Sun tunnel condensation: keeping the tube dry

Air carries water vapour, and warm air carries more of it than cold air. When warm air touches a surface colder than its dew point, the surplus vapour condenses on that surface. That is the whole mechanism.

A sun tunnel gives this process an ideal stage. The dome and the top of the tube sit in the open air above the roof. Below them, the tube crosses the loft, which in winter is cold and ventilated on purpose. At the bottom is the diffuser, set into a ceiling below a bedroom, a landing or a bathroom that is heated and lived in. The tube is a thin-walled metal or plastic channel joining two very different climates. If it is left bare, its inner face can be damp in January while the room beneath it feels perfectly comfortable.

Two ways of getting wet are worth telling apart, because they need different cures.

• Condensation on the inside of the tube, which forms when room air finds its way up the tube and meets a cold wall. The droplets run down and may gather at the diffuser.
• Condensation on the outside of the tube in the loft, which forms when damp air from the house rises into the roof space and meets the cold metal. The droplets drip onto the insulation or the ceiling below.

The first is a problem of air movement through the tube. The second is a problem of moisture in the loft. Both come back to the same two levers: keep warm air away from cold surfaces, and keep the cold surfaces from being quite so cold.

A household makes a surprising amount of water vapour every day, simply by living in the house. Breathing, cooking, washing, drying clothes and running a shower all add to the air, and that air drifts upwards.

The rooms beneath a sun tunnel tell you how much risk the tube carries. A landing or hallway is usually the gentlest case: the air is shared, moves about and is rarely saturated. A kitchen is wetter, particularly when pans are boiling with no lid. A bathroom is the most demanding of all, because a hot shower can fill a small room with vapour in minutes. That is why a windowless bathroom is one of the best uses of a sun tunnel and also the room where the condensation detail needs the most care. Our page on sun tunnels for windowless bathrooms looks at the room itself, and the answer on fitting a sun tunnel where there is an extractor fan deals with how the two live side by side.

Moisture also arrives from outside the room. A loft with gaps around pipes, a hatch that closes loosely or recessed lights that open straight into the roof space lets house air climb into the cold zone. That air is the source of most drips on the outside of a tube. So before any work is planned, it helps to know whether the loft is already collecting household damp. Staining on rafters, damp patches on the underside of the felt and condensation on nails are all signs. We look for them at the survey, because a tube through a damp loft behaves differently from the same tube through a dry one.

A sun tunnel is a short chain of parts, and condensation gathers at the joins and at the coldest points. Knowing the chain makes the cure easier to follow.

Starting at the top, there is a dome, set on a flashing or a kerb that is dressed into the roof covering. The dome is the coldest component, because it is in direct contact with outside air and often with clear sky. Below that is the tube, a reflective run that carries the light down through the loft. It may be a rigid aluminium pipe or a flexible ducting form, and the choice affects both light and moisture, as the page on rigid or flexible sun tunnels explains. At the bottom is the ceiling collar and diffuser, the visible part in the room.

Each junction is a place where air can pass or where a cold bridge can form:

1. The dome to the flashing, where the roof's own weatherproofing has to be kept separate from the tube's airtight seal.
2. The tube sections joined end to end, where a loose joint lets loft air in and room air out.
3. The tube to the ceiling collar, where the plasterboard meets the metal.
4. The diffuser to the collar, which is the last barrier between the tube and the room.

A well-made sun tunnel is watertight at the roof and airtight at the ceiling. Those are two separate jobs, done by two separate sets of details, and either can be done badly while the other is done well. How the roof end is kept dry is covered in our answer on how a sun tunnel is kept watertight on the roof. This guide is about the second job, which is keeping room air where it belongs.

The tube crosses the coldest part of the house, so it is where insulation does the most good. A bare metal tube in a winter loft is at almost the same temperature as the air around it, and any damp air that touches it has nowhere to go but onto the wall.

The principle is to wrap the tube so its inner face stays close to room temperature. In practice that means a sleeve of insulation around the full length of the tube from the ceiling to the roof line, with the insulation fitted snugly and taped at each joint so there are no bare patches. The sleeve should be continuous. A gap of even a hand's width behind the insulation leaves a cold stripe on the tube, and condensation will find it. It should also sit over the top of the joints, not stop short of them, because the joints are where the tube is thinnest and most likely to cool.

The sleeve has a second duty. It lets the loft insulation carry on around the tube without a hole in it. A sun tunnel cut through a well-insulated ceiling leaves an opening the width of the tube, and the loft insulation around it should close right up to the sleeve, not stop a few centimetres away. If the ceiling insulation is thin or has been disturbed by the work, heat from the room rises through the gap, warms the tube and then condenses on the cold dome. Our page on sun tunnels, loft insulation and heat loss deals with the thermal side in detail, including how the glazing at each end of the tube sits against Building Regulations Part L.

