If silverfish have started appearing in your bathroom or kitchen this month, the insect is not really the problem. Silverfish are a moisture readout. They need consistently damp, humid air to survive, so a sighting is usually telling you that somewhere in that room is holding more water than it should. Treat the damp and the population stops being sustained. Treat only the silverfish and they come back, because the conditions that suited them are still there.
Key takeaways
- Silverfish are harmless to people. They do not bite and they do not spread disease, so a sighting is a nuisance and a damp signal rather than a health risk.
- September is when they become obvious, because falling outdoor temperatures cool indoor surfaces and push relative humidity up in rooms that are not ventilated.
- Where you find them is diagnostic. Silverfish in a bathroom, under a sink, in a cellar or behind a bath panel are pointing at the moisture source.
- They damage belongings rather than buildings, feeding on the starches in paper, wallpaper paste, book bindings, cardboard and natural fibres.
- Ventilation, leak repair and getting stored paper off damp floors do more than a spray on its own, because a surface treatment cannot reach insects living in voids.
What is behind the current increase?
Metro reported on 3 September 2026 that a run of warm, wet late summer weather has turned UK homes into good silverfish habitat, and that September is considered silverfish season because it combines warmth with high humidity. The report gives the level they favour as ideally above 50 per cent humidity, and describes them as being found in dark, damp places such as bathrooms, cloakrooms, cellars, basements and attics. Worth adding some professional context to that figure: 50 per cent is nearer the floor at which silverfish can survive, and the band they actively thrive in is closer to 75 to 95 per cent relative humidity. That distinction matters for what follows, because it is not a level a whole room usually sits at.
According to the same report, silverfish do not bite people and do not spread disease. That is worth stating plainly at the top, because the reaction to finding one is often out of proportion to what it actually means.
Metro described them as wingless, nocturnal insects, silver or grey, with two antennae and a tapered body just over a centimetre long. That description is the reason most people never see them in daylight, and it matters for what follows.
Why does September make silverfish visible?
This is where we would put the emphasis differently from the coverage. The report attributes the increase to silverfish seeking warmth and shelter indoors as temperatures drop. In our experience that is not usually what is happening, and the real mechanism is more useful to understand.
Warm air holds more moisture than cold air. Through the summer, the moisture your household generates every day from showering, cooking, drying clothes and simply breathing gets carried in warm air and largely goes unnoticed. In September the outdoor temperature starts to fall while the household is still producing the same amount of moisture. Indoor surfaces such as external walls, window reveals, tiled floors and the cold spots behind furniture begin to drop below the temperature of the air in the room.
When moist air meets a cooler surface, the relative humidity at that surface rises sharply, and it is the humidity at the surface that an insect lives in, not the comfortable average for the room as a whole. So a house that felt fine in July can develop pockets well up into that thriving band in September without anything having changed except the weather outside. The room average can read perfectly comfortable while the air against a cold external wall or behind a bath panel is sitting far higher.
Silverfish do not migrate indoors the way rodents do. In most cases they were already present in small numbers in a cellar, a subfloor void or behind a bath panel. What changes in September is that the conditions improve, the population grows, and they start ranging further in search of food, which is when they cross a bathroom floor at night and get noticed. Autumn insect activity in general follows this pattern, and we covered a related version of it in why cluster flies swarm your home in autumn.
What are silverfish actually eating?
Silverfish feed on starches and sugars, which in a house means a specific and slightly surprising list:
- Paper and cardboard, including stored documents, books, magazines and the boxes in a loft or garage.
- Wallpaper paste, which is why loose or peeling wallpaper edges in a humid room are a classic sign.
- Book bindings and photographs, where the glue is the attraction rather than the paper.
- Cellulose fabrics such as cotton, linen and viscose, and silk where it has been starched or stained with food or sweat.
- Dried food residues, including flour, cereal dust and crumbs in the back of a cupboard.
The damage is to your belongings, not to the structure of the building. Silverfish do not gnaw joists, chew cables or undermine brickwork. What they do is graze the surface of paper and fabric, leaving irregular holes, notched edges and a thinned, scraped appearance, and they can quietly ruin a box of documents or a stored garment over a long period because the activity is slow and out of sight.
Where you find them tells you where the water is
This is the most practical thing to take from a sighting. Silverfish travel from harbourage to food and back, and because they need humidity they do not go far from it. So the location of a sighting narrows down the source:
- Bath or shower room, especially at night on the floor. Look at the seal around the bath or tray, the bath panel, and whether the extractor fan actually clears the room after a shower.
- Under a kitchen or utility sink. Look for a weeping trap, a slow waste connection, or a washing machine or dishwasher fitting that drips intermittently rather than obviously.
