Do insects get inside sun tunnels?

A sun tunnel is one continuous, sealed path: dome, tube, diffuser. It is not an open pipe with a mesh over each end. Every joint in that run is designed to keep out weather, and keeping out weather also means keeping out anything small enough to fly or crawl.

At the top, the dome sits on a flashing collar that ties into the roof covering, whether that is tile, slate or a flat roof membrane. In the middle, rigid sections overlap and lock, or a flexible tube is drawn tight and taped at each connection. At the bottom, the diffuser clips or screws into a ring set into the plasterboard. None of those joints is left open to the loft space.

The dome is the single point where the tunnel meets the weather, so it carries the seal that matters most. It sits under the rim on a gasket, and the flashing wraps the join so rain runs over it rather than into it. That same gasket is what stops anything working its way in around the edge. A dome that has been seated properly, with the gasket unbroken and the flashing dressed down correctly, leaves no gap for an insect to use.

See Sun Tunnel Brands and Flashings for how different kits handle this join, and What Flashing Does a Sun Tunnel Need on a Slate Roof for the detail on pitched slate.

Between dome and diffuser, the tube is either rigid, in short lengths that push together, or flexible, drawn taut and taped at the joints. Neither is perforated, and neither is designed to breathe. Read Inside a Sun Tunnel: Dome, Tube and Diffuser for how the three parts work together, and Rigid Tube or Flexible Tube for how the two types differ where the route bends through a loft.

The inside of the tube is reflective, coated to bounce daylight down rather than let it scatter, and that finish also gives an insect nothing to grip on the rare occasion it did get past a seal. In practice the tube is sealed at both ends and along its length, so this is a moot point on a job fitted correctly.

At ceiling level, the diffuser sits in a ring fitted into the plasterboard opening. It clips or screws home against that ring, closing the gap between tube and room. It is the seal people are most likely to check, because it is the one within reach, but it is also the one least likely to fail: it is indoors, out of the weather, and rarely disturbed once fitted. Cleaning it is straightforward. See How Do You Clean a Sun Tunnel for the method.

When someone finds insects in a loft, the sun tunnel is rarely the route. The more likely gaps are:

• unscreened soffit and fascia vents, fitted for airflow rather than sealed against insects
• gaps around older ridge tiles or hip tiles that have lifted or slipped
• a damaged flashing on a different roof penetration, such as a vent pipe or an old aerial fixing
• gaps around loft hatches or old cable entries that were never made good

A sun tunnel adds one more roof penetration, but it is a factory-sealed unit designed for exactly that job, unlike a vent or an old fixing that was never meant to keep insects out in the first place.

A properly installed sun tunnel keeps its seal for years, but two things can change that. A dome gasket can perish over time, particularly on a roof that gets full sun for most of the day. See Does a Sun Tunnel Dome Turn Yellow Over Time for how the dome material ages and what to look for. A cracked dome, from an impact rather than normal weathering, breaks the seal along with the plastic and should be replaced rather than left.

The other case is a joint disturbed by later work in the loft: insulation topped up carelessly, a cable run pulled through near the tube, or a tube knocked while something else is being moved. If a diffuser will not sit flush, or the dome looks lifted at one edge, that is worth checking rather than ignoring, because the same gap that lets in cold air will let in anything small enough to use it.

Every seal described above depends on the unit going in correctly the first time: the flashing dressed to the roof covering, the tube joints made up square, the diffuser ring set true in the plasterboard. We install to current Building Regulations and handle the Building Control notification where the work is notifiable, and every quote comes with a sightline sketch, a drawing of what you'll actually see from where you sit, so there are no surprises once the tube is in place. See What Building Control Expects of a Sun Tunnel and The Sightline Sketch: How We Plan the View.

If you're weighing up a sun tunnel against a small rooflight, or want to know how the finished job looks from below, the Sun Tunnel Installation hub covers routes, sizes and where each type suits a room. For the wider cost picture across rooflights and sun tunnels, What Affects the Cost of a Rooflight sets out the general ranges.

Can flies get in through the diffuser?

Not through a diffuser that is seated correctly in its ring. The join is against the ceiling, indoors and out of the weather, so it is the least exposed seal in the whole run. If a diffuser has never sat flush, that is a fitting issue worth having checked rather than something you should expect to live with.

Do sun tunnels attract insects to the roof?

No more than any other part of the roof. The dome is clear plastic during the day and does not throw light outward at night, so it does not act as a lure the way an open window or an outside light does. Add-on kits that include a night light sit inside the tube, behind the diffuser, not exposed at the dome.

What does it mean if I find an insect inside the tube?

It means a seal has failed somewhere along the run, most likely the dome gasket or a joint that has come apart. It is not something a sealed, correctly fitted sun tunnel should allow, so it is worth having the dome and the nearest joints checked rather than treating it as normal.

Does a cracked dome let insects in?

Yes. A crack breaks the seal as well as the plastic, so both weather and anything small enough to use the gap can get through. A cracked dome should be replaced rather than left, and the flashing checked at the same time in case the same impact disturbed it.