The dome end deserves a little extra. Where the tube meets the dome, a short section of insulation inside the roof space helps the top of the tube stay above the dew point, while the dome itself is usually a double-skinned unit that carries its own small air gap. That air gap is the dome's own insulation, and it is the reason a good dome is less prone to misting on the underside than a single-skin one.

Insulation slows heat loss, but sealing stops the vapour. If warm air never reaches the cold wall of the tube, nothing condenses. That makes airtightness the real hero of a dry installation.

The job starts at the ceiling. The collar that joins the tube to the plasterboard should fit tightly and be sealed to the board with a flexible sealant or a foil tape made for the purpose. A hole in a ceiling is a chimney for the damp air below, and a sun tunnel is exactly that shape. The tube sections should overlap in the direction that sheds water, be fixed with screws or clips and be taped at every joint with a metallised tape that will stay bonded through the heat of summer and the cold of winter. Ordinary cloth tape dries out and lets go.

The bends matter too. Every angle in a tube adds joints and surface area, and so it adds places for vapour to get through. A straight run is the driest run. When a bend is needed to clear a rafter or a chimney breast, the joint should be supported, taped and insulated like any other. Our guide to tunnel length, bends and how much light arrives explains the trade-off between a direct run and a more awkward route.

A flexible tube, made of ribbed foil ducting, can be a little more vulnerable than a rigid one, because the ribs create small pockets where droplets can gather, and the thin skin cools quickly. It can be fitted dry and stay dry, but it needs to be pulled taut, supported at intervals so it does not sag into a low point, and wrapped along its whole length. A sagging section collects any droplets that form and holds them. A rigid tube sheds them and lets them run back to the diffuser or the dome, where they evaporate.

The diffuser is the face of the sun tunnel in your ceiling, and it plays a bigger part in keeping the tube dry than its looks suggest. It is the closing seal between the room and the tube.

A good diffuser has a clean, flat fit against the collar, with a gasket or a tight clip-in seat, so that little air flows through it. Some designs have a second, sealed pane a short way inside the tube, which gives a pocket of still air between the room and the tube. That pocket acts like a small piece of double glazing and keeps the diffuser surface warm, which reduces the chance that vapour settles on it. Others have a plain opal disc that clips in without a seal at all. The second kind is cheaper and it is perfectly fine in a dry hallway, but it lets more air through and so it suits a bathroom less well.

Finish matters as well. A smooth, light ceiling paint around the collar is easy to wipe and does not harbour mould if a little moisture lands there. Matt emulsion on the ceiling near the tube is better than a shiny finish. Our page on sun tunnel diffusers and ceiling finishes goes through the choices, and explains how each one changes the quality of the light in the room as well as the moisture behaviour.

A well-built tube is half the answer. The other half is how much water the room puts into the air. Reducing the vapour at the source is cheaper and more reliable than trying to keep it out of a tube.

Building Regulations Part F covers ventilation, and its purpose is to keep indoor air healthy and to control moisture. A bathroom should have an extractor fan that removes steam while it is being made, and the fan should duct to the outside, not into the loft. A fan that vents into the roof space loads the very zone we are trying to keep dry, and it is one of the commonest causes of a dripping tube. The fan should run during the shower and for a while afterwards, so that the air in the room is changed before it has time to rise through the tube. Many fans have a timer or a humidity sensor that does this without any effort from the people using the room.

In kitchens, a cooker hood that extracts to the outside does the same work. Where a room has no extraction at all, a window opened for a few minutes after cooking or bathing is better than nothing. For a hallway or landing, background ventilation through the house, with trickle vents or airbricks kept clear, usually keeps the air moving enough to avoid damp.

Door habits help too. A bathroom door left open after a shower lets the steam travel through the whole house and settle wherever the surfaces are coldest, and a tube in a landing ceiling is one of those places. Closing the door until the fan has done its work keeps the moisture in the room where the fan can reach it.

A sun tunnel cannot be judged on its own, because it sits inside a loft that has its own moisture balance. A dry, well-ventilated loft with a good barrier between it and the house makes the tube's job easy.

The roof space is ventilated on purpose, through eaves vents or ridge and tile vents, so that any vapour that does get in can leave. Those routes should stay open. Loft insulation pushed hard into the eaves can block the airflow at exactly the point where it is needed, and a blocked eaves vent turns a dry loft into a damp one within a season. When we fit a tube, we check that the insulation around it does not crowd a vent, and that any cables, pipes or hatches near the work are not leaking house air upwards.

