The short answer
Not much. A sun tunnel opens a small hole in the roof, usually a few hundred millimetres across, and the tube is lined to carry light rather than heat. A room under one stays far cooler than a room under a large rooflight. Some warmth does arrive on a bright July afternoon, so the dome, the tube and the diffuser are worth choosing with that in mind.
Why the opening size matters most
Summer heat comes through roof glazing in proportion to the area of glass the sun can reach. A sun tunnel has very little of it.
A skylight or rooflight lets the sun shine straight into the room. The glass is large, the sun falls on floors and furniture, and those surfaces re-emit the warmth as heat. A tunnel works differently. Its dome is small, often about the size of a dinner plate or a little wider, and the light has to travel down a narrow tube before it reaches the ceiling. Much of the sun's energy is absorbed or scattered on the way. What arrives at the diffuser is soft, even daylight with no beam to sit on the carpet.
That is the main reason a bathroom, landing or hallway fitted with a tunnel does not turn into a greenhouse. There is nothing to warm a settee or a bed. If you are weighing a tunnel against a larger opening for a dark room, our comparison of a sun tunnel or rooflight for a dark room sets the two side by side.
Where the warmth does come from
A tunnel is not a perfect heat barrier. Three parts can pass some warmth into the room, and each can be managed.
- The dome. Clear acrylic passes most of the sun's energy. On a cloudless afternoon the dome itself gets warm, and that warmth moves down the tube. Some domes are tinted or carry a coating that holds back part of the heat, at the cost of a slightly dimmer result on a grey day.
- The tube wall. A thin metal tube in a hot roof space can warm the air around it. A tube that runs through a loft heated by a summer sun will radiate a little of that heat downwards, so the insulation around it counts. The same tube in a cool, ventilated loft does almost nothing.
- The diffuser. This is the ceiling end. A single-layer diffuser is fine in a bathroom. A double-layer one, or one that is sealed around the edge, slows heat and cold alike.
A rigid tube with a highly reflective lining loses less energy on the way down than a long flexible one, which matters for light as much as heat. The choice between the two is covered in our guide to rigid or flexible sun tunnels.
A rough guide by room
How warm a room feels depends on what sits under the tunnel and which way the roof faces. This table sets out the usual pattern.
| Room | Summer effect | Worth doing |
|---|---|---|
| Windowless bathroom | Barely noticeable | Standard diffuser |
| Landing or hall | Barely noticeable | Standard diffuser |
| Loft bedroom, south roof | Some warmth at midday | Coated dome, sealed diffuser |
| Kitchen, bright corner | Mild, near the diffuser | Check position against worktops |
A south or west facing slope gets the strongest afternoon sun in Sussex and Surrey, so a tunnel there sends more energy down than one on a north slope. A bedroom under a hot slate or tile roof is already the warmest room in the house, and a tunnel will not make that worse by any amount you would feel, but it will not cool it either. Our page on whether a sun tunnel can go on a north-facing roof explains how orientation changes the brightness.
Keeping the heat out in practice
A few specification choices make the difference between a tunnel you forget about and one you notice in August.
First, insulate the tube where it crosses the loft. Quilt or a purpose-made sleeve around the tube keeps roof-space heat off it in summer and keeps room heat from leaking into the loft in winter. Part L of the Building Regulations sets the fabric standard, and an insulated tube keeps the ceiling line in step with the rest of the loft floor. Our page on sun tunnels, loft insulation and heat loss covers the winter side.
Second, choose the dome for the roof it sits on. A dome on a hot south slope can carry a solar control finish. The same dome on a shaded north slope gains nothing from one and loses brightness. If you are unsure whether glass or acrylic suits the job, read which is better, a glass or acrylic sun tunnel dome.
Third, pick the diffuser to suit the room. A frosted, opal diffuser spreads the light so that no one patch of ceiling glows. It also hides the tube end, so a warm spot is less likely to be noticed.
How we write it down
Heat is a question for the survey, not a guess made from a brochure.
When we visit, we look at the slope, what lies above the ceiling and what the room is used for. A bathroom on a north slope needs little. A loft bedroom under a south slope needs a little more thought about the dome and the tube. Those decisions go into the quiet spec that comes with every quote, alongside the glazing, the seals and the upstand, so you can see what was chosen and why. The work is installed to current Building Regulations, and we handle the Building Control notification where it applies. The job carries our 10-year workmanship guarantee.
More on summer and sun tunnels
Will a sun tunnel make my loft hotter?
No more than the roof already does. The tube carries light down and is not a heat source of its own. An insulated tube keeps the loft air and the room air apart.
Can I close off the light if the room gets too bright?
Some kits include a damper that narrows the tube, and some diffusers are fitted with a shade. Our answer on whether you can dim a sun tunnel explains the options.
Does a tunnel let in less heat than a roof window?
Yes, because the opening is smaller and the light is scattered on its way down. A roof window looks across the whole room, so the sun can reach the floor directly. See our roof window installation page if you need a larger, opening unit.
Will it be cold in winter instead?
A well-fitted tunnel has a sealed, insulated tube, so the ceiling stays close to the temperature of the room around it. A bare tube through a cold loft is the weak point, which is why we insulate it.
To find out how a tunnel would behave over your own ceiling, get a quote and we will note the roof slope and the tube run in your quiet spec. The full service is on our sun tunnel installation page.