A sun tunnel for a bathroom with no window: bringing daylight to a room the roof forgot

A bathroom is small, it is used for short spells, and it usually sits in the middle of the house. Those three facts are exactly what a light tube bathroom installation is good at.

A rooflight needs a roof directly above the room, a ceiling void to form a light well, and usually some framing in the loft. A tunnel needs a hole about the width of a dinner plate and a clear route for the tube. That matters because the typical windowless bathroom is an internal room on a landing, an en-suite carved out of a bedroom, or a ground floor shower room tucked behind the stairs. Few of them have a neat rectangle of roof overhead.

The size of the room also works in the tunnel's favour. A dome and tube of modest diameter carries enough daylight for a room of a few square metres, where the same tube would only soften the light in a large kitchen. You can see how that works in our explanation of how a sun tunnel brings daylight into a dark room, and how much arrives is covered in how much light a sun tunnel gives.

Then there is the matter of use. Nobody reads in a bathroom for hours. You want to see your face clearly when you shave or put on make-up, you want the tiles to look clean, and you want the room to feel less like a cupboard first thing in the morning. A tunnel does all of that without a single watt, and it gives a natural colour that warm bulbs never quite match.

There is a limit, and it is better to say so early. A tunnel gives daylight, so it gives nothing at night. A bathroom still needs its electric light, and we plan the two together. The daylight question is covered in what a sun tunnel gives after dark.

The shower is the wettest spot in the house, and the tunnel should be kept out of its direct path. Position is the single choice that matters more than any other for how the diffuser looks in five years' time.

Hot shower air rises in a column. It meets the ceiling directly above the tray, spreads out, and condenses on the coolest surface within reach. If the diffuser sits right over the shower, it takes the full load of that steam, and its plastic or glass face will bead with water every morning. So we plan the diffuser to land beside the shower, not above it, and ideally near the middle of the room where the light can reach both the basin and the mirror.

For a typical layout, three rules guide the position:

1. Keep the diffuser clear of the area directly above the shower tray or bath taps, where the steam rises thickest.
2. Centre it over the space you stand in to use the basin, so that the light falls on your face and not on the back of your head.
3. Keep it away from the line between the shower and the extractor fan, so that steam is not dragged across the glass on its way out.

In a very small room, such as a 1.5 by 2 metre en-suite, you cannot avoid the shower completely. There the answer is a smooth, sealed diffuser that steam can be wiped from, together with a fan that is sized for the room. We walk through the wider problem in preventing condensation in a sun tunnel, which also covers the cold tube in the loft.

Something else that we check is the loft end of the tunnel. A bathroom lets off more moist air than any other room, and the tube passes through a cold roof space. A rigid, well-sealed tube with no gaps at the joins keeps that moist air inside the room, and away from the cold metal of the tube wall. Our note on rigid or flexible sun tunnels explains why the joints matter so much here.

A windowless bathroom needs a mechanical fan, and the fan and the tunnel can live in the same ceiling as long as each has its own clear route. They do not compete, but they do need to be planned together.

Building Regulations Part F expects a bathroom to have extract ventilation. Where there is no openable window, a fan is the only way to meet that, and the usual rate set for a bathroom is 15 litres per second for an intermittent fan. The fan has to duct to the outside, it has to run when the room is in use, and it should keep running for a while after you leave. A timer or humidity switch does that job. The tunnel takes none of this away, because a sealed diffuser is not a vent and the tube is not an air path. A fan with its own duct stays a separate system from the tunnel.

The practical question is the geometry. Both need a path through the ceiling and into the loft. The fan wants a short, straight run of duct to a wall or roof terminal, with as few bends as possible. The tunnel wants a straight tube up to the dome, and it can take a bend or two if it must. If the two cross in the loft, the duct is usually the one that gives way, because it is lighter and easier to route.

