The short answer
Yes. A sun tunnel collects daylight from the whole sky, not only direct sunshine, so it keeps working under cloud. On a grey day the light it delivers is softer and lower, and it changes through the day with the sky. A windowless hall or bathroom that would otherwise need a bulb at midday usually stays usable without one, though it will not glow like a sunny afternoon.
Where a tunnel's light actually comes from
It is easy to picture a sun tunnel as a pipe for sunbeams. It is better thought of as a pipe for sky.
The dome on the roof is clear, and it looks up at a wide patch of sky. Daylight reaches it in two forms. One is direct sun, a hard beam from a single direction. The other is diffuse light, the general brightness of the sky that comes from everywhere at once, because cloud and air scatter the sun's light in all directions. Under a thick grey overcast there is no beam at all, yet the ground is still lit. That is diffuse daylight, and the dome gathers it.
The light then goes down a tube with a mirror-like lining and spreads into the room through a diffuser in the ceiling. Nothing in that path needs a beam. The lining bounces whatever arrives, and the diffuser scatters it. Our page on how a sun tunnel brings daylight into a dark room follows that path from dome to ceiling.
Because diffuse light arrives from the whole sky, the orientation of the roof matters less on a cloudy day than on a sunny one. On a clear day a south-facing dome gathers more beam. Under cloud, all slopes see about the same sky.
What a grey Sussex morning looks like under one
Expect a room that is lit evenly and gently, without bright patches or hard shadows.
A dull November morning across the Weald is a fair test. The sky is a flat lid of cloud and the garden looks muted. In a hallway with no window, the tunnel gives a pale, steady wash that is enough to find your way around, put shoes on and read a label. It is not the brilliant pool you get in June. It is closer to the light you would get standing in a north-facing doorway.
The light also changes as the cloud does. A break in the cloud brings a brief brightening, then it softens again. That slow variation is part of why a tunnel feels natural. It is tied to the weather outside, which a bulb never is.
In winter the sun sits low and the days are short, so there is less daylight to collect whatever the weather. A tunnel still helps, but the hours it is useful are the same hours the day is bright. Our page on how much light a sun tunnel gives covers what to expect across the year.
What makes a cloudy-day tunnel better or worse
When the sky gives less, the build has to lose less. Four things decide how much of the grey light reaches you.
| Choice | Why it matters under cloud |
|---|---|
| Diameter | A wider dome catches more sky |
| Length and bends | Each bend and metre loses some light |
| Tube lining | A highly reflective lining loses less |
| Dome and diffuser | Clean, clear surfaces pass more |
Diameter is the biggest lever. A wider tunnel gathers more, and on a dull day that extra catch is the difference between a useful room and a dim one. The trade-off is the space it takes in the roof and the ceiling, which is why choosing a sun tunnel diameter is worth doing with the room measured.
Length is next. A straight, short run carries light well. A long, bent one gives a good share of it up on the way down, and that loss is plainest on overcast days when there is little to spare. Our page on tunnel length, bends and how much light arrives explains the balance. Where the loft is deep or the route awkward, a rigid tube with a mirror finish tends to beat a flexible one for light, as set out in rigid or flexible sun tunnels.
A dirty dome, moss or a stained diffuser will cut the light further. Cleaning is simple and is covered in our answer on how to clean a sun tunnel.
Rain, cloud and quiet
Cloudy days are often wet days, and a dome has to handle both.
Cloud and rain arrive together in a Sussex autumn. A sun tunnel is a small opening, and a well-made dome and tube do not amplify rain the way a large flat pane can. Still, the dome is a thin shell on a hard surface, and a lightweight one can tick or drum in a downpour. The way the dome is seated, the tube is fixed and the loft space is lined all affect what you hear in the room below. We deal with that in rain on the dome: keeping a sun tunnel quiet.
The quiet side of the job goes into the quiet spec that comes with every quote, along with the size and route of the tunnel. For a room under a busy flight path, the same spec records how we have dealt with noise from above.
Is a tunnel enough, or do you need a window-type rooflight
For a hall, landing, cupboard-sized room or bathroom, a tunnel is generally the right tool. For a room that needs real brightness, a larger opening may be better.
A tunnel gives a modest amount of light at a point. A rooflight gives a large, bright panel. If you want to read, cook or work in the room all day, the extra area of a rooflight counts for more when the weather is poor. Our comparison of a sun tunnel or rooflight for a dark room helps you choose. Where the room is an internal bathroom or a dark stair, see sun tunnels for windowless bathrooms.
The last step is a survey of the actual roof and ceiling. Get a quote and we will look at the route, the diameter and what the room needs on its dullest day.
Cloudy-day sun tunnel questions
Will a sun tunnel make a room brighter than a bulb on a grey day?
Not always, and it does not need to. The point is to give natural light at no running cost, and it does that in proportion to the sky. A switched light still serves evenings and the darkest afternoons, and the tunnel covers the hours in between.
Does a tunnel work in the evening after the sun sets?
No, there is nothing to collect once the sky is dark. The answer on sun tunnels at night explains the light kits that fill that gap.
Can I make the light softer or dimmer?
Yes, with the right diffuser or a damper. See how to dim a sun tunnel for the options.
Does a north-facing roof make a difference?
Less than on a sunny day, since cloud spreads the light across the whole sky. The page on fitting a sun tunnel on a north-facing roof gives the detail, and our sun tunnel installation page covers the wider service.