Water on the inside of a diffuser, or a stain in the tube, is the commonest complaint on a badly installed sun tunnel. It is almost never the dome leaking. This page is what is actually happening and the four things that stop it.
What the tube is, thermally
Start here, because it explains everything that follows.
A sun tunnel is a metal or plastic pipe passing from a warm room, through a cold roof space, to a cold roof. The pipe is thin, it is a good conductor, and the air inside it is sealed.
Which makes the tube a cold surface with a warm room at one end of it. The general physics of why water forms on cold surfaces is set out on the condensation page for lanterns and it is the same mechanism here. What is specific to a tunnel is the geometry: a long cold pipe, sealed, with a humid room at the bottom.
The two ways water gets in
| Where it appears | Likely cause | What it points to |
|---|---|---|
| Inside the diffuser, misted | Room air in the tube | The ceiling seal |
| Running down the tube inside | Room air condensing high up | Ceiling seal, no insulation |
| Droplets under the dome | Roof space air in the tube | The dome or the upper joint |
| A stain on the ceiling around the trim | Water outside the tube | Roof space condensation or a leak |
| After rain only | Water ingress | The flashing, not condensation |
| Cold mornings only | Condensation | Sealing and insulation |
The timing test
The single most useful diagnostic and it costs nothing.
Water that appears after rain, and only after rain, is ingress. Water that appears on cold clear mornings, and especially after a shower or a bath, is condensation. The two want completely different remedies, and treating one as the other wastes a visit.
Ask the household to note the date and the weather each time it happens. A fortnight of that answers the question more reliably than an inspection does.
The ceiling seal, which is the main event
Where the tube meets the ceiling is the joint that matters most.
A bathroom pushes warm humid air upward. If there is any path at the ceiling trim, that air enters the tube, meets the cold pipe, and condenses on the inside of the diffuser where the household sees it.
A properly sealed diffuser assembly, sealed to the plasterboard rather than merely resting on it, closes that path. It is a small detail, it is invisible once the trim is on, and it is the difference between a dry tunnel and a wet one in a bathroom.
The dome seal
The other end, and it does two jobs.
The dome keeps rain out, which is obvious. It also keeps roof space air out, which is less so. A roof space is cold, and in winter it can be humid; air from it inside a tube will condense against the upper section on a cold night.
Which means the flashing keeps water out and the dome seal keeps air out, and they are separate details even though they sit within a hand’s width of each other.
Insulating the tube
The third measure and the one most often skipped.
A bare tube through a cold roof space is a cold surface for its whole length. Wrapping it in insulation raises the temperature of the tube wall, so even where a little room air does get in it is less likely to reach dew point.
Insulation also stops the tunnel behaving as a thermal bypass through the roof insulation, which is a heat loss issue as well as a condensation one. Two problems, one wrap.
Where the insulation has to stop
A detail that matters if the tunnel is insulated properly.
The insulation wraps the tube where it passes through the cold space. It must not obstruct roof ventilation at the eaves and it must not be packed so tightly against the dome collar that it interferes with the fixing.
Nor should it be crammed into the space around the tube at ceiling level in place of a proper seal. Insulation stops heat; it does not stop air. The two jobs need two products.
Sealed systems, which help
Some tunnels are sold as sealed units and it is a genuine advantage.
A system designed as a closed tube, with gasketed joints at both ends and a sealed diffuser, has no straightforward path for humid air to enter. That is the design answering the problem rather than the installer having to.
Where a bathroom is the room being served, a sealed system is worth specifying rather than treating as an upgrade.
Bathrooms specifically
The room where all of this actually bites.
A shower puts a large quantity of water vapour into a small room in a few minutes, and the tunnel is at the highest point of the ceiling, which is exactly where that vapour goes.
Which is why a tunnel in a bathroom wants the sealed diffuser, the insulated tube, and a functioning extract, and why a tunnel in a hallway is far more forgiving of an ordinary installation.
The extract is part of the answer
Worth stating because people treat it as separate.
Condensation in a bathroom is a humidity problem before it is a tunnel problem. A room that ventilates properly after a shower gives the tunnel far less to deal with.
Where an extract fan is undersized, ducted badly, or wired to a switch nobody uses, the tunnel is being asked to survive conditions no sealing detail was designed for. Fixing the extract is frequently the cheaper half of the remedy.
Water outside the tube
A different fault that presents similarly and is worth separating.
Where the stain is on the ceiling around the trim rather than inside the diffuser, the water is in the roof space, not the tunnel. That is either roof space condensation collecting on the cold tube and running down its outside, or a genuine roof leak finding the opening.
