The short answer
A sun tunnel is a tubular rooflight. A small dome on the roof collects daylight, a reflective tube carries it down through the loft, and a diffuser in the ceiling spreads it into the room. It suits a landing, bathroom, hallway or cupboard that has no outside wall, where a full roof window would not fit or is not wanted.
How a tubular rooflight works
A sun tunnel has three parts. At the top is a clear dome, set on a base that is flashed into the roof covering. Below it runs a tube with a highly reflective inner surface, which can be rigid or flexible. At the bottom is a diffuser, a frosted panel or fitting set into the ceiling of the room, which spreads the light around.
Daylight enters the dome, bounces down the tube, and leaves through the diffuser. The tube turns what would be a hole in the roof into a light pipe, so the light can travel past the loft space and rafters to reach a room that is some way below the roof surface. The tube length depends on the distance between roof and ceiling.
Light is the point, not heat or view. There is no outlook, and the output is less than a full roof window of the same area, since some light is lost on the way down. In return, the opening in the roof and the ceiling is small, and the installation is much less invasive than a conventional window.
Some models add an electric light for evenings, and some have a vent. How well the roof junction is made decides whether it keeps working for years.
Where a sun tunnel suits
The best sites are small rooms with no external wall. A landing or hall that gets no daylight, a bathroom or en suite in the middle of the house, a walk-in wardrobe, a utility space or a cupboard all benefit. In each case a roof window would be too large or the room is too far from the roof slope.
The roof structure above the room matters. A tube needs a clear path through the loft space between the ceiling and the roof, and rafters, ceiling joists, pipes and wiring may be in the way. A flexible tube can bend around a small obstruction, though bends reduce the light. We check the route from the loft side before suggesting a position.
The roof covering matters too. The base has to fit the tile or slate, with a flashing that suits it, and a fixing to the rafters. It is easier on a pitched roof with standard tiles than on an old stone slate. Our answer on whether a roof window fits any roof sets out the pitch and covering limits that apply to openings in general, and the data sheet of the product you choose gives its own range.
Where a room needs a bigger, brighter opening with a view, a roof window or a dormer is a better choice than a tunnel.
Planning and Building Regulations
On a house, a sun tunnel dome is usually an alteration to the roof, and Class C of Part 1 of Schedule 2 to the General Permitted Development Order 2015 is the relevant class. It excludes anything standing more than 0.15 metres out from the original slope, so a low-profile dome is the one to choose. A tall dome that stands further out needs a planning check.
Class C is for houses. Flats carry no permitted development rights at all, so a flat needs a planning application for anything other than a straight swap. A listed building also needs listed building consent, and a conservation area or Article 4 direction can change the position, which our answer on roof windows in a conservation area explains. A tunnel on a rear slope is far less likely to be questioned than one on the front.
Building Regulations apply to the opening and to the energy performance of the unit. Approved Document L gives rooflights a limit of 2.2 W/(m²·K) and roof windows 1.4, and which figure applies to a tubular unit is a question for your building control body, with the data sheet in hand. Cutting through a rafter or ceiling joist is structural, and the tube passes through the loft insulation, which needs to be kept continuous.
Installing one properly
The work has two ends. On the roof, the covering is lifted around the position, the base is fixed to the timbers and flashed into the tiles or slates, and the dome is fitted. In the room, the ceiling is cut, the tube is connected and sealed, and the diffuser is fixed. The tube is insulated through the loft so cold cannot reach the ceiling and cause condensation at the diffuser.
The flashing is the make-or-break detail. As with any rooflight, the junction with the covering is where leaks begin, and a dome base with the wrong flashing for the tile is no better than a bad window. Our guide to the roof windows and skylights service describes how we approach it.
Condensation is the second risk. Warm air in a bathroom can reach the cold tube, and moisture then collects inside it. A sealed tube and an insulated run help, and the room needs ventilation.
At the survey we check the route in the loft, the covering, the pitch and the room, and photograph each. You see what is possible and what is not. A single written price follows, and the product is named in the scope so you know what is going on the roof.
