Sun tunnels against a full rooflight

Where a rooflight wins

Light. Far more, and not close. A modest rooflight delivers several times what a 350mm tunnel does.

View. Sky, weather, changing light. A tunnel gives daylight without any view at all.

Ventilation. An opening rooflight ventilates from the highest point of the room. A tunnel is sealed.

The feel of the room. A rooflight changes a space architecturally. A tunnel lights it.

Where a tunnel wins

Where a rooflight cannot go. The decisive case. An internal room with a floor or a loft between it and the sky.

Cost. Substantially less, because there is no structural opening, no trimming and no large glazed unit.

Disruption. Half a day to a day, one neat ceiling aperture. A rooflight is days and a structural opening.

Planning sensitivity. The roof collector is small and low. On a listed building a tunnel is sometimes acceptable where a rooflight is not.

Heat. A tunnel contributes very little solar gain, which in a small internal room is an advantage.

How to decide

Is there roof directly above the room? If yes, a rooflight, unless planning or budget rules it out. If there is a floor or a loft in between, a tunnel.

Both

Common in a single storey extension: rooflights over the main space and a tunnel into the internal WC or utility off it.

Two products, two jobs

A sun tunnel and a rooflight both put daylight into a room, and beyond that they have very little in common. Treating the tunnel as a budget rooflight is the commonest way people end up disappointed, and treating the rooflight as the automatic answer is how money gets spent where it did not need to be.

The honest way to choose is to be clear about what each actually delivers.

What a rooflight gives you

A great deal of light. A rooflight faces the sky directly, and on an overcast day the zenith is several times brighter than the horizon. That is why a rooflight admits roughly twice the daylight of a vertical window of the same area, and why the advantage is greatest on exactly the days you most want it.

A view. Sky, weather, moving cloud, and at night stars. That changes how a room feels in a way that light alone does not.

Ventilation, if it opens. An opening at the highest point of a room lets warm moist air leave and draws cooler air in through anything open lower down. Nothing else in a room does that as well.

Presence. A rooflight is a feature of the room. It changes the ceiling.

What a sun tunnel gives you

Soft, even daylight. A disc of light in the ceiling, brighter in the middle of the day and dimmer at each end, with no glare and no shadows cast.

Access to rooms nothing else can reach. This is the whole point. An internal bathroom, a WC, a hallway or a landing with no external wall and no available roof slope has no rooflight solution at all, and a tunnel brings genuine daylight into it.

Very low cost. The roof component is small and most of the work is from inside, so the access that dominates a rooflight quotation barely features. On a bungalow it is frequently a half day job with no scaffold at all.

Near invisibility. On a low profile unit the roof component sits close to the covering and reads as a very small rooflight, frequently invisible from any public vantage point on a rear slope.

Complete privacy. There is no line of sight through a mirrored tube, which in a bathroom is a genuine advantage over glazing.

What a sun tunnel does not give you

No view. No sense of sky. No ventilation whatsoever, because there is no opening and no airflow. And nothing at all after dusk, which is why the optional light fitting inside the diffuser is worth specifying at the outset.

It also gives considerably less light than a rooflight. A 350mm tunnel has an aperture of under a tenth of a square metre, and no amount of reflective efficiency turns that into a window. What it gives, on a dull afternoon in a windowless bathroom, is enough daylight not to reach for the switch. For the money that is a good proposition; it is not a substitute for glazing.

Five situations where the tunnel is the better answer

1. The room has no roof directly above it. An internal room, or one under another storey. There is no alternative and the tunnel is the answer.

2. A bungalow. The tube drops straight from the roof into the ceiling below, frequently under a metre. Light loss compounds with length, so a short straight run performs at the top of what the product can do, and every room in a bungalow is under the roof so the daylight can go where it works best.

3. Where a rooflight will not be consented. The intervention in historic fabric is one opening between rafters rather than a trimmed structural opening, and it is largely reversible. On a listed building that is an argument worth putting to a conservation officer, and we have seen tunnels consented where rooflights were not. It still needs consent: a tunnel is not exempt for being small.

