The diffuser is the only part of a sun tunnel anybody sees, and it does more than cover the hole. It decides how the light spreads, how bright the ceiling looks, and whether the tunnel stays dry. This page is what the differences actually are.
What it does
Three jobs, and only one of them is appearance.
It spreads the light arriving down the tube across the room rather than letting it fall as a column. It seals the bottom of the tube against room air. And it finishes the opening.
Which means choosing one on looks alone tends to disappoint on the first two.
Flat against domed
| Flat | Domed | |
|---|---|---|
| Appearance in the ceiling | Flush, discreet | Sits proud slightly |
| Light spread | Narrower | Wider |
| Suits | Modern interiors | Maximum spread |
| Dust on the face | Collects less | Collects less again |
| Low ceilings | Better | Slightly intrusive |
Most installations use a flat diffuser and most people are pleased with it. A domed one earns its place where the room is wide and the spread matters more than the flush finish.
Prismatic against opal
The optical difference, and it is the one that changes the room.
A prismatic diffuser has a moulded pattern that refracts light outward, spreading it across the room and away from the column below the tube. It gives a wider, more even distribution.
An opal diffuser is a translucent white surface that scatters light softly. It looks calmer, glows more evenly across its own face, and spreads slightly less.
In a bathroom or a hallway either works. In a wider room, prismatic distributes better; where the diffuser is directly in view from a seating position, opal is more comfortable to look at.
How much light each loses
Worth knowing, because every diffuser costs something.
Light passing through any diffuser loses a fraction of itself. A clear cover would lose least and would produce a bright column and a dark room, which is not what anybody wants.
Which means the loss is the price of the spread, and the useful comparison between diffusers is not which is brightest at the diffuser face but which lights the room best.
Sealed and insulated types
The specification that matters in a bathroom.
Some diffusers are designed as a sealed assembly with a gasket, and some include an insulating element or a second inner lens. Both raise the temperature of the visible surface and cut the amount of room air that can reach the tube.
In a bathroom that is the difference between a dry tunnel and one that mists on cold mornings, and it is covered in detail on the condensation page.
Double diffusers
Two lenses with a gap between them, and it is worth understanding what it buys.
The gap is a thermal break, in the same way that two panes of glass are. It reduces the chance of the visible surface reaching dew point and cuts heat loss through the opening.
It costs a little light. On a bathroom or a room where the tunnel passes through a very cold roof space, that trade is usually worth making.
Colour and the light it gives
Diffusers are not all neutral and it shows.
A slightly warm diffuser makes daylight read as warmer than it is, which some people prefer in a bathroom. A neutral one delivers something closer to the actual daylight arriving.
Where a tunnel and a rooflight light the same space, a neutral diffuser matches better. Where a tunnel is the only daylight in a small room, either is fine and it is a matter of taste.
Diameter and trim
The dimensional side, and the trim is the part that decides how it looks.
The diffuser matches the tube diameter. The trim ring is what covers the cut edge of the plasterboard, and its width determines how forgiving the installation is of an imperfect opening.
A slim trim looks better and demands a neater cut. A wider one is more tolerant. On a ceiling being made good anyway, the slim one is worth the extra care.
Square diffusers
Available on some systems and worth considering.
A square diffuser on a round tube reads as a deliberate ceiling detail rather than a circular hole, and in a room with square downlights or a rectilinear plan it can sit better.
Optically there is little in it. It is an appearance choice and it is a legitimate one.
Where it sits in the ceiling
A detail that is decided at installation and is difficult afterwards.
The diffuser is where the light arrives, so its position decides which part of the room is lit. In a bathroom, over the middle of the floor rather than over the bath is generally right; in a hallway, at the point somebody stands rather than at the far end.
The tube can be offset a modest amount to put the diffuser where it should be rather than where the roof happens to allow, and that is worth doing.
Cleaning
The maintenance question, and the answer is: rarely, and easily.
A sealed tunnel keeps dust out, so the inside of the diffuser stays clean. The room side collects what any ceiling surface collects, and it wipes.
Most diffusers unclip for cleaning, which is worth checking before ordering rather than discovering that removing it means a tool and a risk of cracking it.
What discolouration means
Worth naming because it worries people. A diffuser that yellows over many years is an ageing plastic and it is a replaceable component. A diffuser that darkens quickly, or shows patches, is generally dust or moisture inside the tube rather than the lens itself.
Which makes discolouration a diagnostic as much as a cosmetic issue: it is frequently telling you the tube is not sealed.
