Sun tunnels in a bathroom or shower room

Condensation

A bathroom produces more moisture per hour than any other room, and the tunnel passes through a cold loft. Get the detailing wrong and moisture condenses inside the tube, runs down, and collects on the inside of the diffuser.

Two details prevent it. Insulate the tube through the cold space so its surface stays above the dew point. And seal the ceiling collar so warm wet air cannot enter the loft around the tunnel at all.

The collar matters more, because it removes the cause rather than managing the symptom.

It is not ventilation

A sealed unit that moves no air. Building Regulations require mechanical extraction in a bathroom regardless of daylight, and a tunnel does not change that.

The reassuring corollary: no steam reaches the loft through it, provided the collar is sealed.

Privacy

Complete. The diffuser is opaque or prismatic, the tube usually turns at least once, and there is nothing to see from either end. In a bathroom that is the whole argument over a rooflight.

Light kit

Worth having, so the same fitting works after dark and the ceiling has one aperture rather than two. Wiring it with the extractor so both come on together is a sensible arrangement.

The fitting sits in a bathroom ceiling and must suit its zone, which is an electrician’s decision and belongs in the specification from the start.

Which diffuser

Prismatic. It spreads light evenly and avoids a bright disc of direct sun landing somewhere inconvenient.

The room they suit best, and the room that punishes bad detailing

Bathrooms are frequently the darkest rooms in a house. An internal bathroom or ensuite with no external wall has no daylight at all and no rooflight solution, because there is no slope above it to put one in.

A sun tunnel is the answer, and for a fraction of the cost of anything else it turns a room you always light artificially into one that is usable on daylight through the middle of the day.

It is also the room where the two details that get skipped on cheap installations cause the most trouble, because a bathroom is the wettest, steamiest space in the building and the tunnel is a cold metal tube running through a cold loft directly above it.

The condensation problem, and why it looks like a leak

A sun tunnel does not produce moisture. What a poorly detailed one does is give existing moisture a cold surface to condense on, inside the tube, where the water then runs down and appears at the diffuser.

From below that looks exactly like a roof leak, and it produces a great many unnecessary roofing investigations.

The tell is timing. Condensation appears in cold weather, is worst shortly after the room has been used, and bears no relation to whether it has rained. If water arrives at the diffuser after a bath on a January evening but the room stays dry through a September downpour, that is what it is.

The two details that prevent it

Insulate the tube wherever it crosses unheated loft space. This keeps the inner surface closer to room temperature so there is nothing cold enough to condense on.

It is a cheap material and it is frequently omitted, particularly on kits that do not include it and on self installed tunnels where nobody knew to buy it separately.

Seal the ceiling ring airtight. The more fundamental fix, because it stops warm moist room air getting into the tube in the first place rather than managing it once it is there.

Sealant or tape at the junction between the ring and the plasterboard, done properly rather than approximately. It also stops warm air leaking from the bathroom into the roof space, which is a genuine heat loss and a route for moisture into the loft.

Both are named lines on our quotations rather than assumptions, because they are the first things dropped from a cheap price and their absence does not show until the first cold spell.

The third factor, which is the room rather than the tunnel

A sun tunnel does not ventilate. There is no opening, no airflow through it, and no capacity to help a room shed moisture. It is a light pipe.

Where a tunnel serves a bathroom, the extract fan has to be doing its job: sized for the room, running on an over run timer or a humidity sensor, and ducted to outside rather than into a loft.

A room producing steam twice a day with a fan that does not run, or runs into a blocked or disconnected duct, will condense on whatever surface is coldest. After a tunnel goes in, that may well be the tunnel, and the tunnel will get the blame for a problem that predated it.

Ducting an extract fan into a loft rather than through the roof or the wall is depressingly common on older installations and it is worth checking while somebody is up there.

Privacy, which is a genuine advantage

There is no line of sight through a mirrored tube, and the ceiling diffuser is opaque or translucent.

So a sun tunnel gives daylight with complete privacy, which in a bathroom is worth more than it sounds. There is no blind to fit, no blind to get wet, and no obscure glazing decision to make.

Compare that with a rooflight in a bathroom, where overlooking from higher ground is occasionally an issue and the answer is obscure glazing specified at order, or a blind that will not enjoy the humidity.

Sizing for a bathroom

Light output goes with the area of the tube rather than its width, so a 350mm tunnel passes roughly twice the light of a 250mm one, not forty percent more.

