Sun tunnels through a cold loft

The situation

A ventilated loft above insulation at ceiling level. In winter that space is close to outside temperature. The tunnel passes straight through it, so the tube is a cold surface with warm humid room air at the bottom.

Left alone, moisture condenses on the inside of the tube, runs down, and collects on the inside of the diffuser. People reasonably conclude the unit leaks.

The two things that prevent it

Insulate the tube through the cold space. Purpose made insulated sleeves are available, or the run is wrapped. This keeps the tube surface above the dew point so moisture never condenses.

Seal the ceiling collar. Where the tunnel passes the ceiling line, the vapour barrier and the ceiling finish are sealed to the tunnel. That stops warm moist air entering the loft around the tube in the first place, which is the real source of the moisture.

Both matter, and the collar matters more. Insulating the tube treats the symptom; sealing the collar removes the cause.

Bathrooms

Where the air is wettest, both details matter most. A sun tunnel from a bathroom into a cold loft with an unsealed collar will drip in the first cold week.

Do not block the loft ventilation

A cold loft depends on a continuous air path from eaves to eaves. A tunnel dropped through the middle of it, especially where insulation is heaped around the base, can obstruct that path.

Insulation is kept clear of the ventilation route at the eaves.

Warm lofts

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

The situation this guide is about

The commonest sun tunnel installation in this country: insulation lying between and over the ceiling joists, a ventilated and therefore cold loft above it, and a tube running from a dome in the roof down through that space to a diffuser in the ceiling below.

Everything that goes wrong with sun tunnels goes wrong here, and all of it is preventable with two details that cost very little.

Why a cold loft is a hostile environment for a metal tube

A ventilated loft sits close to outside temperature. That is deliberate: the ventilation keeps the roof space dry by carrying away any moisture that gets into it.

A sun tunnel puts a metal tube through the middle of that space, and metal is an excellent conductor. Left bare, the inner surface of the tube sits at roughly loft temperature, which in January is close to freezing.

Now consider what is at the bottom of the tube: a bathroom, a WC or a kitchen, producing warm moist air, with a diffuser opening into it.

Warm moist air meeting a cold metal surface condenses. Water forms on the inside of the tube, runs down it, and arrives at the diffuser.

Why it gets reported as a leak

Because from below that is exactly what it looks like: water appearing at a rooflight in the ceiling.

It produces a great many unnecessary roofing investigations, and occasionally a great deal of unnecessary work on a roof that was never at fault.

The tell is timing. Condensation appears in cold weather, is worst shortly after the room has been used, and bears no relation to whether it has rained. A leak follows rain, usually driven from one direction, and leaves a mark that stays after everything dries.

If water arrives at the diffuser after a bath on a January evening but the room stays dry through a September downpour, it is condensation and the fix is inside the loft rather than on the roof.

The first detail: insulate the tube

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

This keeps the inner surface of the tube 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. It is one of the two lines to insist on in a quotation.

The second detail: seal the ceiling ring airtight

The more fundamental measure, because it deals with the cause rather than the consequence.

An unsealed ceiling ring lets warm moist room air move up into the tube. Insulating the tube manages that air once it is there; 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. Air leakage 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. Sealing the ring closes that path.

Air leakage carries far more moisture into a construction than diffusion through an intact membrane does, which is why a taped seal and a loose one behave completely differently.

The third thing, which is the room rather than the tunnel

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 a 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.

A room producing steam twice a day with a fan that does not work will condense on whatever is coldest, and after a tunnel goes in that may well be the tunnel, which then gets blamed for a problem that predated it.

Restoring the ceiling insulation

Loft insulation cut for the tunnel leaves a gap around it, and a gap in a ceiling insulation layer is a thermal bridge and a convection path.

It is dressed back up to the tube rather than left short, so the insulation line is continuous around the penetration.

Not blocking the loft ventilation

A cold loft is ventilated deliberately, at the eaves and frequently at the ridge, to keep it dry.

A tunnel running across the space should not obstruct that airflow, and insulation pushed into an eaves gap while working around the tube is a common and avoidable error. Blocking eaves ventilation to tidy up around a sun tunnel creates a damp loft, which is a worse problem than the one being solved.

Where the loft is not cold

Worth distinguishing, because the detailing changes.

In a converted loft or a room in the roof, the insulation sits at rafter level and the space the tube crosses is inside the insulated envelope. That 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 at rafter level. Both have to be maintained and sealed to the tube, and getting that wrong puts moisture inside a warm roof construction, which is worse than the same error in a ventilated loft.

Fixing an existing installation

Straightforward, and it is done entirely from inside the loft.

Insulate the tube along its full run through the cold space. Seal the ceiling ring properly, from above where access allows or from below where it does not. Dress the ceiling insulation back up to the tube.

None of that requires touching the roof, and all of it is considerably cheaper than the roofing investigation people frequently commission first.

Where flies or insects are appearing in the diffuser, that is the same problem from another angle: a tunnel is a sealed system, so anything inside got in through a gap, and the same gap will let moisture in.

The two lines to ask for on 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 environment the tube passes through

A ventilated cold loft is, by design, close to outside temperature. That is what keeps the roof structure dry, and it is deliberate.

Which means a tunnel crossing it is a tube of cold metal or plastic running from the roof down to an opening in the ceiling of a warm room.

Everything that goes wrong with sun tunnels comes from that fact.

The two things that must be done

Seal the diffuser to the ceiling. The more important of the two, because it stops warm damp room air entering the tunnel at all.

A gasket or a continuous silicone bead onto a clean surface. Not a decorative bead run over an existing gap after the trim is up.

Insulate the tube over its whole length through the loft, so that even if some air gets in, the inner surface is not cold enough to condense on.

Continuous, without compressing to nothing at the joints or where it passes a rafter. Purpose made insulated sleeves exist and are worth using over an improvised wrap.

Where the ceiling insulation meets it

The loft insulation has to be dressed up to and around the tube rather than cut away to make room.

A gap in the loft insulation around a tunnel is a cold patch in the ceiling of the room below, and it produces the same cold band and condensation that a bridged rooflight reveal does.

Taping the joints

Rigid tunnels come in sections and the joints need taping.

An untaped joint does not leak rain, but it lets moist loft air into the tube, and loft air in winter carries plenty of moisture from the house below. That condenses inside the tube just as readily as air from the room.

Working around what is in the loft

Water tanks sit precisely where hallways are, which is precisely where tunnels want to go. Cabling, structural members and stored belongings all complicate the route.

Survey the loft before choosing the diameter and the ceiling position, because it is entirely possible to want a size or a route the space will not take.

And keep the run short and straight. Bends cost more light than length does, so a route a metre longer but straight will outperform a shorter one with a dogleg.

The diagnostic afterwards

Water at the diffuser after rain is a leak on the roof. Water in cold weather without rain is condensation in the tube, and no amount of work on the flashing will change it.