Where Sun Tunnels Work Best

The commonest candidate is the first-floor landing in a house where the stairs rise between two walls. A landing like that often has no window, and the bedroom doors are shut. Daylight reaches it only by chance, so a light has to be on all day.

A tunnel solves that cleanly. The landing ceiling usually sits directly under the loft, the route is short, and the diffuser throws light across the landing and down the stairwell. If the stairs turn, the light reaches the upper flight best and the lower flight less well, so the lower hall may still want a light fitting.

Hallways on the ground floor of a bungalow work in the same way. The ceiling is under the roof space, the front door may have little glass, and the corridor to the bedrooms is windowless. A single tunnel placed over the middle of a short corridor lights the area around it, while two spaced along a long hall spread the light more evenly than one.

Before anything is ordered, we check what sits in the loft above the landing. A cold water tank, a run of pipes or a bracing member can sit on the line the tube needs, and a bend added to go round them reduces the light that arrives.

Internal bathrooms and cloakrooms are the second large group. A bathroom with no outside wall relies on an electric light and an extractor. A tunnel adds daylight for the hours when the room is in use.

Privacy is built in. The diffuser is a frosted panel, so nobody can see in or out, and there is no overlooking from a neighbouring roof. This is a clear advantage over a clear pane of glass in the ceiling slope.

A tunnel does not ventilate. The tube is sealed, so the room keeps its extractor fan, and the two work side by side. Where a ventilation kit is wanted as part of the tunnel, we confirm at the survey whether the model supports one. Opening roof windows can help ventilate a room, but they do not suit every bathroom, and our comparison page, sun tunnel or roof window?, sets out the difference.

Condensation is a bathroom matter whatever the roof does. Warm, damp air meets cold surfaces, and that happens near a poorly sealed tube as readily as near a window. The seal at the ceiling and the insulation around the tube in the loft are both parts of the installation, and both are checked.

A kitchen in the middle of a house, or in an extension that has been built onto an older core, may have only one outside wall. The work surfaces furthest from that wall sit in shadow. A tunnel over the dark end of the room gives a softer pool of light than a spotlight does.

Utility rooms and boot rooms follow the pattern. They are often tucked between a kitchen and a garage, with a ceiling under the roof and a small floor area. A tunnel of modest diameter lights a small room fully, and it needs no wall space and no electrical work.

The limit shows in a large kitchen. A tunnel lights an area around the diffuser, and a big room gets a bright pool rather than light across the whole floor. For an open-plan space that wants daylight over a wide area, a lantern or a flat rooflight is the better answer. See roof lantern or flat rooflight for that comparison.

Rooms under a flat-roofed extension are a separate case, because the roof structure is different and the readings change. Our page on a sun tunnel through a flat roof covers it.

Walk-in wardrobes and small dressing rooms are at the quiet end of the house, and they have a habit of ending up with no window. Colours are hard to judge by artificial light, so daylight is welcome. A small diameter tunnel is enough, and the diffuser sits above the middle of the floor.

Store rooms, cupboards that people walk into and the small room at the top of a stair can be treated the same way. If you only need to see what is on the shelf, a short tube and a small dome do it. If you want a room where somebody sits and works, a window and a view will suit better.

In a loft conversion, a tunnel is rarely the main source of light. A tunnel needs a void above the room, and a converted loft mostly has the roof slope as its ceiling, so a room that is itself in the roof is normally lit by a roof window. A small en suite with a flat ceiling and a void above it can be the exception. Between them, you can light every level of a house with a mix of the two.

The same dark rooms turn up in the same kinds of house. A bungalow has a central hall with bedrooms off it, and the hall has no wall to the outside. A 1930s semi has a landing above the front door and a bathroom tucked over the porch or the kitchen. A terrace has a long, narrow plan in which the middle rooms have neighbours on both sides. An extension built on the back of an older house often leaves a utility room or cloakroom in the join between the two.

None of these needs a tunnel, and many have a window that does the job. The ones that benefit are the rooms where the plan has left no wall to put glass in. When we visit, the first thing we ask is which rooms you walk into and switch a light on at midday.

Chalet bungalows are the mixed case. The rooms upstairs are in the roof and want windows. The landing and the ground-floor hall below them are different: the stair void may lie directly under a roof slope with a short route, and a tunnel can reach it. Which of the two a given room needs is a question of what lies directly above it, and that is exactly what the loft survey records.

The dome sits on the roof, so what happens up there matters as much as the room below. These features shape how much light the dome collects.

• A clear view of the sky. A chimney stack, a taller neighbouring roof or a mature tree can shade the dome for part of the day.
• The slope it sits on. A slope that faces the sun for longer collects more direct sun, but the dome also gathers light from the open sky, and a tunnel is chosen for the room below rather than the compass.
• The pitch. Domes are made with flashings to suit a range of pitches, and each model has its own rated range: the VELUX pitched-roof sun tunnel, for example, is rated for 15 to 60 degrees. We measure your slope and match the flashing to it.

Tiles and slates are fine for a tunnel, and so are many other coverings. The flashing is dressed to suit the covering, and the dome stands clear of the surface. Whatever the covering, the survey records it.

The tunnel route is read from the loft. We trace the line from the underside of the roof covering to the ceiling, count any bends and note what is in the way. The loft readings go on the survey sheet along with the roof ones, and the process is described in our measured survey page.

For flat roofs the logic changes slightly. The dome sits on a kerb, and the tube follows a short route through the roof structure. This is a different set of readings, but the same principle holds.

Every tube brings the same trade-off. Light output falls as the tube gets longer, and each bend costs more again. A room in a bungalow, with a short straight run, gets the most from a tunnel. A landing under a deep, complicated loft gets less.

To cope with an awkward route, a tube can be made wider, or flexible sections can be added. A wider tube carries more light but needs a wider gap between rafters. Take rafters at 600 mm centres as an example: the gap is a little under 600 mm, so a tube has to be smaller than that. Specific diameters and their limits are covered on our diameter answer, and the longest runs are discussed on how long a sun tunnel can be.

Our rule is to quote only after the loft has been looked at. A tunnel that suits a landing in one house can be a poor choice in the next, and the readings are what tell the two apart.

A tunnel is the wrong answer when the room needs something it cannot give.

• Rooms in the roof. A bedroom or study under the slope needs a roof window, which gives a view, opens and can serve as an escape window in a bungalow.
• Large open-plan rooms. These need more glass than a tunnel can supply.
• Rooms where you want to see the sky. A diffuser is a light fitting, not a window.
• Rooms with no usable loft or roof space above. Without a route there is nowhere to run a tube.

The overlap is where most of the thinking happens. A house with a bedroom in the roof and a dark landing below it will often take a roof window for the bedroom and a tunnel for the landing. Both can be measured on the same survey, and each is sized to its own room. The planning position for both is in our planning guide, and the survey covers what applies to your roof.

To describe a dark room and ask what would suit it, request a survey. Mention what sits above it, so the survey can check the loft route for that room.