Condensation inside a tunnel run

The mechanism

The tube passes through a cold loft. In winter that space is close to outside temperature, so the tube is a cold surface.

Warm humid air from the room reaches it, either through an unsealed ceiling collar or through the diffuser seal, and condenses on the inner surface. It runs down and pools on the diffuser.

Telling it from a leak

Does it happen when it has not rained? Then it is condensation. A leak follows rain and is worse in driving rain from a particular direction.

The two fixes

Seal the ceiling collar. The more important of the two. Where the tunnel passes the ceiling line, the vapour barrier and ceiling finish are sealed to the tube so room air cannot enter the loft around it.

Insulate the tube through the cold space, using a purpose made sleeve or by wrapping it, so the surface stays above the dew point.

Both are part of a proper installation, both are invisible afterwards, and that is exactly why they get skipped.

Where it matters most

Bathrooms first, then kitchens and utility rooms. The wettest air in the house meeting the coldest surface available.

Fixing an existing one

Straightforward and worth doing properly. Get into the loft, insulate the run, and seal the collar at ceiling level. A couple of hours and it is solved permanently.

Replacing the unit will not help, because the new one will do the same thing on the same detailing.

Warm lofts

Where insulation is at rafter level, the loft is inside the thermal envelope and the tube is not passing through cold space. Insulating becomes unnecessary, though the collar is still sealed.

The commonest complaint about sun tunnels

Water appearing at the diffuser, and the homeowner concluding the roof is leaking.

It usually is not. What is happening is that warm moist air from the room has got into the tube, met a cold metal surface, condensed, and run back down to the only opening available.

It is entirely preventable, the prevention costs very little, and the cure on an existing installation is done from inside the loft without touching the roof at all.

Why the tube is cold

A sun tunnel crosses a ventilated loft, and a ventilated loft sits close to outside temperature. That ventilation is deliberate: it keeps the roof space dry.

The tube is metal, which conducts well, so a bare tube’s inner surface sits at roughly loft temperature. In January that is close to freezing.

At the bottom of that tube is a bathroom, a WC or a kitchen: the rooms with no windows and the rooms that produce the most moisture. The physics does the rest.

How to be sure it is condensation

Timing tells you, and it takes no equipment.

Condensation appears in cold weather, is worst shortly after the room has been used, and bears no relation to whether it has rained. Water after a bath on a January evening, with a dry September downpour, is condensation.

A leak follows rain, usually rain driven from one particular direction. It arrives at one point rather than evenly and it leaves a mark that stays after everything has dried.

Insects or flies collecting in the diffuser point the same way as condensation: a tunnel is a sealed system, so anything inside got in through a gap, and the same gap admits moist air.

The two details that prevent it

Insulate the tube. Along its full run through unheated space, from where it leaves the ceiling insulation line to where it meets the roof.

This keeps the inner surface closer to room temperature, so there is nothing cold enough for moisture 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 measure, because it addresses the cause rather than the consequence.

Insulating the tube manages moist air once it is inside. Sealing the ring stops it arriving. Sealant or tape at the junction between the ring and the plasterboard, done properly rather than approximately.

It has a second benefit: warm air leaking from a heated room into a cold loft is a genuine heat loss, and it carries moisture into the roof space where it can condense on the underside of the covering.

The third factor, which is the room

A sun tunnel does not ventilate. There is no opening, no airflow, 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 working: sized for the room, on an over run timer or humidity sensor, and ducted to outside rather than into the loft.

That last point is worth checking while somebody is up there. Ducting a bathroom extract into a loft rather than through the roof or the wall is depressingly common on older installations, and it puts the room’s moisture directly into the space the tunnel crosses. The tunnel then condenses on air the fan delivered.

Where the joints matter

A rigid tunnel is assembled from sections, and each joint should be taped and sealed.

An open joint in the loft admits nothing on its own, and it lets moisture that has formed inside the tube escape into the loft rather than staying where it can be diagnosed. It also lets loft air, which is cold and sometimes dusty, into the light path.

