Opening rooflights for ventilation and cooling

Warm air is lighter than cool air, so in any heated or sun-baked room it collects against the ceiling. Where there is an opening at the top, that air drifts out, and the space it leaves behind pulls in cooler air from wherever it can find it. Engineers call this the stack effect, and skylight stack ventilation is simply that effect put to work through a roof opening, after the way a chimney draws. The taller the column of air between the low opening and the high one, the stronger the pull.

A rooflight set in a pitched roof or on a flat roof is as high as an opening in the house can usually be. Compare it with a sash window in a side wall: that opening is only a metre or two above the floor, so hot air banked up under the ceiling stays where it is. The rooflight reaches the layer where the heat actually sits.

The effect is strongest when the room is hot and the air outside is cooler, which is most evenings in a Devon summer, even after a warm day. It is weakest on still, humid afternoons when the air outside is as warm as the air inside. In that case a rooflight still lets heat out, but it will not do the work of a fan.

A rooflight can only push air out at the rate that air can come in. Open it in a sealed room and the flow is feeble. Give it an inlet and it starts to behave like the top of a chimney.

The inlet can be almost anything on a lower level: a window on the ground floor, a door left ajar to a cooler hallway, a casement on the shaded side of the house. The further the inlet is below the rooflight, the more pull the system gets. A loft room with a window opened on the floor below and a roof window above it has a stack of two storeys to work with. A single-storey extension with a lantern has less height but a flat ceiling that traps a lot of heat, so even a modest difference helps.

Two arrangements work best:

• a low opening and a high opening on the same side or in the same space, so air has a short, direct route;
• openings on opposite sides of the house, so a breeze can cross the building and the rooflight can add to it.

If you are planning a new rooflight, think about the inlet at the same time. A rooflight over a landing is useful when the stairwell connects ground floor and bedrooms, because the whole stair becomes the shaft.

Position matters more than size. A small opening at the top of a room will clear more heat than a large one halfway down a slope, and the choice of slope and ceiling changes what is possible.

On a pitched roof, a roof window as high up the slope as the room allows will draw best, because it sits nearest the ridge. Windows fitted lower down the roof still ventilate, but the warmest air lies above them and has a longer way to travel. Where a loft room has two roof windows on the same slope, opening both is less effective than opening one high and one low, with the low one acting as a small inlet.

On a flat roof, an opening lantern, flat glass rooflight or a vented lantern sits above a ceiling that is often fully flat, with no sloping part to bank heat against. That makes the rooflight the main exit for hot air, so it deserves a position near the middle of the room or above the part that overheats, such as the cooking zone in a kitchen extension.

Orientation affects the other side of the story, which is how much heat comes in through the glass. A south or west facing rooflight collects far more sun than a north facing one. If your room already gets too hot, the glazing specification matters as much as the opening: our guide to solar control glass for rooflights covers the coatings that cut the heat before it needs to be vented.

Approved Document F covers ventilation in dwellings. It deals with two kinds: background ventilation, the small constant supply that keeps a home's air fresh, and purge ventilation, the larger opening you use to clear fumes, smells or heat quickly. An opening rooflight can contribute to purge ventilation, since it can be opened wide on demand.

Part F also sets how much opening area a habitable room needs, and the calculation depends on the room, its size and how the rest of the house is ventilated. We don't set that out here, because it changes from one layout to another. For a new rooflight in an existing home the practical point is simpler: a rooflight that opens is a recognised way to give a room a means of purge ventilation, and one that does not open leaves the room relying on what it already has.

Cutting through a finished roof to fit a unit is building work, so Building Control approval is required, and we submit the notification. Ventilation is one of the things that is looked at alongside the structure and the thermal performance. If the room is a loft conversion, the roof window may also need to serve as an escape window, which sets its own size and position rules under Approved Document B. The two roles can be met by the same window, but they should be planned together from the start rather than discovered at inspection.

How the rooflight opens decides how often it will be used, and that decides how much cooling you get. A window you can't reach will stay shut.

Manual

A pole, a handle or a cord on a roof window. Cheap to run and simple, and entirely suitable where you can reach it comfortably, such as a roof window over a bedroom wall or a bathroom.

Electric

A motor opens and closes the sash from a wall switch or remote. This suits rooflights set high above a stair, over a kitchen island or in a double-height space, where nobody would climb up to open them. Electric units can also be tied to a rain sensor so they close themselves when it starts to fall, which we cover in our guide to rain sensors on opening rooflights.

