A sun tunnel is a daylight system that brings light from the roof into a room with no external wall, through a highly reflective tube. On the roof sits a dome or flat lens; in the ceiling below sits a diffuser that spreads the light evenly through the room.
It is also called a light tube, light pipe or tubular skylight. It is the answer for places where neither a roof window nor a facade window is possible: an internal bathroom, a landing, a hallway, a cloakroom or a kitchen at the back of the house.
Build-up
A sun tunnel always has three parts:
- The roof terminal. A dome in acrylic or polycarbonate, or a flat lens. For a pitched roof there are flashing kits matched to each tile profile; for a flat roof, a version on an upstand.
- The tube. A rigid tube or a flexible duct with a mirrored inner layer. The reflectance determines the yield — good rigid tubes reach over 98%, and then the light loss per metre is small. Flexible ducts reflect less well and lose noticeably more light through their corrugations.
- The ceiling terminal. A diffuser that spreads the collected light, usually an opal or prismatic panel.
Sizes and output
Common diameters run from around 25 to 55 cm. A practical rule of thumb: about 25 cm for a small space such as a WC or store cupboard, about 35 cm for a bathroom or hallway, and 50 cm and up for a kitchen-diner or a larger landing.
The achievable tube length depends on the system. Rigid systems bridge several metres using extension sections; flexible systems are meant for short runs — beyond about two metres there is little light left. Every bend costs extra light, so keep the route as short and as straight as possible.
What a sun tunnel does and does not do
- It does: deliver usable daylight even on a grey day, without having to open up the roof structure for a full roof window.
- It does not: give a view, ventilation or air change. An internal bathroom still needs mechanical extract.
- Watch solar gain: a sun tunnel facing south can bring in a surprising amount of heat in summer. Versions exist with a dimmer or an adjustable closing ring.
Condensation and insulation
The weak point is where the tube passes through the insulation layer. An uninsulated tube through a cold loft is a thermal bridge: condensation forms on it, runs down to the diffuser and is then blamed on a leak. Choose a system with an insulated tube jacket, or insulate the tube within the unheated zone, and make sure the ceiling junction is sealed airtight. That stops warm, moist indoor air from creeping up alongside the tube.
Installation and maintenance
The roof penetration is the critical point: the junction with the roof covering or tiling must be made with the flashing kit matched to that roof. On a pitched roof this means working from the roof; leave it to someone who can do so safely.
Maintenance is limited: clean the dome annually with water and a soft cloth, without abrasives. The diffuser usually comes away on a clip or twist fitting so dust and insects can be removed. As with a dome rooflight, the plastic of the roof terminal is a wearing part that yellows over time.
Related terms
- Dome rooflight
- Continuous rooflight
- Light hood
- Daylight
- Thermal bridge
- Roof window
- Light transmission
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.