Replacing one
Straightforward on most systems. The diffuser is a consumable in the way a lamp is, and a replacement restores the appearance of a tunnel that is otherwise sound. On a current model it is an ordered part; on an older one it may need a compatible alternative.
Where the tunnel is old enough that no part exists, replacing the diffuser assembly with a current one that fits the tube is generally possible.
Diffusers with light kits
The interaction worth knowing about. Where a light kit is fitted, the diffuser is being lit from behind at night as well as from above by day. An opal diffuser handles that better, because a prismatic one lit by a point source can show the lamp as a bright spot through the pattern.
Which is a reason to decide the light kit before the diffuser rather than after.
Fire performance
A consideration in specific positions. A diffuser is a plastic element in a ceiling, and where that ceiling has a fire resisting function, on a protected stairway or between dwellings, the whole tunnel penetration is a question for the building’s fire strategy rather than a product choice.
In an ordinary domestic bathroom or hallway ceiling it does not arise. Where it might, it is established before the opening is cut.
Acoustics
Rarely raised and occasionally relevant.
A tunnel is a pipe connecting a room to a roof, and a diffuser is the only thing closing it. On a shallow roof under heavy rain, some households notice a faint sound through the tunnel.
A sealed diffuser, and a double one especially, reduces it. It is a minor effect and it is worth knowing exists rather than being surprised by it.
What to specify, by room
| Room | Diffuser | Why |
|---|---|---|
| Bathroom | Sealed, double or insulated | Humidity |
| Hallway or landing | Flat prismatic | Spread along the space |
| Small internal cloakroom | Flat, sealed | Tidy, forgiving |
| Wide room | Domed prismatic | Maximum distribution |
| Room where it is in view | Opal | Comfortable to look at |
| Tunnel with a light kit | Opal | Even glow at night |
Trim finishes
A choice on some systems and it is worth knowing it exists.
The standard trim is white, which disappears into a white ceiling and is right almost everywhere. Some ranges offer a brushed or coloured ring, which reads as a deliberate fitting rather than a hole in the plaster.
In a bathroom with brassware or handles in a particular finish, matching the trim to it is a small detail that makes the tunnel look specified rather than installed. In a hallway, white and invisible is generally better.
Sloping ceilings
The case where the diffuser needs thinking about rather than ordering.
Where a tunnel lands in a sloping ceiling rather than a flat one, a standard flat diffuser sits at the slope’s angle and throws its light down the slope rather than into the room. Some systems offer an angled adaptor that brings the face back to horizontal.
Where no adaptor exists, a domed diffuser spreads more forgivingly on a slope than a flat one does. It is worth raising at survey rather than discovering at fitting.
Matching diffusers across a house
A small point that shows in a hallway with more than one tunnel.
Two tunnels in the same sight line want the same diffuser, in the same profile and the same colour. Where one was fitted years ago and a second is being added, a visible mismatch is the thing people notice rather than the extra light.
Where the original cannot be matched, replacing both with a current pair is generally the better answer and it is a small cost against the appearance of the ceiling.
Getting the diffuser flat
The installation detail that decides whether it looks right.
A diffuser sitting slightly proud on one side, or rotated so a moulded pattern runs off the room’s axis, is immediately visible from below in a way it never is from a ladder. The opening has to be cut square and the assembly seated evenly.
It is worth stepping back into the room and looking up before the trim is finally fixed, because a few millimetres are much easier to correct then than afterwards.
The one that comes in the box
Worth a sentence, because most tunnels are fitted with it.
Standard diffusers from the established manufacturers are good and most installations need nothing else. The reason to choose is a specific condition: a bathroom, a very cold roof space, a light kit, or a room where the diffuser is in direct view.
Absent one of those, the supplied diffuser is the right answer and there is nothing to decide.
Questions worth asking before ordering
- Is the diffuser sealed, and is a double or insulated version available?
- Prismatic or opal, and which suits how this room is used?
- Does it unclip from the room for cleaning?
- Is a replacement lens available separately in years to come?
- Is a light kit going in, now or later?
- Is there a second tunnel it needs to match?
What we specify on a sun tunnel
We specify a sealed or double diffuser on any tunnel serving a bathroom, an opal lens wherever a light kit is going in or the diffuser will be in view, and prismatic where the room is wide enough for the spread to matter. We set the diffuser position from where the room needs light rather than where the tube most easily lands, and we check the lens unclips before ordering.
It is the only part of the tunnel anybody sees, and it decides how the tunnel behaves. It is worth five minutes at specification.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.