Do sun tunnels lose heat?

Where the loss happens

Through the collector. A small glazed or acrylic dome, typically 250mm or 350mm across. That is a fraction of the area of even a modest rooflight, so the absolute loss is small.

Through the tube walls. The larger part on a badly detailed installation. An uninsulated aluminium tube running through a cold loft is a long thin radiator, losing heat from the warm air column inside it.

Through the diffuser. Small, and sealed units are available where it matters.

Insulating the run

The single thing that controls it. A purpose made insulated sleeve, or wrapping the tube where it passes through cold space.

It also prevents condensation inside the tube, which is the more common complaint, so the same detail solves both problems.

The ceiling collar

Sealing it stops warm room air entering the loft around the tunnel. That is an air leakage path as well as a moisture one, and air leakage loses more heat than conduction does.

The comparison

A 350mm tunnel has a collector area of about 0.1 square metres. A modest rooflight is around 1.5. So even at similar U-values the tunnel loses roughly a fifteenth as much.

Against the daylight it delivers, that is a good trade.

Solar gain

Correspondingly small, which is usually an advantage. A tunnel into a small internal room will not overheat it the way a rooflight can.

The practical answer

Insulate the run and seal the collar, and the heat loss is not something you will notice.

Less than a rooflight, more than a ceiling

A sun tunnel is a small opening in the thermal envelope, so it loses some heat. How much depends almost entirely on two things that have nothing to do with the dome.

The insulated tube

A metal tube running from a cold roof space down to a warm ceiling is a conductor. Insulating it where it crosses unheated loft space is not an optional extra: it is the single thing that decides whether the installation performs.

Without it, the tube is a cold surface in contact with the room below, and it produces two problems rather than one. Heat leaves through it, and warm moist air condenses on it, sending water down to the diffuser where it gets reported as a leak.

The airtight ceiling seal

The larger loss in most poorly fitted installations, and the one nobody thinks about.

An unsealed ceiling ring lets warm room air move into the tube and up into the roof space. That is air leakage rather than conduction, and air leakage is a far more effective way to lose heat from a house than conduction through a small area of glass.

Sealing the ring is quick and it is a named part of what we do rather than something left to chance.

The dome and diffuser

Modern kits use a double glazed or insulated dome and, on better systems, a sealed double glazed diffuser at the ceiling. That combination performs respectably: comparable per square metre with a decent rooflight, over a much smaller area.

Because the area is small, the absolute loss is small. A 350mm tunnel is under a tenth of a square metre. Even at a poor U-value the number is trivial against a whole house.

Heat arriving through the dome

Slight, and occasionally welcome. A south facing tunnel into a small internal ensuite genuinely warms it on a hot afternoon, which is either a bonus in March or a nuisance in July depending on the room.

Because the aperture is small, there is nothing like the overheating risk of a rooflight, and solar control glazing is rarely worth specifying on a tunnel.

What to insist on

Insulation to the tube where it crosses unheated space, and an airtight seal at the ceiling ring, both named on the quotation. Those two lines are the first things dropped from a cheap price and they are the whole of the thermal performance.

Where the tunnel serves a bathroom, check the extract fan is working as well. A tunnel does not ventilate, and moisture that cannot leave will find the coldest surface in the room.

Two separate losses

Through the dome and diffuser. The glazed elements are small, so in absolute terms this is modest. A sun tunnel presents far less glazed area to the sky than a roof window does.

Through the tube. This is the one that catches people out. The tube passes through cold loft space, and unless it is insulated it acts as a chimney of cold surface running from the roof down to a hole in your ceiling.

An uninsulated tube in a cold loft will feel cold at the diffuser on a winter night, and it will produce condensation inside itself.

What fixes it

Insulate the tube along its whole length through any unheated space, and seal the diffuser at the ceiling so warm damp air from the room cannot enter the tunnel.

Both are inexpensive at installation and awkward afterwards, because reaching the tube means going into the loft and working around it.

A double glazed diffuser at the ceiling improves matters further and is worth specifying where the tunnel serves a heated space rather than a hallway.

How it compares

Against a roof window of similar light output, a well insulated sun tunnel loses less heat, simply because there is much less glass involved.

Against no opening at all, it obviously loses more. Whether that matters depends on the space: for a windowless internal hallway, the daylight is usually worth the small thermal cost, and the alternative is a light burning all day.