Do sun tunnels cause condensation?

Why it happens

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 below reaches it, and moisture condenses on the inner surface, runs down and collects on the diffuser.

It looks exactly like a leak, and people reasonably conclude the collector is letting water in.

The two fixes

Insulate the tube where it passes through the cold space, using a purpose made sleeve or by wrapping the run. That keeps the tube surface above the dew point.

Seal the ceiling collar so warm moist room air cannot get into the loft around the tunnel at all. This is the more important of the two, because it removes the cause rather than managing the symptom.

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

Where it matters most

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

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.

Fixing an existing one

Usually straightforward and worth doing properly. Get into the loft, insulate the run, and seal the collar at ceiling level. That is a couple of hours and it solves it permanently.

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

Only badly fitted ones

A sun tunnel does not produce moisture. What a poorly detailed one does is give existing moisture a cold surface to condense on, inside the tube, where the water then runs down and appears at the diffuser looking exactly like a leak.

It is the single most common complaint about sun tunnels and it is entirely preventable.

What is actually happening

The tube is metal and it passes through an unheated loft, so its surface sits at loft temperature, which in January is close to outside.

If warm moist air from the room below can get into the tube, it meets that cold surface and condenses. Bathrooms and kitchens produce the most moisture, and those are exactly the rooms sun tunnels serve.

The two details that prevent it

Insulation to the tube wherever it crosses unheated space. This keeps the inner surface closer to room temperature so there is nothing cold enough to condense on. It is a cheap material and it is frequently omitted, particularly on self installed kits that do not include it.

An airtight seal at the ceiling ring. This stops room air getting into the tube in the first place, which is the more fundamental fix. Sealant or tape at the junction between the ring and the plasterboard, done properly rather than approximately.

Both are named lines on our quotations rather than assumptions, because they are the first things dropped from a cheap price and their absence does not show until the first cold spell.

The third factor, which is the room

A sun tunnel does not ventilate. There is no opening and no airflow through it, so it cannot help a room shed moisture.

Where a tunnel serves a bathroom, the extract fan has to be doing its job. A room producing steam twice a day with a fan that does not run, or runs into a blocked duct, will condense on the coldest surface available, and after the tunnel is fitted that may well be the tunnel.

Telling it apart from a leak

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

If water appears at the diffuser after a bath on a January evening but not during a September downpour, it is condensation.

Putting right a tunnel already installed

Straightforward and it is done from inside the loft. Insulate the tube along its full run through the cold space, and seal the ceiling ring properly from below or above.

Neither requires touching the roof, and both are considerably cheaper than the roofing investigation people frequently commission first.

The tunnel does not create moisture, it provides a cold surface

A tunnel passes through unheated roof space. Unless it is insulated, its inner surface sits at loft temperature, which in winter is close to outside.

If warm damp air from the room can get into that tube, it meets the cold surface and condenses. The water then runs down the inside and arrives at the diffuser, where it drips.

It looks exactly like a leak, and it is not. Somebody sent up to the roof will find nothing wrong.

Sealing and insulating, in that order

Seal the diffuser to the ceiling. This is the more important of the two, because it stops room air entering the tunnel in the first place. Silicone or a proper gasket around the trim, not a decorative bead over the top of a gap.

Insulate the tube along its whole length through unheated space, so the inner surface is not cold enough to condense on.

Both are cheap at installation and awkward afterwards, because reaching the tube means getting into the loft and working around it.

Where the risk is highest

Bathrooms and kitchens, obviously, where the moisture load is greatest. A tunnel into either should be sealed and insulated as a matter of course rather than as an upgrade.

And any house with high internal humidity generally, which includes many older properties with limited ventilation.

The other measures

A double glazed diffuser raises the internal surface temperature at the ceiling and is worth specifying where the tunnel serves a heated space.

Taping the joints between rigid tube sections stops moist loft air entering from outside the tube as well.

And ventilation in the room itself, because a room that cannot lose moisture will find the coldest surface available whether that is a tunnel or something else.