An opening roof lantern, with vents that close when it rains

A lantern is normally fixed glass. Opening is an added feature, built into one or two of the sloped faces rather than the whole unit.

A vent panel is a hinged sash set into a slope of the lantern, framed in the same aluminium as the rest. It is hinged along the top edge, so when it lifts it opens outward like a small awning. That matters in wet weather, because the lip of the panel sheds rain away from the opening rather than channelling it in. The glass in the sash is a sealed double unit, matching the fixed panes.

There are two ways to work a vent. A manual one uses a pole or a winder handle, which suits a lantern within reach. An electric one uses a small motor, or actuator, fixed to the frame. It pushes the sash open and pulls it shut on a control, which is the only sensible choice when the lantern sits four metres above the floor. Electric vents can be wired to a wall switch, a handset or a smart control panel, and several can be linked to work together.

Not every lantern is a good candidate. A narrow lantern may not have a slope wide enough to carry a sash without leaving the vent looking clumsy. Our guide to roof lantern glazing bars shows how the bar layout limits the panes a vent can replace.

A lantern puts glass at the top of the room, which is exactly where warm, damp air collects. A vent lets that air leave.

Warm air is lighter than cool air and rises. In a kitchen extension, the steam from a pan, the heat from an oven and the warmth of a sunny afternoon all climb and sit under the glass. An open vent at the ridge gives that air a way out. If a door or window is open lower down, cooler air comes in to replace it, and the room sets up its own gentle draught without a fan. Our answer on whether a lantern makes a kitchen too hot covers the heat side, and orientation plays a part there too, as explained under lantern orientation and the summer sun.

Ventilation is a matter for Building Regulations as well. Approved Document F governs ventilation in new work, and a new extension needs background ventilation and a way to purge the room quickly of moisture and fumes. An opening vent in the lantern can help meet the purge requirement where its free area is enough. The designer of the extension decides that, and we work out the opening size with them, so it is not left to chance.

The quiet angle runs both ways. A closed lantern with a laminated acoustic inner pane keeps noise out, and an open vent lets some back in. That is why the choice of vent belongs in the written quiet spec, where we note the noise heard at survey and whether the room will be vented at those times.

A flat roof sheds no rain quickly, and an open vent is an open door. A sensor closes it for you.

A rain sensor is a small unit, usually mounted on the lantern or the roof beside it, that detects moisture and sends a signal to the motor. The vents close, often within seconds. It is a simple piece of kit, and in a kitchen with stone worktops and a wooden table beneath the lantern it is worth having on every electric vent. Some systems also close on wind, or on a timer, and can be overridden from the wall.

The sensor does not make a wet room out of a dry one. It is there for the moment you leave the house with the vents cracked open and the weather turns. It is worth checking each spring, by dampening the sensor face with a cloth and watching the vent shut.

The pitch of the roof alters how this works. A lantern on a flat roof has the sensor and vents at roughly the same level as the roof finish, so the sensor sees the weather just as the vent does. Our page on flat roof rooflight installation explains how upstands and falls combine around a lantern.

A vent adds moving parts, a power supply and extra seals. Each needs planning before the lantern is ordered.

Start with the power. An electric vent needs a low-voltage or mains feed, and a run of cable from the lantern to a controller. That is best decided when the extension is wired, since chasing a cable into a finished ceiling is a nuisance. The electrical work follows Part P of the Building Regulations, which covers electrical safety in dwellings, and it is done by a competent electrician.

Next come the seals. A vent sash closes onto a gasket all round, and if that gasket is firm and continuous the closed lantern behaves like a fixed one, in both warmth and sound. A cheap, loose vent can whistle in wind, so the gasket and the catch are specified with the same care as the glass. A lantern with vents is also heavier than a fixed one, which affects the kerb it sits on, as set out on our page about kerbs and upstands.

If you want the same idea without a lantern, our electric and opening rooflight installation page covers flat vents and hinged rooflights on their own.

Can I add a vent to a lantern I already have?

Usually not as a simple add-on. The vent is part of the frame and glazing, so a fixed lantern is normally replaced with a vented one, which is best done at the same time as any other change to the roof.

How many vents does a lantern need?

That depends on the room, the size of the lantern and the orientation. One vent may do for a small garden room. A long kitchen lantern often suits two, one at each end, so air moves through the space.

Will an open vent let in rain when I am home?

A top-hinged sash sheds light rain well, and the sensor closes it in heavier weather. You can also shut it from the wall whenever you wish.

Do vents make a lantern less energy efficient?

A well-gasketed vent adds little when closed, though the frame around it adds a small amount of extra edge length. Our answer on lantern energy efficiency explains how the whole unit is judged.

If you would like a lantern that opens and closes with the weather, get a quote and we will write the vent positions into your spec. The full service is on our roof lantern installation page.