Opening rooflight ventilation: letting warm air out through the roof

Heated and humid air rises, and the top of a room is where it gathers. A vent in the ceiling plane lets that air leave without having to be pushed sideways first.

Stand in a kitchen after a pan of pasta has boiled. The steam lifts, spreads across the ceiling and settles there. A wall window opens at head height at best, so the warmest layer above it stays where it is. An opening skylight sits above that layer. Once it is cracked open, the air that has been collecting under the ceiling starts to drain upwards through the gap.

This is called the stack effect. The taller the column of warm air and the bigger the difference between the inside and outside temperature, the harder it pulls. Because the air that leaves has to be replaced, cooler air is drawn in from somewhere lower: an open door, a window, a trickle vent. The rooflight does not blow anything. It simply gives the rising air an exit and lets the house supply the rest.

A few consequences follow from the physics:

• The higher the opening sits above the floor, the stronger the pull.
• Air needs a way in as well as a way out. A rooflight opened in a sealed room will do very little.
• The effect is strongest when the room is warmer than the air outside, and fades on a still, mild evening when the two are close.

Building Regulations Part F separates the slow, constant air change a home needs from the fast clearing of a room after cooking or decorating. An opening rooflight belongs to the second group.

Approved Document F describes three kinds of ventilation. Whole dwelling ventilation is the low background flow that keeps air fresh all day, usually through trickle vents or a mechanical system. Extract ventilation removes moisture and smells at the source, from a hob, a shower or a utility room. Purge ventilation is the big, occasional flush: you open everything and the air in the room is replaced quickly.

An opening skylight is a purge device. It is there for the steamy hour, not the whole day. The guidance treats an openable window or rooflight as acceptable purge ventilation for a habitable room when its openable area is at least one twentieth of the floor area of that room. A kitchen of 20 square metres therefore wants around one square metre of genuinely open area. The word that matters is open. A large rooflight that tilts up by a hand's width gives much less free area than its glass size suggests, and the figure that counts is the gap the air can pass through.

Extract rates for kitchens and bathrooms are set separately. The published figures are 30 litres a second from a cooker hood that extracts to the outside or 60 litres a second from any other kitchen fan, and 15 litres a second for a bathroom. A rooflight does not replace those fans where the rules call for them. It sits alongside them and deals with the moments when a fan alone is not enough.

The way a rooflight opens decides how much air it moves, how it behaves in a crosswind and how easy it is to seal when it shuts. The four common types give quite different free areas.

A sliding unit gives the biggest clear opening because one pane moves completely clear of the other. A hinged unit is simpler and sheds rain more readily, but the gap at the open edge is a wedge rather than a window. If a room relies on the rooflight for its purge air, the wedge needs to be wide enough to reach that one twentieth figure. We work through the choice in more detail in the guide to hinged or sliding opening rooflights.

A flat-roofed kitchen extension is where opening rooflight ventilation gets the most use. The room has a hob, a kettle, a dishwasher and a lot of glass, and the ceiling is often the highest point in the house.

Think of a 1960s semi with a kitchen added at the back. The old rear wall is now an internal wall and the new room is open to the garden on one side. Cooking heat and steam rise to a flat ceiling with a lantern or a single rooflight in it. If that glazing is fixed, the hot air has nowhere to go, the room warms by the afternoon and the extract fan works alone. If one panel opens, the same air leaves by the shortest route.

Position matters. The opening sits best over or just beyond the cooking area so the steam has the shortest climb. A vent directly above the hob can bring its own problems with rain and with greasy vapour, so it is usually better placed a metre or two to one side, drawing the plume across the ceiling. We go into the practical layout of these rooms in the guide to an electric rooflight over a kitchen extension.

Rain is the other half of the kitchen question. A rooflight that has to be shut every time a shower passes soon stops being used. That is where a motor and a sensor help, which is covered in the page on rain sensors on electric rooflights.

The stair is a chimney already. Warm air from every floor drifts up it and collects at the top, which is why the landing is often the warmest and stalest place in the house on a summer evening.

A rooflight above the stairwell opens that column at the top. Open it and the landing starts to draw air up from the hall, which pulls cooler air in through the open ground floor windows. Nothing mechanical is involved. It works the way a traditional lantern or cupola worked on a Victorian house, only with modern seals and a proper upstand.

Height is the reason this works so well. A stairwell in a three-storey house can be seven or eight metres from the hall floor to the glass, and that long column gives a strong pull. The opening also sits out of reach, so it is worth thinking early about how it will be operated. A manual unit on a pole is fine for some households. For a high, awkward position a motor with a wall switch is more realistic, and the guide to electric rooflights for stairwells and landings covers the controls and the safe reach.

A loft conversion sits directly under the roof covering, so it takes the day's sun first and keeps it longest. An opening skylight here is the simplest form of cooling the room can have.

The lower part of a sloping ceiling stays cooler while the upper part fills with warm air. One opening rooflight high on the slope lets that warm layer out. A second opening lower down, or a wall window on the far side, sets up a through draught that feeds it. The pair work much better than either does alone, because the air has a clear route in and a clear route out.

The rules for overheating, set out in Part O of the Building Regulations, reward exactly this arrangement in new dwellings, though they do not apply to extensions. In practice the choice comes down to the free area that opens and whether the openings are in different faces of the building. We explore the summer case separately in the guide on whether an opening rooflight helps cool a house.