4. Where the budget will not stretch to a rooflight. Not a compromise so much as a different scale of intervention, and in a windowless hallway it is transformative.

5. Where privacy is absolute. A bathroom overlooked from higher ground, where obscure glazing would work and a tunnel removes the question entirely.

Where the rooflight wins

Any room where you want a view, ventilation, a sense of sky, or a lot of light. Any habitable room being converted, where the window may also carry a means of escape duty that a tunnel cannot. Any kitchen or living space, where the difference in light output is the difference between a bright room and an adequately lit one.

And any room where the roof slope is directly above and accessible, because at that point the tunnel’s main advantage, reaching where glazing cannot, does not apply.

Getting the tunnel specification right

Three decisions decide whether a tunnel performs or disappoints, and the cheapest option is wrong on two of them.

Rigid rather than flexible. A rigid mirrored tube reflects 98 percent or better at each bounce. A flexible foil concertina reflects perhaps 80 percent and scatters light at every fold, and over a three metre run that difference compounds into most of the light. The practical result is a flexible tunnel delivering something like a third of what a rigid one of the same diameter and length gives.

Where the route genuinely will not take a rigid tube, a flexible one is better than nothing and we will fit one having told you what to expect. What we will not do is fit a flexible tube on a long run and let you find out in November.

Diameter, which matters more than people expect. Light output goes with the area of the tube rather than its width, so a 350mm tunnel passes roughly twice the light of a 250mm one, not forty percent more, and a 530mm doubles it again.

As a guide: 250mm suits an internal WC, an ensuite or a short hallway. 350mm suits a landing, a family bathroom or a small kitchen, and it is the size we fit most. 530mm is for a room you actually live in.

The cost difference between one size and the next is small; the light difference is substantial. Nobody has ever told us their sun tunnel was too bright.

Route length and bends. Every metre of tube costs light and every bend costs more than a metre. A straight four metre run beats a three metre run with two bends, and moving the ceiling position slightly to achieve a straight drop is frequently the better trade.

Two smaller rather than one large

Worth considering seriously in any room bigger than a WC. Two pools of light spaced apart read as ambient daylight; one larger tunnel in the middle reads as a spotlight with the corners of the room still dark.

Where two are fitted on the same visit, the second costs meaningfully less than the first, because the access, the protection and the setting up are shared.

The details that prevent trouble

Two, and both get skipped on cheap installations.

Insulate the tube wherever it crosses unheated loft space. A metal tube through a cold loft is a cold surface, and warm moist air from the room condensing on it sends water down to the diffuser looking exactly like a leak.

Seal the ceiling ring airtight, which stops room air getting into the tube in the first place and stops warm air leaking into the roof space.

Where a tunnel serves a bathroom, the extract fan also has to be doing its job. A tunnel does not ventilate, and a room producing steam twice a day with a fan that does not run will condense on whatever is coldest.

The comparison in cost terms

A sun tunnel is a fraction of the cost of a rooflight, and the gap is widest on single storey property where the tunnel needs no scaffold and the run is short.

On a two storey house with a ground floor room, where the tube runs through a loft and then a first floor void, the gap narrows: more tube, more joints, more time, and sometimes boxing in through a first floor room. At that point it is worth asking whether a rooflight in a different room would serve the house better for similar money.

How to decide in five minutes

Look up. If there is a roof slope directly above the room and you want a view, ventilation or a lot of light, you want a rooflight.

If there is no slope above the room, or the room is internal, or the only available position is on a slope that will not be consented, you want a tunnel.

If there is a slope, the room is modest, and what you actually need is enough daylight not to reach for the switch, the tunnel will do it for a fraction of the cost and it is worth pricing both.

Send a photograph of the room, one of the loft space above it, and a note of what the room is used for, and we will tell you which we would recommend and why. Where the honest answer is that a tunnel will not give you what you are hoping for, we will say so. It is a good product within its limits and a disappointing one outside them.