to keep working.

The decision that matters for an electric unit is made before the plasterboard goes up: where the cable comes from and where the switch sits. Retrofitting a supply into a finished ceiling is disruptive, and solar versions exist largely because of that problem.

What we check on a ventilation flap at handover

On any unit we fit, the flap is operated through its full travel before we leave, the filter is confirmed as seated along its whole length, and the sash is checked to close and lock with the flap in the closed position. A flap that fouls the sash is usually a filter sitting proud of its channel.

We show the household the vent position and explain that it is designed to be left open. That sounds trivial. It is the single most useful thing to hand over on a roof window, because a vent nobody knows about is a vent nobody uses.

The ten-year workmanship guarantee we give covers the installation and the perimeter detailing rather than a foam filter strip, which is a consumable like a lightbulb. We will tell you which part number it is so you can get the right one.

What to ask before you order

Four questions settle the ventilation side of a roof window specification, and they are quick to answer.

• What is the equivalent area of the vent on that exact model code, not the range?
• Does the room have any other background ventilation, or is this window carrying it?
• Is a filter fitted, and is a replacement strip available as a stock item?
• If a flush or conservation-pattern unit is being specified for appearance, does it still meet the ventilation figure the room needs?

Get those on paper and the vent stops being an afterthought. On our surveys we quote a range by email first, then a fixed price once we have been on the roof and seen what the room and the covering actually require. Call 01865 704245 or email info@heritageskylightsoxford.co.uk and we will tell you what the room needs before anybody orders glass.