The short answer
Yes. A sun tunnel works on cloudy days, because it collects diffuse daylight from the whole sky as well as direct sun. On an overcast day it will not be as bright as on a clear one, but the room will still be lighter than it would be with no opening, and often lighter than you expect from the look of the sky.
The difference between a bright day and a grey one is one of degree, not a switch from on to off.
Why a cloudy sky still gives light
Cloud does not block daylight. It scatters it, so that light reaches the ground from every part of the sky and not from the one direction of the sun. That is why a grey day has no hard shadows.
A sun tunnel dome is a clear plastic bubble, or in some models a flat clear panel, and it takes in light arriving from many angles. Direct sun is a bonus, but the dome does not depend on it. The same tube carries whatever the dome collects, bright or dull, down to the diffuser.
Devon has plenty of bright overcast skies through the year, particularly in the damp months, so a tunnel does real work on days when a window on the north side of the house would look dim.
How much dimmer is a cloudy day
A heavy overcast sky delivers a fraction of the light of a sunny one, and the tunnel's output follows. A tunnel that lights a bathroom brightly at noon in June will give a softer, flatter light on a dull day in January. The room will not look dark, but you may turn the lights on earlier in the afternoon.
It is the same for any rooflight or window, and the tunnel keeps up better than most people expect because it has no frame or wall reveal in the way of the sky. We do not quote a lumen figure for a given sky, because it depends on the model, the length of the tube and the day. Our answer on how much light a sun tunnel gives goes through the numbers in more detail.
What makes a cloudy-day result better or worse
Four things change what the room gets when the sun is hidden.
- Tube length
- Every metre of tube costs a share of the light. On a bright day you may not notice, but on a dull one the loss is more obvious. See how long a sun tunnel can be.
- Bends
- Each elbow absorbs light, so a straight tube does better in poor light than one that angles round an obstacle.
- Diameter
- A wider tube collects more from a dull sky, because the dome is larger. In weak light, the extra size helps more than it does in strong sun.
- Dome and tube surface
- A clean dome and a polished rigid tube pass more light. A rigid tube usually does better than a flexible one, which our guide to rigid or flexible sun tunnels explains.
Roof direction matters less than you might think
A skylight on a north slope sees no direct sun in the middle of the day, but it still sees the open sky. In diffuse light the direction of the roof counts for little, because the light comes from everywhere. That is one reason a tunnel works for the dark landing in the middle of a terrace, where there is no good wall to put a window in.
The direction does decide how much heat and glare reach the room on a bright day, but a tunnel's small dome limits both, so it rarely overheats the way a large south-facing roof window can.
Winter light in Devon
The sun stays low over Devon in winter, and days are short, so much of the daylight a house gets between November and February is diffuse. A tunnel that is fitted straight and kept short does its most useful work then, bringing a steady, even light into a room that would otherwise need a bulb on all day. Keeping the dome clean helps too, because a film of green algae or dirt on a dome that sees little rain can take a visible share of weak light. Most homeowners can reach a low dome on a flat roof only with care, so ask us to look at it when we are on site.
Evenings and the night
A sun tunnel is a daylight device. When the sky is dark, it gives no light, and at dusk it fades with the sky. If you want a light in the same fitting, some models take a light kit, and we cover that in can a sun tunnel have a light fitted.
How the ceiling end shapes the light
The diffuser at the ceiling spreads the light into the room. Its shape and the finish round it change how that feels. A flat diffuser sitting in a plain ceiling gives an even pool. A ring with a clean plastered edge and a crisp paint line looks like part of the ceiling, not an add-on. On a grey day, when there is little contrast to hide a rough finish, an untidy edge is easy to see, so we take care with it. Our finish schedule records how the ceiling around the diffuser is formed and decorated.
Is a tunnel the right choice for a dull room
For a small, dark space such as a landing, hallway, bathroom or cloakroom, yes. For a larger room or one where you want a lot of daylight, a rooflight or roof window will deliver more, particularly on a dull day. We compare the two in sun tunnel or rooflight: which brings more light. If you are unsure, send a photo of the room with your request, and ask for a quote. We will look at the roof, the route and the room and recommend the product that suits, and our sun tunnel installation page covers the planning position and the fitting.