250mm suits an internal WC or a small ensuite: enough to walk in and see without reaching for the switch.

350mm suits a family bathroom and is the size we fit most. This is where a room stops reading as artificially lit and starts reading as daylit.

530mm is generous for a bathroom and worth considering on a long run or where the ceiling is high.

The one caution is proportion. The diffuser diameter matches the tube, so a 530mm unit puts a substantial disc in the ceiling, and in a small WC that can look wrong. It is the single situation where the smaller size is right despite the lower output.

Light after dark

There is none. A sun tunnel tracks the day and gives nothing at night, which in a bathroom used morning and evening is half the time you need it.

Most units take an optional light fitting inside the diffuser, so the same ceiling opening does both jobs. In a small bathroom ceiling already carrying an extract fan, that matters: the alternative is a daylight disc and a separate light fitting competing for space.

Specify it at the outset. The cable is run while the ceiling is open, and adding it later means opening the ceiling again. If you are unsure, have the cable run and capped.

Bathroom electrical zones apply to any fitting, and a ceiling fitting above a bath or shower has particular requirements. That is settled at design stage with the electrician rather than on the day, so tell us if the tunnel is going over a shower.

The route, and what usually gets in the way

Bathrooms are frequently in the middle of a plan, which is why they have no external wall, and the middle of a plan is where the loft is busiest.

Water tanks, soil pipes and vent pipes all serve the bathroom and all live directly above it. Purlins and structural members cross the space. Cables run to the fan and the light.

Every bend in a rigid tube costs light, more than a metre of straight tube does, so a route that has to dogleg around a tank is a route worth reconsidering. Moving the ceiling position half a metre is frequently the better trade, and in a bathroom the diffuser rarely needs to be dead centre.

Send a photograph of the ceiling and one of the loft above it, and we can usually confirm the route and the diameter without a visit.

Listed and conservation property

Worth mentioning because bathrooms in older houses are frequently the rooms with no daylight, and a full rooflight may not be consented.

A sun tunnel is the least visible daylight option there is: the roof component on a low profile unit sits close to the covering and reads as a very small rooflight, and on a rear slope it is frequently invisible from any public vantage point. The intervention is one opening between rafters rather than a trimmed structural opening, and it is largely reversible.

That is a genuine argument to put to a conservation officer, and we have seen tunnels consented where rooflights were refused.

It still requires listed building consent on a listed building. It is not exempt for being small.

Why it suits the room so well

Bathrooms are frequently the darkest room in the house, because privacy has reduced the window to a small obscure pane or removed it entirely.

A sun tunnel gives daylight with no overlooking at all and no window to obscure. There is nothing to see in through a tube from the roof, so the privacy question disappears completely.

It also puts the light where it is useful, over the basin or the shower, rather than at the edge of the room where a small window sits.

The moisture problem, which is the whole design issue

A bathroom is the most humid room in the house, and a shower produces a large volume of water vapour very quickly. It rises straight to the ceiling.

The tunnel passes into cold roof space. If that warm damp air can get into the tube, it condenses on the cold inner surface, runs back down, and drips at the diffuser.

It looks exactly like a leak, and somebody sent to the roof will find nothing.

The three measures, in order

Seal the diffuser to the ceiling. The most important, because it stops room air entering the tunnel at all. A gasket or a continuous silicone bead onto a clean surface, not a decorative bead run over an existing gap.

Insulate the tube over its whole length through unheated space, continuously and without compressing at the joints.

Specify a double glazed diffuser, which raises the internal surface temperature at the ceiling. In a bathroom this is worth the small loss of light.

All three are cheap at installation and awkward afterwards, because reaching the tube means getting into the loft and working around it.

Ventilation is a separate requirement

Standard sun tunnels are sealed and passive. They do not ventilate, and they do not replace an extract fan where the regulations call for one.

Some manufacturers offer a version with a ventilation function, and where that is used it has to satisfy the extract requirement on its own terms rather than by assumption.

The realistic arrangement is a tunnel for daylight and a mechanical extract for moisture, running long enough to be useful.

The light kit

A diffuser with an integrated fitting gives daylight by day and electric light at night from one aperture, which is elegant in a bathroom.

The caution is that it must not compromise the seal. Use a manufacturer designed integrated kit rather than cutting a hole in a standard diffuser.