On a flexible tube the concertina folds make a continuous seal harder to achieve and easier to compromise, which is one more reason to fit rigid wherever the route allows.

Warm roofs and converted lofts

The picture changes and it does not go away.

Where the insulation sits at rafter level, the tube passes through insulated construction rather than cold space, which reduces the condensation risk on the tube considerably.

What it introduces instead is a more demanding detail where the tube crosses the insulation and the vapour control layer. Both have to be maintained and sealed to the tube.

Get that wrong and moisture goes into a warm roof construction, where it condenses out of sight and shows up eventually as damp insulation or decayed timber. That is a worse outcome than the same error in a ventilated loft, where at least the moisture ends up somewhere it can escape.

Fixing an existing installation

Straightforward and cheap, and done entirely from inside.

Insulate the tube along its full run through the cold space. Seal the ceiling ring properly. Tape any open joints. Dress the loft insulation back up to the tube so the ceiling insulation line is continuous around the penetration.

None of that requires roof access, scaffold or a tower, and all of it is considerably cheaper than the roofing investigation people frequently commission first.

While you are up there, check where the bathroom extract discharges.

What not to do

Do not seal the loft ventilation to reduce the cold air around the tube. A ventilated loft is ventilated deliberately, and blocking eaves or ridge paths to tidy up around a tunnel produces a damp loft, which is a considerably worse problem.

Do not drill drainage holes in the tube. It relocates the water and leaves you with a tube that is now open to loft air.

Do not start on the roof. Establish whether it follows rain or cold weather first. Two of the possible answers do not involve the covering at all.

The two lines to insist on in a quotation

Insulation to the tube where it crosses unheated space, named rather than assumed.

An airtight seal at the ceiling ring.

Both are the first things dropped from a cheap price, and neither shows until the first cold spell, by which time nobody connects water at the diffuser with a line missing from a quotation months earlier.

The mechanism, step by step

A tunnel passes through unheated loft space. In winter the loft is close to outside temperature, so the tube wall is cold.

Warm air in the room below holds water vapour. If that air can get into the tube, it rises, meets the cold inner surface, and gives up its moisture onto it.

The water runs down the inside of the tube and arrives at the diffuser, where it pools and eventually drips.

To the owner this appears identical to a leak. Water is coming through the ceiling around a roof penetration. The difference is that a roofer sent to find it will find nothing wrong.

The diagnostic

Timing, and it costs nothing.

Water during or shortly after rain is ingress. Water in cold weather without rain, particularly on a still frosty morning after a night of cooking or bathing, is condensation.

A second clue: condensation appears at the diffuser and the ceiling immediately around it, evenly. A leak from a disturbed flashing tends to track and stain in one direction.

The two fixes, in order of importance

Seal the diffuser to the ceiling. This stops room air entering the tunnel in the first place, and it addresses the cause rather than the symptom.

It needs a proper gasket or a continuous silicone seal around the trim, bedded onto a clean surface. Not a decorative bead run over the top of an existing gap.

Insulate the tube along its full length through any unheated space, so the inner surface is not cold enough to condense on even if some air does get in.

Purpose made insulated sleeves exist. Where they were not fitted, a wrap of insulation secured properly does the job, provided it is continuous and does not compress to nothing at the joints.

The third measure

Tape the joints between rigid tube sections. An untaped joint lets moist loft air into the tube from outside, which condenses just as readily as air from the room.

And a double glazed diffuser raises the surface temperature at the ceiling, which helps where the tunnel serves a heated space.

Where the risk is highest

Bathrooms and kitchens, obviously. A tunnel into either should be sealed and insulated as standard rather than as an upgrade.

Also any house with high internal humidity and limited ventilation, which describes a great many older properties.

The room itself

A space that cannot lose moisture will find the coldest surface available. Where the tunnel has been sealed and insulated and condensation persists, the ventilation in the room is the remaining variable.

Ruling out the roof first

Before anybody goes up, check the pattern. Condensation appears evenly around the diffuser and follows cold weather. A leak from a disturbed flashing tracks in one direction and follows rain.

Establishing which you have costs nothing and saves paying somebody to look for a fault that is not there.