The ceiling below is the barrier that matters most. Every unsealed penetration, such as a light fitting, a pipe entry or a loosely fitted hatch, lets warm air into the loft. A sun tunnel is another penetration, and the collar seal is what stops it being one of the leaky ones. If a survey turns up other gaps, it is sensible to close them at the same visit, because they help every cold surface in the roof, and not only the tube.

Some lofts have been converted, boarded or had their insulation changed over the years, and the airflow can be very different from what was originally intended. A loft that has a warm-roof build, where insulation sits between and over the rafters, behaves differently from a cold loft with insulation on the floor. The tube detail changes with it: in a warm roof the tube sits inside the insulated envelope and stays warmer, while in a cold loft it needs its own sleeve. We choose the build to match the loft we find, and we write it down.

The place and the season of the droplets tell you most of what you need to know about the cause. The table below sets out the common patterns and what each one suggests.

Timing is a clue as well. Condensation that appears on cold, still mornings and clears by lunchtime is nearly always a temperature effect, and it responds to better insulation. Condensation that stays for days, or that comes with staining and a musty smell, suggests a steady source of moisture, and the right move is to find the source. If a dome has misted between its own two skins, that is a different fault: the sealed unit has failed and the dome needs to be replaced, which our misted and failed rooflight replacement service covers.

Quiet is the reason we keep returning to the build of a rooflight, and it applies to a sun tunnel as much as to a lantern. The steps that keep a tube dry are also the steps that keep it still.

A tube wrapped in a snug insulation sleeve is damped, and it is less free to ring when rain strikes the dome or when a plane passes overhead. A rigid tube with taped joints does not rattle. A collar seated firmly in sealed plasterboard does not hum. A diffuser with a gasket does not tick as the temperature changes. So when we talk about a dry tube at a survey, we are also talking about a hushed one, and we write the choice into the quiet spec that comes with the quote.

Rain on the dome is the clearest example. A hard, thin dome over a bare tube can drum in a downpour, and the tube carries that sound down into a landing or a bedroom. A thicker dome, a wrapped tube and a sealed diffuser turn the same shower into a soft background. Our page on keeping a sun tunnel quiet in the rain walks through that side of the build.

A sun tunnel that stays dry is the result of a set of small decisions made in the right order. The plan for each one is written down before any work begins.

At the survey we look at the loft first. We check the ceiling insulation, the eaves vents, the state of the roof timbers and any sign of existing damp. We look at the room below and ask what it is used for, because a bathroom calls for a different specification from a landing. We measure the route of the tube and note any bends that can be avoided by moving the position a little. And we listen to the room, because the noise it hears shapes the choice of dome and tube as well.

On the day, the order of work follows the moisture. The roof opening is made and the flashing dressed first, so the house is weathertight. The tube is then built from the top down or the bottom up, depending on the roof, with taped joints and a continuous insulation sleeve. The collar is sealed into the ceiling and the diffuser fitted last. We make good the ceiling, clean the dome and tube and leave the loft tidy, with the loft insulation brought up to the sleeve all the way round. Where the work is notifiable, we handle the Building Control notification, and the installation is done to current Building Regulations. The work carries our 10-year workmanship guarantee, described on the guarantee page.

These are the points people raise most once they have seen droplets on a tube or are planning for one. Each answer comes back to the same principle of warm air, cold surface and a good seal.

Is a bit of misting on a new tunnel normal?

A little moisture in the first weeks can be the building drying out. Plaster, paint and new sealant all release water, and a tube may show it on a cold morning. If the droplets are gone within a few weeks and do not return, the installation is behaving. If they persist through the heating season, there is a gap to find.

Will a bathroom fan stop the tube dripping?

It will help a great deal, provided the fan ducts to the outside and runs long enough after the shower. The tube still needs airtight joints and a sealed collar, because the fan cannot remove every trace of vapour. Fan and seals work together.

Can I fit insulation around a tube myself?

The loft work is simple to describe, but the quality is in the detail: a continuous sleeve, taped joints and no gaps behind the wrap. It is better done when the tube is installed, when every joint is open to reach. A quote from us includes the sleeve, and it is part of the written specification.

Does a dome that mists underneath need replacing?

If the misting is between the two skins of a double dome, yes, because the sealed unit has failed and cannot be dried out. If the moisture is only on the room side of the tube, the dome is fine and the cure is further down the chain. A survey tells the two apart quickly.

If you are planning a sun tunnel, or living with one that drips, the overall service is set out on our sun tunnel installation page. When you are ready to talk through your loft and the room below it, get a quote. We will survey the roof space, choose the insulation, seals and diffuser to suit, and set out the specification in writing.