Do sun tunnels work on a north facing roof?

What the collector gathers

Daylight from the whole sky dome, not direct sunbeams. On an overcast day, which in this country is most days, there is no directional sun at all and every orientation performs the same.

A domed collector is designed for that. It gathers from a wide angle, including low light early and late, which is why domes are used rather than flat panels.

The difference in practice

A south facing collector gets bright peaks when the sun is on it, which a north facing one does not. Averaged over a year the north facing tunnel delivers less, and considerably less less than a north facing rooflight would.

For a hallway or a bathroom, where you want reliable useful light rather than sunbeams, that is a reasonable trade.

The advantage

North light is even and consistent. No moving bright disc travelling across the floor, no glare, and no solar gain.

In a small internal room, consistency is worth more than peak brightness.

If you have a choice

Take south or east facing if the route allows it, and do not distort the run to get it. A straight north facing run beats a bent south facing one, because bends cost more light than orientation does.

That is the useful rule: route first, orientation second.

Compensating

Where north facing is the only option and the run is long, step up a size. A 350mm north facing tunnel comfortably outperforms a 250mm south facing one.

Yes, better than most people expect

A sun tunnel collects diffuse skylight as well as direct sunlight, and in an English climate diffuse light is most of what there is for most of the year.

A north facing dome never sees direct sun, so it loses the bright shaft you get from a south facing tunnel on a clear afternoon. What it gives instead is steady, even light through the whole day with no peak and no glare, and on an overcast day it performs almost identically to a south facing one, because on an overcast day the whole sky is the light source.

What you actually notice

Less variation. A south facing tunnel is dramatic at midday and dimmer at each end of the day. A north facing one is consistent, which in a bathroom or a hallway is arguably the more useful behaviour.

No solar gain either, which matters in a small internal room. A south facing tunnel into a windowless ensuite genuinely warms it on a hot afternoon.

Where orientation does matter

Less than run length, number of bends, and whether the tube is rigid or flexible. Those three decide far more about the result than which way the roof faces.

If you have a genuine choice between a north facing position with a short straight run and a south facing position needing two bends and an extra metre of tube, take the north facing one. The straight run will deliver more light.

Where the north facing route is the longer one, consider stepping up a diameter to compensate. Output goes with area, so 350mm passes roughly twice the light of 250mm.

Shading is the real question

Orientation matters much less than obstruction. A dome under a tree canopy, behind a chimney, or shaded by a taller neighbouring building loses far more than a north facing position does.

Look at what is above the dome position rather than which way the slope faces. A clear patch of sky is what the product needs.

Practical note

North facing slopes hold moss and algae more readily than south facing ones, because they dry more slowly. Since the whole product depends on collecting light, a dirty dome costs output directly, so a north facing tunnel benefits from being cleaned when the gutters are done rather than left to the rain.

That is a five minute job and it is the only maintenance the installation really asks for.

Why orientation matters less here than you would expect

A dome collects light from the whole sky, not only from direct sun. On an overcast day, which in this country is most of them, the sky is a uniform light source and a north facing dome collects essentially the same as a south facing one.

What a south facing dome adds is direct sunlight on clear days, which produces bright moments rather than a higher average.

So a north facing tunnel gives a steady, even, cool toned light through the day, with less variation. In a hallway or a landing that is arguably the better result, because you are after consistent illumination rather than drama.

What to do about it

Go up a diameter size. It costs little and it is the single most effective way to increase output on any run.

And keep the run short and straight, which matters far more than which way the roof faces. A single bend costs more light than several metres of straight tube, so a north facing tunnel running straight up will comfortably outperform a south facing one that has to dogleg around a tank.