01The tube
A pipe from cold air to a warm ceiling
A sun tunnel is a small hole, so it cannot chill a whole room. What it can do is chill the ceiling directly under it, and that comes down to how the tube is built and wrapped.
Think about what a sun tunnel really is. It is a dome outside, a tube through the loft or roof space, and a diffuser (the frosted or clear disc) on your ceiling. In winter the loft is as cold as the garden, and the tube runs straight through it. If that tube is bare, the metal wall inside the loft cools to the loft temperature, and the diffuser at the bottom is the only thing between that cold column and the warm room.
That is why some people notice a slightly cool spot under a tube. It is small, since the opening is small, but it is real. The heat does not leave through a lot of glass. It leaves through a narrow, cold bridge from the room into the roof space. So, do sun tunnels make rooms cold? Not if the tube is dealt with properly. The good news is that the fix sits in the build, not in giving up on the idea.
The cold in a sun tunnel does not come from the dome. It comes from a bare tube standing in a freezing loft.
02Where heat goes
The three places a sun tunnel loses warmth
Dome, tube and diffuser each leak a little in a different way, and a sound installation deals with all three.
Start at the top. The dome is plastic or glass and is the part exposed to the outside air. A twin-skin dome, with two layers and a pocket of air between them, loses less than a single skin, and we say which the product has before you choose. Next is the tube wall, the part a rooflight or skylight does not have. In a warm, boarded-out roof space the tube is less of a problem, but in an ordinary cold loft the walls of a bare tube turn chilly. Last is the diffuser, the bottom seal of the whole system. A diffuser that is sealed to the ceiling, and with a second layer or a sealed lens in it, stops the cold column from breathing into the room.
| Part | How it loses heat | What helps |
|---|---|---|
| Dome | Direct contact with outside air | Twin-skin or glass dome |
| Tube wall | Conducts cold from the loft | Insulating sleeve around the tube |
| Diffuser | Cold air drifting down the tube | Sealed fit and a second lens |
03Wrapping it
How we keep the tube warm in a cold loft
Insulation around the tube, and a clean seal at the ceiling, turn a possible cold spot into a spot you will not notice.
Where the loft is insulated at ceiling level, which is the usual set-up, the tube passes through that blanket and out into the cold space above. The sensible approach is to bring the insulation up around the tube so there is no gap in the layer. We wrap the tube in the loft with an insulating sleeve, taking it as far up as the tube runs in cold air, and we make sure loft insulation closes back around the tube at ceiling level rather than being cut short. If the roof space is boarded and used for storage, we plan the route and the sleeve together so nothing gets squashed flat.
The seal at the ceiling matters as much. A tube that connects to the plasterboard with gaps lets room air rise into the tube and meet cold surfaces. We seal that joint, so the warm air stays down in the room where you paid to heat it.
- Survey the loft and trace the route from roof to ceiling
- Choose a dome and tube suited to the roof and the loft
- Fit the tube, then sleeve the part that runs through cold air
- Close the loft insulation back around the tube without gaps
- Seal the diffuser to the ceiling and check it is airtight
If you want to see how the whole assembly fits together first, how sun tunnels work shows the three parts in order.
04Damp
Condensation on the diffuser, and what it tells you
Droplets or a damp ring around the diffuser mean warm air is meeting a cold surface. That is heat loss showing up as water.
Most homes make a lot of moist air in the kitchen, the shower and the washing line indoors. If that air reaches a cold surface it turns to water. In a bathroom, a sun tunnel with a poorly sealed diffuser can mist or drip on a cold morning because moist air has found its way up the tube. The cure is not a bigger fan on its own. It is the same warm build we have been describing: a sealed diffuser, an insulated tube and a loft blanket that closes up properly. You can read how it applies to flat roof units on do flat rooflights get condensation.
- A cold spot on the ceiling under the diffuser compared with the ceiling beside it
- Water drops or a damp ring around the diffuser edge
- Dark staining on the plasterboard next to the opening
- Cold air you can feel when you hold a hand up to the diffuser
- A tube in the loft with no insulation and no sleeve
Will a sun tunnel leave a cold spot on my ceiling?
Will a sun tunnel put my heating bill up?
Only a very small amount, because the opening is small. A tube that is wrapped and sealed loses little, and the daylight can mean the light stays off for longer. If warmth is a worry, compare it with a glazed option on do rooflights lose more heat than windows.
Is a rigid tube warmer than a flexible one?
Not by itself. Warmth depends on the sleeve, the seal and the loft insulation around either. A flexible tube has more surface and more folds to wrap, so we take care with it. The two types are set side by side on rigid vs flexible sun tunnels.
Can I add insulation myself afterwards?
You can ask us to include it at fitting, which is the neater route. Pushing insulation in later around an existing tube often leaves gaps, and squashing loft insulation makes it work less well.
Does the diffuser keep the cold out?
A sealed diffuser stops the air in the tube moving into the room, and a second lens adds a layer of still air. It does a good job, but it works best when the tube above it has been insulated too. See the wider picture on sun tunnels, and compare with a glazed skylight or rooflight if warmth is your first worry.