The remedy is roof ventilation or roofing work, and no amount of resealing the diffuser touches it.
Roof space ventilation
The background condition that decides how forgiving the installation has to be.
A well ventilated cold roof carries moisture away and stays close to outside conditions. A sealed, unventilated one accumulates humidity, and every cold surface in it, tunnels included, collects water.
Where a tunnel has gone into a roof space with blocked eaves ventilation, the tunnel is showing a symptom of the roof rather than causing one.
The reflective surface, and why dryness matters
The reason this is more than cosmetic.
The inside of a sun tunnel tube is a highly reflective surface, and its performance depends on it staying clean and bright. Repeated wetting dulls it, allows dust to adhere, and in time can mark it permanently.
Which means a tunnel that runs wet loses light over the years as well as looking unpleasant. The sealing is protecting the product, not just the ceiling.
Dust, which is the other reason to seal
A quieter failure than water and equally worth preventing.
An unsealed tube in a roof space draws in dusty air, and the dust settles on the reflective surface and on the inside of the diffuser. It happens slowly and nobody notices until the tunnel is visibly duller than it was.
A sealed tube stays clean for its life. It is one of those details where doing it correctly means the customer never thinks about it again.
Flexible tubes and condensation
A specific weakness worth naming. A flexible tube has a corrugated inner surface and more joints, which gives water more places to sit and makes sealing along its length harder to guarantee. It also performs less well optically, which is covered on the rigid and flexible comparison.
Where a run is straight enough for rigid, rigid is the drier as well as the brighter choice.
Falls, and where any water goes
A minor detail with a real consequence.
Where a tunnel has a slight offset or an angled section, any water that does form runs to the low point. If the low point is a joint, it finds it; if the low point is the diffuser, the household sees it.
Which is another argument for joints that are gasketed rather than taped, and for keeping the run as vertical as the roof allows.
The first winter
Worth setting expectations, because one observation is not a fault.
A newly plastered ceiling, a newly built extension, or a house that has just had work done carries construction moisture, and for a season everything cold in it collects water. A tunnel fitted into that will occasionally mist in its first winter and be dry ever afterwards.
Which means a single misting in the first few months is worth noting rather than acting on. A pattern across a full winter, or any misting in a second one, is a fault.
What we check on an existing wet tunnel
- When does it happen: after rain, or on cold mornings?
- Is the water inside the diffuser, or on the ceiling around it?
- Is the ceiling trim sealed to the plasterboard, or resting on it?
- Is the tube insulated through the cold roof space?
- Are the joints gasketed, or taped?
- Is the dome seated correctly and the flashing sound?
- Does the room’s extract work, and does anybody use it?
- Is the roof space ventilated at the eaves?
What a remedy usually turns out to be
On the tunnels we are called to, it is generally two of the following.
Sealing the diffuser assembly to the ceiling properly, which is the commonest single fix. Insulating a bare tube. Replacing taped joints with a gasketed connection. And, surprisingly often, dealing with an extract fan that has never worked.
Rarely is it the dome, which is what the household expected it to be.
Whether it can be fixed without removing the tunnel
Usually yes.
The ceiling seal is reachable from the room by removing the diffuser. The insulation and the joints are reachable from the roof space. Neither requires taking the tunnel out or disturbing the roof covering.
Where it does mean removal is a tunnel whose joints have corroded or whose tube has been permanently marked, and that is a longer running problem rather than a new one.
Drying a tube that has already been wet
The question that follows the repair. Once the seals are right, a tube that has held water needs to dry before anybody judges the result. On a sealed system that means opening it, drying the inner surface, and closing it properly rather than sealing the moisture in.
The reflective surface is wiped rather than scrubbed, because it is a coating and an abrasive cloth marks it permanently. Where it has already corroded or stained, the section is replaced rather than cleaned.
What good looks like
A tunnel that never shows water, in any weather, in any season.
That is achievable and it is normal on a properly installed sealed system. A tunnel that mists occasionally on the coldest mornings is telling you something is not sealed, and it will get worse rather than settle.
What we always specify on a sun tunnel
We specify sealed systems on any tunnel serving a bathroom, seal the diffuser assembly to the plasterboard rather than resting a trim against it, insulate the tube through the whole cold section, and use gasketed joints rather than tape. On an existing wet tunnel we establish the timing before anything else, because rain and cold mornings mean two different jobs.
A dry tunnel stays bright for its life. A wet one dulls, and the water is only the first thing you notice.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.