Do Sun Tunnels Work on Cloudy Days?

The name suggests direct sun, but a sun tunnel works on daylight. On an overcast day the cloud scatters the sunlight across the whole sky, and that scattered light still reaches the dome. A clear dome collects what falls on it from a wide angle, so the sky does not need to be bright blue.

That scattered daylight is what lights most rooms most of the time. In a British climate a lot of days are grey, and a windowless room fitted with a tunnel is lit by diffuse daylight on those days. Light levels in the room drop as the cloud thickens, and rise again when it thins.

Nobody can promise a fixed amount of light, because it depends on the sky, the season, the tube and the room. We do not quote figures for it.

The dome itself is a clear or lightly textured acrylic or polycarbonate shape that sheds rain and lets light in from several directions at once. Some domes carry a ring or prism inside that bends low-angle light down the tube, which helps in winter when the sun sits low and the sky is often grey.

Direct sun gives a bright, defined pool of light. A tunnel on a sunny day throws a clear, strong glow from the diffuser. Under cloud, the light is softer and the glow spreads more evenly, with no sharp edge.

That softer light is often more pleasant in a bathroom or a hall. It does not produce glare, and it shows colours accurately. The room feels lit rather than spotlit.

The difference between a bright day and a dull one is real, though. A room with a sun tunnel on a dark December afternoon is dimmer than on a bright June morning. That is true of any window, so the right comparison is a room with the tunnel against the same room without it.

• Clear sky: strong output, highest when the summer sun is high.
• Thin or broken cloud: output that rises and falls as the cloud moves, with softer light.
• Thick, low cloud: a clear drop, but still a lit room.
• Dusk, dawn and night: little or none from the dome.

Seasons shape the picture as much as cloud does. Winter days are short and the sky is low, so a tunnel delivers less in December than in June, whatever the cloud. If the room matters most in winter, say so at the survey; it can move the choice to a wider tube.

The sky is only one factor. The dome diameter decides how much light enters, so a wider dome collects more on a grey day. Our guide to what diameter sun tunnel you need explains the sizing.

Tube length and bends decide how much of that light survives the trip. A short, straight run loses little. A long run with elbows loses more, and on a dull day that loss shows. The page on how long a sun tunnel can be sets that out.

Roof direction counts less than it does for a window. A dome on a north-facing slope still gathers daylight from the open sky above it. Shade does matter: a dome beside a tall tree or a chimney stack will see less sky, and the survey checks what stands around the position.

The diffuser at the ceiling spreads the light and gives the room its look. Clean domes and clean diffusers pass more light, so an occasional wipe of the diffuser, as the maker advises, helps.

A room painted a light colour returns more of the light that reaches it. Dark walls absorb it, so the same tunnel gives a lighter feel in a pale room, and a light floor carries the glow further.

A sun tunnel is chosen for a room with no other daylight. Landings, hallways, bathrooms, utility rooms and walk-in wardrobes are the usual places. They are rooms people pass through or stay in briefly, and a soft level of daylight makes them pleasant without any lamp on during the day.

For a room you work or read in all day, a roof window brings more light and gives a view. The comparison is set out in our sun tunnel or roof window guide, and the rooms that suit a tunnel best are covered in where sun tunnels work best.

Some systems offer an optional electric light fitting in the diffuser, so the same ceiling opening serves at night. Not every make has one, and it needs a wiring connection in the loft, which should be done by a qualified electrician. Under Part P in England, adding a light to an existing circuit is not notifiable work on its own. It becomes notifiable where it needs a new circuit, or where it falls in the special location around a bath or shower: within 0.6 m of the bath or tray and up to 2.25 m above the floor, or 1.2 m from a shower head with no tray. A bathroom diffuser can fall inside that space, so the position is checked first.

One thing to avoid is judging a tunnel by a photograph taken on a sunny day. Ask the installer how it performs in a room like yours under grey cloud, and expect an answer in terms of what the tube and diameter can do, with no promise of a specific brightness.

The fair test is to stand in the room on a grey day with the lights off. A hall with no window is dim enough to need a lamp at midday. With a tunnel, it is lit by daylight. That is the gain, and it holds under cloud.

We measure the roof, the loft and the ceiling position at the survey, and note the shade around the dome. The written quote then states the diameter and tube type planned for the room. Details of the work are on the sun tunnel installation page.