Can you put a sun tunnel in a bathroom?

A bathroom boxed into the middle of a first floor, or tucked between two bedrooms, usually has nothing but an electric light. A frosted diffuser overhead gives it a soft, even daylight that is kind to a face at the mirror and shows whether the tiles are clean.

A sun tunnel also copes with the shape of the room. The dome sits wherever the roof allows and the tube finds its way down, so the diffuser can go over the bath or the shower tray rather than wherever a window would be. The diffuser is frosted, so nobody can see in from outside, and nothing needs a blind.

A bathroom that sits directly under a roof slope is the easy case. If another bedroom or a loft room lies between the bathroom and the tiles, the tube would cross it, and we would look at a different route or a different product.

A shower fills a small room with warm, wet air, and that air looks for any gap in the ceiling. If it climbs into a cold tube in the loft, it condenses on the lining and the light dims. So two things matter: the joint between the diffuser and the ceiling must be airtight, and the tube in the cold loft must be insulated.

We seal the diffuser to its ceiling ring and tape every joint along the tube. The insulation around the tube helps keep the lining above the temperature at which moisture settles.

Ceiling finishes in a bathroom deserve a thought as well. Paint made for damp rooms, or tiles carried up to the diffuser ring, stands up to the steam better than ordinary emulsion.

Extraction and daylight are two jobs with two ducts. The extractor fan carries damp air to the outside through its own pipe. Venting it into the light tube would fill the lining with moisture, so the two never share a path. Where a fan is already fitted, it stays as it is on its own duct to the outside, and the tube does not replace it. If your bathroom has no fan, the survey is the moment to raise it, since a room that has gone without may be a room that mists the mirror and grows mould in the corners.

When a fan is already there, we check where its duct runs in the loft so the tube's route avoids it. Where a new fan is wanted, we can allow for it in the layout of the ceiling, as both come through the same ceiling and need their own space.

After dark a tube does nothing, so the bathroom still needs its ordinary light. A lamp fitting can be built into the same ceiling ring as the diffuser, which gives a single round panel for both day and night. A lamp inside a bathroom ceiling means electrical work, and its position relative to the bath or shower is something we check before we agree it.

Cutting a new opening in a roof is building work, and we handle the Building Control notification where the work is notifiable. For the planning rule that applies to the dome on the roof, see our skylight planning guide.

If you want to know how the light behaves in other rooms, our hub on sun tunnels goes through landings, halls and wardrobes. For a very deep route, read how long a tube can be in our answer on run length. To start, book a free survey and say which bathroom is dark.