Ventilation flaps and filters

What it is for

Background ventilation. A continuous trickle of outside air that dilutes moisture and stale air in the room, running all the time rather than in bursts when somebody opens a window.

In a loft bedroom that matters more than people expect. Two adults asleep produce a significant amount of moisture overnight, and a sealed room with a cold roof window is exactly where that condenses.

The filter

Behind the flap is a filter catching dust, pollen and insects. It works, and it clogs.

A clogged filter reduces airflow to almost nothing, and the usual first sign is condensation appearing on the glass on cold mornings when it never used to. People conclude the window has failed.

The filter pulls out on most models and can be vacuumed or rinsed and dried. Once a year is plenty. It is the single most neglected part of a roof window and it takes about two minutes.

Trickle vents and the regulations

Current requirements expect background ventilation in habitable rooms, and where a roof window is the main opening its ventilation flap may be counted towards that.

Where a window is being replaced in a habitable room, the replacement generally needs to provide background ventilation no worse than what it replaced. In practice any modern unit with a flap satisfies this.

Leaving it open

The flap is designed to be left open permanently, and on most units it is weathered so rain does not enter through it. That is the intended use, not an oversight.

Closing it in winter to keep heat in is a false economy: the heat saved is small and the condensation caused is not.

The part of a roof window nobody looks at

Open the sash of almost any pitched roof window and look at its top edge. There is a flap, usually with a removable strip covering a filter behind it.

That is the trickle ventilator, and it provides background ventilation with the window closed and locked. It is operated either by turning the handle to an intermediate position or by a separate catch, depending on the model.

It is one of the more useful features on a roof window and it is routinely ignored, painted over, blocked or left clogged.

Background against purge

Building Regulations separate two provisions, and the distinction explains why both a flap and an opening sash exist.

Background ventilation is a small continuous provision, keeping air moving and controlling moisture without you having to think about it. That is what the trickle ventilator does.

Purge ventilation is the ability to clear a room rapidly: after a shower, while cooking, or on a warm evening. That is what opening the sash does, and it is calculated on free open area rather than on the size of the unit.

The two are not interchangeable. A trickle vent will not clear a bathroom after a bath, and an opening sash left shut provides no background ventilation at all.

Why background ventilation matters more than it used to

Because homes have got tighter.

Older houses leaked air constantly through gaps in the fabric, around windows, under floors and through chimneys. That was wasteful and it meant moisture produced indoors left continuously.

Modern homes, and older homes progressively draught proofed and double glazed, do not leak in that way. Moisture produced by cooking, washing, drying clothes and breathing stays in the building until something removes it, and it condenses on the coldest surface available.

In a room with a rooflight, that is frequently the rooflight, because it faces the cold night sky. So the window gets blamed for revealing a problem it did not cause.

A trickle ventilator left open through the heating season removes a surprising amount of that moisture, and the heat lost through it is small by comparison.

The filter, and what happens when it clogs

Behind the flap on most designs sits a filter, there to stop insects and coarse dust entering the room.

Over a few years it clogs. When it does, the ventilation stops working, and the room gets damper without anybody connecting the two things. People then report condensation on the glass, and the cause is a filter nobody has touched since the window was fitted.

Cleaning it is a minute’s work once a year. Remove the strip, take the filter out, vacuum it or rinse and dry it depending on the type, and refit.

Do it at the same time as cleaning the gasket, so it is not forgotten. Both are part of the same short annual routine.

The mistakes we find

Painted shut. Redecorating a room and painting the frame without masking the ventilation flap seals it permanently. It also frequently seals the filter housing so the filter cannot be removed.

Blocked deliberately. Somebody feels a draught, stuffs the vent, and the room starts condensing. The draught was doing a job.

Never opened. On many windows the flap is closed as delivered and nobody realises it is there. Where a room has been condensing since the day the window went in, this is worth checking before anything else.

Confused with the handle position. On some designs the handle has an intermediate ventilation setting that holds the sash slightly open on a catch. That is a different thing from the trickle vent, and both are useful.

The ventilation catch position

Worth understanding because it is genuinely useful and it doubles as a safety feature.

Almost every roof window has a setting where the sash is held slightly open on a catch, providing more airflow than a trickle vent while the window remains secure against a casual push. It cannot be opened further without operating the handle deliberately.

