Where the water actually comes from
Condensation in a sun tunnel is almost always moisture from inside the house, not rain from outside, and it forms wherever warm damp air touches a surface below its dew point.
Every home carries water vapour from showers, cooking, drying clothes and simply breathing. Warm air holds more of it than cold air. When that air drifts up through the ceiling diffuser into the tube and reaches a part that is colder than its dew point, the vapour gives up water as fine droplets. It is the same effect you see on a cold bathroom mirror or the inside of a single-glazed window on a January morning.
A tubular skylight is a particularly clear example because of where it runs. The diffuser sits in a warm, heated room. The dome sits on the roof in outdoor air. In between, on most pitched roofs, the tube passes through a loft that is deliberately kept cold, because the insulation lies on the ceiling below it. So a single sealed tube joins a warm zone to a cold one, and the cold part is where any stray moisture will settle. Our guide to how sun tunnels work shows the three parts in order.
The three places condensation can show
If a sun tunnel does get condensation, it tends to appear in one of three spots, and each points to a different cause.
- Inside the dome. A light mist on the underside of the dome on a cold, still morning can be normal, and it usually clears as the sun warms the roof. Persistent heavy droplets suggest room air is reaching the top of the tube too freely.
- Along the tube in the loft. Beads of water on the outer skin of an uninsulated tube come from damp loft air meeting cold metal. Water on the inside comes from room air rising up the tube.
- On the diffuser. A damp or cloudy diffuser in the ceiling is usually a sign of a humid room, such as a bathroom without enough extraction, rather than a fault in the tube.
Water that forms high up can run down the reflective lining and collect on the diffuser, so where you see it is not always where it started. At a survey we look at the whole run before deciding what is going on.
How insulation keeps the tube warm enough
The main defence is to stop the section of tube in the loft from getting cold in the first place, which means wrapping it in insulation where it crosses the cold space.
Several manufacturers supply an insulated sleeve for the loft section of their tubes, and others expect the installer to wrap the run on site. Either way, the aim is the same: keep the surface of the tube closer to room temperature so that warm air inside it stays above its dew point. The sleeve also helps with the ceiling insulation itself, because cutting a hole through the quilt and leaving a bare metal tube creates a cold bridge beside it. The companion answer on whether a sun tunnel can go through loft insulation covers how the quilt is made good around the tube.
The type of tube affects how easy this is. A rigid tube is a smooth, straight metal pipe and takes a sleeve neatly. A flexible tube is a folded foil that already loses more light, and its ridged surface is harder to wrap evenly. The rigid vs flexible sun tunnels guide compares the two in more detail. Short runs matter as well: the less tube crossing the loft, the less cold surface there is, which is one more reason to position the unit carefully, as set out in where to put a sun tunnel on the roof.
Sealing the tube at both ends
Insulation keeps the tube warm; sealing keeps damp room air out of it, and both need to be done for the result to last.
Each joint between tube sections should be taped with the manufacturer's foil tape, so that loft air cannot get into the run and room air cannot escape into the loft. At the ceiling, the diffuser should seal to its ring so that the tube is a closed column rather than a chimney drawing up steam from a shower. Some diffusers are made with two layers, with a still pocket of air between them that acts much like a double-glazed window, which keeps the inner face warmer and helps where a room is humid. Our guide to sun tunnel diffusers describes the types.
At the roof, the dome and its flashing keep rain out. A properly lapped flashing on a pitched roof, or a correctly formed upstand on a flat roof, is a separate job from condensation control, but a poor one can let water in that looks like condensation from below. That is why we check both before calling it one or the other.
Rooms that need more care
Bathrooms, shower rooms and kitchens produce far more moisture than a hall or landing, so a sun tunnel in one of them needs the room's own ventilation to be working as well.
Building Regulations Approved Document F expects bathrooms and kitchens to have extract ventilation, and a fan that actually runs long enough after a shower makes more difference to a damp diffuser than anything in the tube. In a windowless bathroom, one option is a combined unit that lights and ventilates through the same opening; the guide to sun tunnel ventilation kits explains how they work. For kitchens, the position of the diffuser matters: set it away from the hob, where rising steam is concentrated, and it stays clearer. The answer on whether a sun tunnel can go in a kitchen covers placement.
| Room | Moisture load | What helps most |
|---|---|---|
| Hall or landing | Low | Insulated, sealed tube |
| Bedroom | Moderate overnight | Sleeve plus good room airing |
| Kitchen | High near the hob | Diffuser away from steam |
| Bathroom | High after showers | Working extract fan |
Upgrading an older tube that mists up
If you already have a sun tunnel that fills with droplets every winter, the usual answer is to replace the loft section or the whole unit with an insulated, well-sealed one, rather than live with it.
Older installations were sometimes fitted with a bare flexible tube laid loosely through the loft, a single-layer diffuser and joints that were never taped. That combination is the one most likely to mist. A replacement lets us upgrade to a rigid, insulated run, a better-sealed diffuser and a correctly formed flashing in one visit, and it is also a chance to look again at the diameter and position for the room, which the sun tunnel diameter guide explains. Our page on sun tunnel installation in Cambridge sets out how that work is planned and quoted, with the written light plan that comes with every quote.
Questions people also ask
Is a little mist in the dome normal?
A light mist on the inside of the dome on a cold, still morning can happen and usually clears as the day warms. If it forms heavy droplets that run down the tube, room air is probably reaching the top of the run, and the seals and insulation are worth checking.
Do tubular skylights cause condensation more than a skylight?
They face a different risk rather than a larger one. A rooflight sits in a warm room with glass exposed to the cold outside, while a tube crosses a cold loft. Both stay dry when insulated and sealed properly. The sun tunnel vs skylight guide compares them more widely.
Does a sun tunnel make a room colder or damper?
An insulated, sealed tube should not make a room noticeably colder or damper. Heat moves through it far more slowly than through an open vent. The answer on whether sun tunnels let heat in covers heat gain and loss in both directions.
Can you add insulation to an existing tube?
Sometimes, if the loft section is rigid, accessible and otherwise sound. Where the tube is flexible, damaged or badly routed, replacing the run is usually the more reliable choice. We decide at the survey, once we have seen it; book a light survey to have it looked at.