Position the fan between the shower and the door, and the tunnel on the far side of the room. Moist air then travels toward the fan and away from the diffuser. We also insulate the loft end of the fan duct, because a cold duct in a loft drips water back toward the fan. Our answer on fitting a sun tunnel in a bathroom with an extractor fan covers the same ground in a shorter form.

The diffuser is the only part of the tunnel you see, so it decides how the bathroom feels. In a bathroom it also has to shrug off steam, splashes and a weekly wipe.

There are three broad styles. A flat, frosted panel gives the evenest light and the cleanest look, and it suits a modern tiled room. A domed or prismatic diffuser spreads light wider, which helps in a deeper room, but its ridges catch limescale and soap. A diffuser with an integral light fitting can carry a lamp, so the same opening serves by night and by day. Each has a place, and the best one depends on the ceiling and the tiles. We go through the choices in sun tunnel diffusers and ceiling finishes.

Whatever the style, the ring where the diffuser meets the plasterboard should be sealed. A sealed ring keeps steam from travelling up the gap and into the tube. We finish it with a bead of decorator's sealant in a colour that matches the ceiling paint, and use a mould-resistant paint on the ceiling around it.

If the diffuser will carry a lamp, the electrical side of the job is treated on its own. In a bathroom, the wiring and the fittings have to suit the zones around the bath and shower, so we treat any integral light as a separate electrical task and do not assume it can be squeezed in. A plain diffuser with a separate ceiling downlight beside it is often simpler, and the light then stays on for the evening.

Cleaning is straightforward. The inside face of the diffuser needs a damp cloth every few weeks in a bathroom, and the dome outside is washed by the rain. The step-by-step is in how to clean a sun tunnel.

A diffuser is frosted by design, which is why a bathroom tunnel gives light without anyone looking in or out. Nothing can be seen through it, from either side.

That matters for a room that a window cannot serve. A bathroom window is usually frosted or covered all year, so the room gets only a dull version of the day. A tunnel gives brighter light than a covered window, and it is private from the start. A neighbour's bedroom window cannot look down into it, and there is no blind to pull.

Glare is the other side of this. On a bright day a tunnel can feel strong, especially in a small white room. The diffuser spreads the light, but the level still rises with the sun. The two ways to tone it down are a smaller diameter in the first place, and a dimming device at the ceiling. The second is covered in whether you can dim a sun tunnel. Choosing a diameter is explained in choosing a sun tunnel diameter.

The direction the roof faces changes the quality of the light, but it does not stop the tunnel from working. A north slope gives a softer, steadier light, which suits a bathroom well. A south slope gives a brighter room on a sunny morning. On a cloudy day, the tunnel gives a pale, even daylight. That pale light is still better than a bulb, and it is explained in do sun tunnels work on cloudy days.

The tunnel runs from the dome to the diffuser, and what lies between them changes the job. A bathroom can sit under the roof slope, under a flat extension roof, or below another bedroom.

Under a pitched roof, the tube goes through the loft and is usually a short, near-straight run. That is the neatest case. We fit the flashing to the tiles or slates, run the tube between the rafters and fit the diffuser from below. The roof side is described in fitting a sun tunnel through a tiled or slate roof.

Under a flat roof, the dome sits on a short upstand, and the tube passes through the roof build-up. A flat roof often has very little depth, so the run is short and the light arrives strongly. The upstand has to be sound and watertight, which is a question of detail, not of size. You can see how that goes in sun tunnels on flat roofs.

A first floor bathroom with a bedroom or a landing above it is a different case. The tube cannot simply run up through the floor above. Sometimes it can pass through a cupboard or a boxed corner, and sometimes it cannot. The question is taken on its own in whether a sun tunnel can pass through an upstairs room. If the route is long or has to bend, the light loses strength, and the trade-off is set out in tunnel length, bends and how much light arrives.

Some bathrooms suit a different solution. Where the room sits directly beneath a flat roof or a loft slope with enough space, a small rooflight gives a bigger spread of light and a view of the sky, and the choice is covered in sun tunnel or rooflight for a dark room. We say which one we would choose for the room after we have stood in it.