In a child’s bedroom that is the setting to use routinely: background ventilation, and a sash that is not free to swing.

In any room it is the setting for a mild day when you want air without leaving a window fully open.

Flat roof rooflights

Fewer have integral trickle ventilators, and fixed units generally have none at all.

That means the background ventilation for the room has to come from elsewhere: wall vents, other windows, or a mechanical system.

On an extension where the rooflights are the main glazing and the rear wall is largely bifold doors, it is easy to end up with a well sealed room and no background provision whatever. That is worth checking rather than assuming, because it is exactly the situation that produces condensation on a new rooflight and a complaint about the glazing.

What a rooflight cannot do

Replace an extract fan.

In a kitchen or a bathroom, a fan sized for the room, on an over run timer or humidity sensor, ducted to outside rather than into a loft, is what actually keeps the room dry. Trickle ventilation supplements it and an opening sash clears the room quickly, and neither substitutes for it.

Ducting a bathroom extract into a loft rather than through the roof or wall is common on older installations and it puts the room’s moisture straight into the roof space.

What to do with yours

Find it. Open the sash and look at the top edge.

Open it, and leave it open through the heating season. The heat lost is small and the moisture removed is not.

Clean the filter once a year, alongside cleaning the gasket and, on a motorised unit, wiping the rain sensor.

And when the room is next decorated, mask the flap and the filter housing before anybody picks up a brush.

What the flap is doing

Most roof windows have a ventilation flap built into the head of the sash, operated by the same control bar that opens the window. It admits a small continuous flow of outside air with the window shut and locked.

That is background ventilation, and it is a different thing from purge ventilation, which is what opening the window does, and different again from mechanical extraction.

Its value is in a loft bedroom, where the alternative is leaving a window open overnight for air. The flap gives you air exchange without an open window, without the rain risk, and without the security question.

The filter nobody knows about

Behind the flap sits a filter, and its purpose is to stop dust, pollen and insects coming through with the air.

It collects all three, and over a few years it blocks. When it does, the ventilation stops working and there is no signal at all. No noise, no visible change, nothing. The flap looks open and nothing passes through it.

A great many houses have roof windows whose trickle ventilation has been doing nothing for a decade.

Cleaning it

Open the sash, rotate it if it is a centre pivot, and look at the head. The filter clips or slides out, usually without tools.

Wash it in warm water, let it dry completely, and put it back the same way round. Some are replaceable rather than washable, and manufacturers sell them cheaply.

Once a year is plenty. In spring, after the pollen season, is the sensible time.

Why it matters more than it sounds

A room that cannot lose moisture will condense it on the coldest surface available, and the coldest surface in a loft bedroom is the rooflight.

So complaints about condensation on a roof window are frequently complaints about ventilation, and a blocked filter is a genuine contributor. Restoring the background ventilation sometimes resolves the condensation without touching the glazing at all.

On flat rooflights

Fixed flat rooflights generally have no ventilation provision at all, so the requirement for the room is met elsewhere.

Where the rooflight is the only opening in a room, a comfort vent or an opening unit is worth specifying rather than assuming the room will manage. That is a decision made at order, not afterwards.

The one thing not to do

Paint over the flap or the filter housing during decorating. Paint on the mechanism stops it operating, and paint in the filter housing is not removable.

Masking properly is most of the work of decorating around a roof window, and rushing it is how a decorating job becomes a repair.

Using the flap through the year

In winter, leave it open. Background ventilation is what removes the moisture a household generates, and it is a small enough opening that the heat loss is modest against the benefit.

A bedroom with the flap open all winter is markedly less prone to condensation on the glass than the same room with it shut, and the difference is noticeable.

In summer it contributes very little, because purge ventilation through an open window does the work instead.

The mistake is closing it in autumn to keep the room warm and forgetting it until spring, which is when condensation complaints arise.

Security

The flap operates with the sash closed and locked, which is the point. It gives ventilation without an open window, so it is usable overnight and while the house is empty.

On a ground floor or accessible rooflight that matters, and it is a genuine advantage over leaving a window ajar.

Noise

An open flap admits sound as well as air, and under a flight path or beside a busy road that is noticeable at night.

Where that is an issue the answer is usually an acoustic laminate in the glass rather than keeping the flap shut, because closing it trades a moisture problem for a noise one.