Sun Tunnel or Roof Window?

A roof window gives a wide, open view of the sky. The glass is large, usually one of the standard sizes, and light enters at the angle the sky provides. In a room with a sloping ceiling the light spreads across the sloping plane and the floor. A large window can light a whole room.

A sun tunnel gives a smaller, softer pool of light. The dome collects daylight, the tube carries it down and the diffuser spreads it into the room. It has no view in either direction, and a diffuser at ceiling height gives even light that reads as a ceiling light which happens to run on daylight. Because the tube is a reflective surface, the brightness reaching the room depends on how far the light has to travel. A short, straight tube delivers more light than a long tube with bends. We never quote a figure for a tunnel in advance, because the result depends on the particular run in your loft.

So the two serve different jobs. A window lights a room and connects it with the sky. A tunnel brings some daylight to a place that otherwise has none.

A roof window also changes with the weather and the hour in a way a tunnel does not. Sun crosses the glass, shadows move across the floor, and the room feels connected to the day outside. Some owners want that and some find it too bright on a south-facing slope, in which case a blind on the window softens the glare. A tunnel is steadier: the diffuser brightens and dims with the sky, but it does not throw a bright square of sun across the carpet.

The way sun tunnels cope with low light is dealt with on our page on cloudy days.

A window lets you look out, and it lets the sky look in. For a bedroom or a study that is a gain. In a bathroom or a toilet, it can be a problem. Obscure glazing deals with it in a roof window, though many people would rather not have a clear pane above a bath. A sun tunnel has no clear view in either direction. The diffuser scatters the light, so it is naturally private.

Fresh air makes the other difference. A roof window that opens helps ventilate the room, and a pivoting sash lets you reach the outside pane for cleaning. A sun tunnel is sealed along its length, so it does not ventilate. If the room needs air moving through it, such as a bathroom or a utility room, a tunnel alone will not do that job. Some tunnel ranges offer a ventilation kit, and we would check what a particular model supports at the survey. Where a bathroom has an extractor already, the tunnel only needs to supply light.

In a bathroom, steam meets cold surfaces whichever unit is fitted. An opening roof window helps ventilate the room, and a sound extractor does the rest.

Quick test

• If you want to look out, choose a window.
• If you want light but not a view, a tunnel is enough.
• If you want to open it, only a window does that.
• If the room is under a loft, not in the roof itself, look at the tunnel first.

A roof window sits between the rafters, but it usually needs more room than the rafters give. VELUX gives width by the first letter of the size code, and the range starts at 55 cm for a C and passes 78 cm for an M and 94 cm for a P. Take rafters set 600 mm apart: the clear gap between two of them is a little under 600 mm, so a window in M or P width needs a rafter trimmed. The cut rafter is carried on new trimmers across the top and bottom of the opening, and forming the opening is a structural alteration that needs Building Regulations approval.

A sun tunnel is a smaller opening. The common domestic tubes, 250 mm and 350 mm across in the VELUX range, are chosen to fit the gap between two rafters, so the tube can usually pass through the roof without a rafter being cut. That is a real saving in structural work. Diameters and gaps are covered in our answer on tunnel diameter.

The loft matters as well. A tunnel needs a clear route through it. Water tanks, ducting, cables and braces can all be in the way, and bends are added to go round them. A roof window opens into a lined well between the roof and the ceiling, so it needs the ceiling underneath to be in the roof slope. A window on a pitched roof cannot light a first-floor landing that has an unconverted loft above it, unless the loft is opened up and a lightwell formed. For that situation a tunnel is far less disruptive.

If the roof is flat rather than pitched, the comparison changes again. That case has its own page: can a sun tunnel go through a flat roof?.

Where a bedroom or study is formed in the roof, the window sits in the sloping ceiling, and the lining of the opening is angled to bring light in. A window works when there is usable floor under the slope and a head height that allows people to stand near it. The height of the sill above the floor is set by the room layout and by the model's pivot position. For a guide to the size codes and how they fit your rafters, see our guide to roof window sizes.

A tunnel has no such requirement. It can sit over any room with a ceiling and a loft above it. Because it is a ceiling fitting at the bottom end, it does not alter head height, and nothing is added to the wall or sloping plane.

Escape is the other headroom point. In a bungalow or chalet bungalow, the upper rooms can use a suitably sized roof window to get out. Above a two-storey house the floor is too high for that, and the loft room needs a protected stair instead. A tunnel has no escape role at all. If the room is a bedroom in the roof, the window is the unit that answers the escape question, and the sizes that suit are set out in our Building Regulations guide.

Both units are alterations to a roof, so both come under the planning rules for rooflights. The national rule for houses is the same for either, and it is set out in our planning permission guide. A flat, a listed building or a house where permitted development has been taken away follows the exceptions in that guide. A sun tunnel dome is smaller than a lantern, but it stands proud of the slope, so its height still has to be checked against the 150 mm projection limit in the conditions.

Building Regulations touch a roof window in two ways, and the guide to rooflights and Building Regulations has the detail. A new opening that needs a rafter cut is a structural alteration. A whole replacement unit is notifiable as a controlled fitting. The thermal standard for a new roof window is covered in the Building Regulations guide. For a tunnel, the fitting arrangement decides what is notifiable, and we confirm that for your roof at the survey instead of guessing in advance.

On heat loss, a window is a single large pane of double or triple glazing in a sealed frame. A tunnel puts a dome and a tube through the loft, so there is a pathway for cold air to the ceiling unless the tube is sealed and insulated properly. The ceiling diffuser acts as a seal at the bottom. Good installation matters on both, because gaps around the frame, the flashing or the tube are where warmth escapes.

Neither unit is a heater. Glazing facing a bright slope will let in warmth in summer. A roof window can be fitted with a blind to control that. A tunnel brings less heat in, simply because its dome is smaller.

A sun tunnel is the lighter job. A hole is cut in the roof covering for the dome flashing, the tube is run through the loft, and the diffuser is fixed to the ceiling. The mess is mostly a round hole in the ceiling plaster, and the room below can be put back in use soon after. Where the tube fits between rafters, no new timbers are needed.

A roof window is more involved. Slates or tiles are lifted, the opening is formed and framed, flashing is dressed to the roof covering, and the inside is lined and plastered. A scaffold tower is often needed to work safely. The finishing, with plaster reveals and paint, takes longer than the fitting.

Neither job should start until the measuring has been done. We take rafter centres, pitch and the loft route on the roof itself, put the readings on a sheet and give you a copy, which is the same process for both. That process is described on our measured survey page.

Replacement later is simpler for the window, which is a standard size and can be swapped like for like. A tunnel's dome, tube and diffuser are separate parts supplied as one kit by the maker.

Put the room first. The same question separates most cases.

Some houses use both. A roof window lights the room in the roof, and a tunnel lights the landing on the floor below. They are fitted on different parts of the same roof, and the survey covers both in one visit.

If you are unsure which suits your room, ask for a survey and describe the room and what sits above it. For rooms that suit a tunnel, our guide to where sun tunnels work best goes through the common ones.