Glass choice and opening go hand in hand here. A rooflight specified with solar control glazing cuts the heat that comes in, and the opening lets out the heat that still does. Use both, and the room stays usable on the warmest days.

An open rooflight is open to sound as well. Closed, the glazing build, the seals and the upstand do the work. Open, none of them can.

That matters in Crawley. Gatwick sits inside the borough, and the Local Plan itself treats the land between the town and the airport as heavily constrained by aircraft noise. Departures and arrivals pass over the loft bedroom a rooflight serves, and the M23, the A23 and the Brighton main line add their own corridor of sound. Open the glass and you hear what the roof was keeping out.

The answer is timing and choice. Open the rooflight when the room is steamy or hot and the sky is quiet. Close it when you want to sleep through a departure. For a bedroom, think about a vent that opens a narrow gap rather than a full lift, since it passes a steadier trickle of air with less noise. The page on night ventilation through an opening rooflight describes those options, and the comparison of opening and fixed rooflights and noise explains the trade.

When we survey a room, we listen first. What we hear goes into the written quiet spec that comes with every quote, and the decision about how far a rooflight should open for that room is part of it. You can read what the document covers on the quiet spec page.

A rooflight that opens is only as good as its closed position. The seal, the upstand and the drip detail have to keep water out the moment the sash comes down.

Upstand height is part of this. A taller kerb keeps rain splash away from the opening edge and gives the membrane more height to rise against the frame. Some opening units ask for a larger upstand than a fixed one of the same size, and that affects the roof build before anyone orders glass. The guide on upstand height for an opening rooflight explains why.

Rain on an open rooflight is a matter of angle and wind. A hinged sash opened by a small amount, on a pitched roof, sheds most light rain. A large opening on a flat roof in a driving shower will let water in. Habits help: close it when you leave the house, and when rain is forecast. For anyone who forgets, the rain sensor closes it for them.

• Open a hinged unit only as far as the room needs.
• Shut the unit before leaving the house or going to bed if rain is expected.
• Check that the lower seal sits evenly when the sash comes down.
• Keep the drip channels and the glazing edges clear of leaves and moss.

A sensible opening is sized from the floor area first and the glass second. Start with the room, work out the free area it should have, then choose the rooflight that provides it.

Take a bedroom of 12 square metres. One twentieth of that is 0.6 square metres, or 6,000 square centimetres. A hinged rooflight opening to a clear gap of 300 millimetres on a 600 by 1,000 millimetre sash would give a free area well short of that, since the gap is a wedge and the sides are partly closed. A sliding or top-hung unit with a larger clear opening gets much closer. Wall windows in the same room add to the total, so the rooflight does not always have to meet the figure alone.

Three checks are worth making at the survey:

1. The floor area of the room under the rooflight, and whether the door to the next room stays open.
2. The clear opening in millimetres at full stay or motor travel, not the nominal size.
3. A second opening somewhere lower, so that air has a way in.

A larger opening is not always better. A very big sash is heavy, catches the wind and needs more careful flashings. Smaller units fitted in pairs can be gentler on the roof and give a more even flow.

The best opening rooflight is the one that actually gets opened. If it is out of reach, stiff or forgotten, the stack effect never gets a chance.

Manual units suit a low ceiling where a person can reach the handle, or where a pole is acceptable. Motorised units suit anything high, anything large and any room where the rooflight should react without being asked. The motor also makes it possible to open a little and often, in the small steps that ventilation actually needs. The comparison on electric or manual opening rooflights sets out the differences in use.

Safety is part of the choice. A sash that opens above a stair or over a child's bed needs a limit on its travel, a stay that holds firmly and a guard against trapped fingers. Those details are chosen at the survey, because they depend on the room and on who uses it.

We start with the room, the air and the sound, and we finish with a rooflight that opens the way you will use it.

At the survey we check the height of the ceiling and how air would move through the floor, where the hob and the shower sit, which way the roof faces and what the room sounds like. We note the free area the room needs, and the type of opening that can give it. All of this goes into the written quote alongside the quiet spec.

On the day the job follows a familiar order. The opening is formed in the roof, the upstand built to the height the unit asks for, the rooflight fitted and sealed, and the controls wired or set. Where the work is notifiable, we handle the Building Control notification, and we install to current Building Regulations. The work carries a 10-year workmanship guarantee, described on the guarantee page. Inside, the reveal is made good so the ceiling looks as if the rooflight had always been there.

If you are weighing up a room, the place to begin is the parent page on electric and opening rooflight installation in Crawley. To talk about a particular room, get a quote, or message us on WhatsApp.

These are the points to settle when you are deciding between a fixed rooflight and one that opens.

Will an opening rooflight get rid of cooking smells?

It helps with the steam and the warm air that carries the smell upwards. For strong or greasy smells, a fan at the hob still does the main work, because it removes them before they spread through the room.

Can one opening rooflight cool a whole room on its own?

Only if air can get in somewhere else. The rooflight lets warm air out and something lower has to let cool air in. Pair it with a window or an open door and the effect is much stronger.

Is it safe to leave one open when I am out?

It is better to close it. Weather changes quickly and an open unit is an open roof. A motorised rooflight with a rain sensor shuts itself if the weather turns.

Does a sash that opens let in more noise than a fixed one?

When it is open, yes, because nothing is left between the room and the sky. When it is closed, a well-made opening unit with good seals performs close to a fixed one of the same glazing build.