The short answer
A well fitted sun tunnel does not cause condensation of its own accord. Condensation appears when warm, moist room air meets a cold surface, and a tube running through a cold loft can become that surface if it is not insulated and sealed properly. So a tunnel can show condensation, but nearly always because of how it was fitted, and the fix is part of the installation.
Bathrooms and kitchens are where it matters most, because they produce the most moisture.
Where the water actually comes from
Air holds moisture, and warm air holds more than cold air. When that air touches a surface colder than its dew point, some of the moisture turns to droplets. You see the same thing on a bathroom mirror after a shower or on a window on a winter morning.
In a sun tunnel there are three places where this can happen:
- on the underside of the diffuser, where the room air meets a pane that is slightly cooler than the room;
- on the inside of the tube, if the tube is cold and room air reaches it;
- on the inside of the dome, where the air in the top of the tube meets plastic that is exposed to the winter sky.
The second and third are the ones people notice, because the water runs down the tube and appears at the ceiling as a drip.
How moist air gets into the tube
Room air reaches the tube through gaps. The usual one is the joint between the ceiling and the diffuser ring, where a gap of a few millimetres lets warm air pass upwards. A second is a loose join between the sections of tube. A third is a missing seal where the tube passes through the plasterboard.
Once warm air is in a tube that is surrounded by cold loft air, the walls of the tube chill and the moisture condenses on them. The moisture runs down the polished surface, collects at the base and stains the ceiling around the opening. That is damp from inside, not a leak from outside, and it is often mistaken for one.
What stops it
Condensation is prevented by dealing with the cold surface and the air path together.
- Insulate the tube
- Where a tube runs through an unheated loft, it is wrapped in insulation, so its walls stay close to room temperature. The wrap has to run the whole length, right down to the ceiling, with no bare patches.
- Seal the joints
- The sections of tube are taped at each join, and the base is sealed to the ceiling, so room air has no route into the tube.
- Choose a sealed or double-skinned dome
- Some domes are made with two skins to reduce heat loss and condensation inside the dome itself, and some tunnels include a sealed diffuser that closes off the bottom of the tube from the room.
- Keep the route short
- A shorter tube has less surface in the cold, so less to insulate. See our note on how long a sun tunnel can be.
Bathrooms and kitchens need more care
The steam from a hot shower or a boiling pan will condense on anything cooler than the room, so a tunnel over a bathroom has more to deal with than one over a landing. Three things help there. Fit a sealed diffuser. Make sure the bathroom has a working extractor fan, because ventilation under Approved Document F is designed to remove moisture at the source. And run the fan for a while after use, so the damp air leaves the building and does not drift into the ceiling.
Sun tunnels are popular for exactly these rooms, since a bathroom often has no external wall for a window. Our guide to sun tunnels for bathrooms, landings and hallways covers the choice of tunnel for each.
What if you already see water
If you already have a tunnel that drips or stains the ceiling, work out which of the three places it comes from before assuming it needs replacing.
- Water on the underside of the diffuser only, on a cold morning, is room moisture and may clear with ventilation.
- Water running down the inside of the tube means the tube is cold and room air is getting in. Insulating and sealing it usually deals with it.
- Staining that appears only after heavy rain points to the roof flashing rather than condensation, which is a fitting question that a surveyor can sort out.
When we replace a tunnel, we fit the new one with the insulation, the sealing and the ceiling ring as part of the job, and the ceiling is made good and decorated to match, which is the part you will see. Our finish schedule records how the ceiling around the ring is formed and finished.
Fitting in a way that keeps the ceiling dry
The best protection against condensation is a tunnel fitted for the building it is in. We look at the loft above the room at survey: whether it is insulated, whether it is boarded, how cold it gets and how much air moves through it. An old house in Devon with a draughty slate roof will have a very cold loft in winter, and the tube needs to be insulated for that.
The dome is flashed for the roof covering, and the tube is routed so that it does not touch cold rafters or the underside of the roof covering. At the ceiling the diffuser ring is sealed and the plaster dressed to it.
Fitting a tunnel is building work in a roof, and where it needs to be notified to Building Control we handle that. The planning position is set out on our sun tunnel installation page.
Next step
If you are planning a sun tunnel over a bathroom or kitchen, or you have one that drips, ask for a quote and send a photo of the room and, if you can reach it safely, the loft above. We will look at how the route is insulated and sealed and write it into the quote.