How much light does a sun tunnel give?

A sun tunnel puts a single, soft source in the ceiling. It looks something like a large recessed downlight, but the light is daylight, so it shifts with the weather. On a bright morning it is crisp and almost white. Under thick cloud it turns grey-white and quiet, and the room feels lit rather than lamp-lit.

The glow is strongest directly below the diffuser and fades outward. In a small space such as a landing, an en-suite or a walk-in wardrobe, that is usually enough to reach every corner. In a big room, the edges stay dim and you will still reach for the switch in the evening or on the darkest days.

A sun tunnel also gives no view and no direct sun patch on the floor. What it replaces is the electric bulb that was on all day, not the window you do not have. People are most often surprised that the room looks like it has a window, even though it does not.

You will see lumen and lux figures in brochures. We leave numbers out because the same product gives different results in different houses. What matters is the list of variables, in rough order of importance.

• Tube width. A wider tube collects and carries more. Going up one size in diameter makes a noticeable difference, more than most other choices. The limit is the space between joists and rafters.
• Length and bends. Light is lost a little at every reflection inside the tube. A short drop loses little. A long one, or one with elbows, loses more. A straight run is the best case.
• Tube type. A smooth rigid tube reflects better than a ribbed flexible one. Our guide on rigid or flexible sun tunnels goes into this.
• What the dome can see. A dome on an open slope collects far more than one half-hidden behind a chimney stack, a gable end or a large tree.

The diffuser matters a little as well. A frosted or prismatic one spreads the light wider, and a clear one throws a brighter spot. Neither adds light, but they change how the room feels.

On the same site, a rooflight gives more. A flat rooflight or roof window opens a large panel of sky directly into the room, and the light reaches a wider area. A sun tunnel squeezes the same sky through a pipe.

That does not make the tube the worse choice. It is the right choice where the rooflight cannot go. A landing in the middle of a first floor, a bathroom with a flat above, a wardrobe in the eaves: these rarely have room for a framed opening and a shaft lining. The tube fits where nothing else will, and a modest light is better than none.

The comparison also changes in winter. Both products collect light from the same grey sky, but a rooflight has a large pane that loses heat to the cold outside, and a tube has a small one. For a small windowless room, that smaller surface is a point in the tube's favour. For a lounge or a kitchen extension, you want the larger source, and our flat roof rooflights and roof lanterns are the better fit.

From October to March, a Nottingham sky is overcast for much of the time. A sun tunnel still works then, because the dome collects light from the whole dome of the sky, not just from the sun. Under cloud the sky is brightest straight overhead, which is exactly where a dome points.

What you lose on a grey day is brightness, not function. The room is lit, but more softly, and the difference between a south slope and a north slope shrinks. The shortest days, in the weeks around the solstice, are the weakest, and a tube that looks fine in April can feel modest in December. That is why we plan around the worst months and not the best. The grey-day daylight page sets out that thinking.

Dirt matters more in winter too. A dome streaked with moss, bird droppings or autumn leaf mould passes less light than a clean one, and a shaded roof collects them faster. A wipe from a ladder, or a rinse with a hose from the ground floor window, once or twice a year is worth doing.

We cannot promise an exact brightness, and anyone who does is guessing. What we can do is check the things that set it. At the free survey we look at the loft route, count the bends, measure the drop, look for shade over the dome position and tell you whether a bigger tube, a different position or a different product would suit the room better.

Where the honest answer is that a tube will give a modest result, we say so and suggest an alternative. Sometimes that is a second tube, sometimes a small roof window.

The written quote follows the survey, and the price is fixed once the roof has been seen. The work meets current Building Regulations, and the 10-year workmanship guarantee is in writing. If you want to know whether planning rules apply to the dome, read does a sun tunnel need planning permission.