A bathroom is a room where sound is part of the experience, since tiles and glass bounce every noise back at you. A dome on the roof above it should not add a soundtrack of its own.

Rain on a thin plastic dome can be heard in the room below, because the tube carries the sound down like a speaking pipe. A bathroom is a hard, tiled space, so a drum of rain arrives clearly. We choose the dome, the tube wall and the collar with that in mind, and we write the choice on the quiet spec that comes with every quote. The details are in rain on the dome: keeping a sun tunnel quiet.

In Crawley there is the aircraft question as well, since Gatwick lies inside the borough and a bathroom is often at the top of the house. We describe it by specification: the dome material and the thickness, and how the tube is sealed at the ceiling. The full answer is in can you hear aircraft through a sun tunnel.

The same thinking applies to the ceiling ring. A loose diffuser can rattle when the fan runs or a door shuts. We fit it snug, with a foam gasket between the ring and the plasterboard, so the only thing the room hears is the water.

A sun tunnel is a small job, but it still touches the roof, the ceiling and the loft insulation, so it sits inside the same rules as other roof work. The main points are fire, insulation and planning.

The tube passes through the loft insulation, and the gap around it has to be dealt with so that warm air does not leak up into the roof space. A bathroom puts more moisture into that air than any other room, so a clean seal at the ceiling matters more here. The approach is covered in sun tunnels, loft insulation and heat loss. Fire is a separate point: the tube and the ceiling ring should not become a path for smoke, and it is considered in whether a sun tunnel affects fire safety in a loft.

We install to current Building Regulations and handle the Building Control notification where the work is notifiable. Where the tunnel sits on a roof slope, planning is usually simple, and the answer is set out in whether a sun tunnel needs planning permission. A listed building or a conservation area can change that, so the position is checked for the address before we quote.

We stand in the bathroom first, then in the loft, and then in the garden. Each visit answers a different question, and together they decide the position, the diameter and the diffuser.

In the bathroom, we look at where the shower, basin and mirror sit, how the fan is ducted, and where the ceiling is cold to the touch. We listen, too. A bathroom beside a bedroom or a landing tells us how much a little extra noise would matter. In the loft, we trace the line from the ceiling up to the roof, looking for rafters, wiring, water pipes and the fan duct. Outside, we look at the roof covering and the direction of the slope.

From that visit you get a written quote and a one-page quiet spec that names the dome, the tube, the diffuser and the seals, and explains why each was chosen for that bathroom. The work is covered by a 10-year workmanship guarantee, and the terms are on our guarantee page. The spec itself is explained on the quiet spec page.

The fitting is quick. A single tunnel is a short job, and the time is mostly in the loft and the ceiling. The plaster is made good, the diffuser goes up, and the room is left clean. The timings are set out in how long it takes to fit a sun tunnel.

These are the points that come up when someone is weighing a light tube bathroom against a rooflight or a window.

Will the diffuser steam up?

A sealed diffuser can mist on the inside face after a long hot shower, because it is cooler than the air. A properly sized fan with a run-on timer clears it quickly. Placing the diffuser beside the shower, not over it, does most of the work.

Can the tunnel go above the bath or shower?

It can, where the room allows no alternative, but it is the harder choice. Steam will reach the diffuser first, so we pick a smooth sealed panel and plan the fan to draw air away from it. If there is a choice, we move the tunnel a little to one side.

Is a tunnel better than a bathroom window?

For a room with no outside wall, a window is not an option, so the choice is between a tunnel, a small rooflight and a bulb. A tunnel is the lightest in build and the simplest to fit. A rooflight gives a bigger spread of light where the roof allows it. Our page on whether sun tunnels are worth it sets out the trade.

Does a sun tunnel replace the bathroom light?

No. It gives daylight only, so you keep the ceiling light for the evening, the early morning and the winter. The two work together, and the tunnel lets the bulb stay off through the daytime.

To talk through your own bathroom, get a quote or start from the sun tunnel installation page.