Solar powered

A small panel in the frame charges a battery that drives the motor, so no mains cable is needed. It can suit a retrofit where running a cable through a finished ceiling would be disruptive.

If you aren't sure which is right, our comparison of electric or manual opening rooflights goes into the trade-offs in more detail. For ventilation specifically, the simple test is this: if you would have to fetch a ladder or a long pole, choose electric.

A rooflight does not have to swing wide to be useful. A small gap at the head of a top-hung window is enough for background airing, and a tilted sash on a flat roof lets heat escape while keeping rain out. Where you want fast, powerful cooling, though, opening angle and opening area are what count. A wide opening gives the stack effect more room to work.

Different designs open in different ways. A centre-pivot roof window turns on a horizontal axis, so the top swings out and the bottom swings in, leaving a gap at both ends. A top-hung window swings out from the bottom like a flap, and a lantern with an opening vent lifts a section of the glazing. Each gives a different opening area for the same frame, and we cover the practical limits in how far opening rooflights open.

One more point on position. Where a roof window is inside a loft room and low enough to reach, you will be able to open it fully by hand. Above head height, a restrictor and a pole or an electric operator take the place of reaching, and the opening angle is set by the mechanism rather than your arm.

A window that is open to the air is open to what comes with it. Three things deserve a thought before you rely on one.

Rain is the obvious one. A rooflight left open in a shower will let water in, and unlike a wall window it can't be shielded by a deep reveal. Rain sensors solve this on electric units. On manual ones, the habit has to be yours, which is a good reason to choose a design with a safe ventilation position, a small gap that stays weather-proof when the sash is not fully open.

Insects come in with the air, especially on summer evenings. Some roof windows can be fitted with an insect screen that sits over the opening. Whether yours can depends on the model, and our short answer on insect screens on opening rooflights covers the options.

Security is the third. A window at roof height is harder to reach than one at the back door, but a flat roof extension is easy to climb on to from a garden wall. If a rooflight is meant to stay ajar overnight, a locking restrictor that holds the sash at a safe gap is better than leaving a handle undone.

An opening rooflight looks different from a fixed one when you stand underneath, and the difference is in the finish. The sash swings, so the lining and its edges have to give the moving frame clearance. A rooflight that opens upward into a plastered opening needs a lining shaped so the sash does not catch on the reveal, and an electric unit has a motor housing and a cable route to be tucked away behind plaster or board.

A quote can gloss over these details, yet they are what you notice every day once the job is finished. The splay that spreads light into the room also gives room for the sash to open. The cable runs to a neat switch rather than loose trunking across the ceiling. The paint line along the frame is set after the sash is fitted, so the gap is even all the way round.

Opening units also tend to be larger than fixed ones for the same light, because the opening mechanism takes frame space, which changes the lining and the trimming around the roof opening. We set out what the lining is made of, how it is shaped and how it meets the ceiling in our guide to how a rooflight is finished inside. The finish schedule sent with each Brightview quote notes the room, the ceiling and roof construction, the shape of the lining and its finish, which means the detail is settled before the roof is opened.

The right setup depends on the room and the roof above it. Three typical cases show how the pieces fit together.

In a loft bedroom, the room under the slope is usually the hottest in the house on a summer night, because heat from the whole building rises into it. A roof window high on the slope, opened at the top, and a window or door open on the floor below will draw air through the stair. Choose a solar control coating if the slope faces south or west, and think about whether you can reach the handle from bed.

In a kitchen extension, the heat comes from cooking and from a large area of glass. An opening lantern or flat rooflight above the kitchen island, with bifold or sliding doors opened as the inlet, gives a clear route for hot air and cooking smells. Electric opening usually makes sense here, as the unit is out of reach.

In a landing or hallway, a roof window or sun tunnel is chiefly there for daylight, but an opening one turns the stairwell into a vent for the whole house. Georgian and Victorian terraces in Exeter often have a tall stair hall with a dark landing, which is just the sort of space where an opening rooflight is worth considering. Listed buildings and conservation areas have their own rules for changes to the roof, so check with Exeter City Council before you plan one.

Whatever the room, ventilation is one part of the specification, along with the glazing, the thermal performance and the finish. The full picture, including the controls, motors and wiring, is on our electric and opening rooflights page, and when you are ready to talk about your own room, ask for a quote and a finish schedule.