- Cellar, basement or an unheated ground floor room. Usually a ventilation problem combined with cool solid walls, sometimes rising damp or a blocked air brick.
- Loft or roof space. Often a roof leak, a bathroom extractor vented into the loft instead of outside, or condensation on the underside of the roof covering.
- Behind or beneath stored boxes in a garage or spare room. Cardboard on a cold concrete floor holds moisture underneath it and is food at the same time, which makes it close to ideal for them.
If sightings are spread across several unconnected rooms rather than clustered in one, that usually points at a whole-building humidity or ventilation issue rather than a single leak, and it is worth having someone look properly.
There is one important exception to all of this. The grey silverfish, also called the long-tailed silverfish, is a related species that has spread through UK and northern European buildings over the last decade or so. It looks very similar to the common silverfish, but it tolerates ordinary indoor humidity, lives happily in dry skirting voids, lofts and bedrooms, and is considerably harder to shift. If you are finding silverfish in genuinely dry rooms, identifying which species you have is the first job, because drying the room out will not deal with the grey one.
Why does spraying alone rarely fix a silverfish problem?
Two reasons, and they are both structural.
The first is that silverfish live in voids. They harbour in the gaps behind bath panels, under floorboards, in wall cavities, behind skirtings and in the spaces around pipework. A treatment applied to the open floor of a bathroom reaches the small proportion of the population that happens to be crossing it, and leaves the rest untouched in the places you cannot see.
The second is that the conditions are what sustain them. If the humidity that supported them is still there after treatment, the survivors breed back and the problem returns, and it is easy to conclude that the treatment did not work when in fact the habitat was never removed. Silverfish are long-lived and slow-breeding compared with most insect pests, so a population rebuilds gradually and can take a good while to become visible again. That delay is often mistaken for a fix.
This is why a serious response deals with the moisture and the harbourage first, and treats second where treatment is warranted at all.
What actually reduces them?
The measures that work are the ones that change the environment:
- Ventilate the wet rooms properly. Run the extractor during a shower and for a period afterwards, not just while you are in there, and check that it is actually moving air rather than simply making a noise. A fan venting into a loft is moving the problem, not solving it.
- Find and fix the leaks. The intermittent ones matter most, because a trap that weeps only when the sink is used keeps a small area permanently damp without ever producing a puddle anyone notices.
- Get stored paper and textiles off cold floors and into sealed plastic boxes rather than cardboard. This removes food and harbourage in one move, and protects the things that were at risk anyway.
- Deal with condensation on cold surfaces, including improving air circulation behind large furniture placed against external walls, and drying washing with the window open or an extractor running rather than in a closed room.
- Reduce clutter in cellars, lofts and utility spaces, which cuts down the undisturbed dark harbourage that lets a population sit unnoticed.
None of that is glamorous, and none of it is instant. It is, however, what makes the difference between a problem that goes away and one that returns every autumn.
When is it worth getting a professional in?
A silverfish or two in a bathroom in September is common and does not need anyone called out. It is worth getting a professional survey when the picture is bigger than that: sightings in several rooms, numbers that are clearly increasing, silverfish in a room with no obvious water source, damage appearing to stored documents, books or clothing, or a situation where you have addressed the ventilation and they are still there.
The other case is where you cannot find the moisture. That is common in cellars, older solid-wall properties and converted lofts, and it is the situation where an inspection earns its place, because identifying the harbourage and the source is the whole job. In a commercial setting, particularly anywhere storing paper records, stock or dry goods, it is worth acting earlier, since the damage lands on the stored material rather than on the building.
You can book online or on web chat at any time, day or night, and technicians attend every day of the year except Christmas Day. If silverfish are appearing in numbers or you want the source found properly, call the number at the top of the page and we will arrange for someone to come and look. You can read more about our silverfish control service.
Frequently asked questions
No. As reported by Metro, silverfish do not bite people and do not spread disease. They are not venomous, they do not carry an infection risk in the way some other pests do, and they pose no particular threat to cats or dogs. The case for dealing with them is about protecting stored paper and fabric, and about what their presence reveals about damp in the building, rather than about health.
It means that somewhere in the vicinity of the sighting is holding enough moisture to support them, which is not the same as structural damp. Very often the cause is everyday condensation in a room that is not ventilated well enough, or a small plumbing leak, both of which are straightforward to put right. Persistent sightings across multiple rooms are more likely to indicate a wider humidity or ventilation issue worth investigating.
Silverfish are nocturnal and avoid light, which is why the usual first encounter is switching on a bathroom light and seeing one moving across the floor or in a bath or basin. A smooth basin or bath is a trap rather than a destination, because they can climb in but cannot get back up the glazed sides, so finding one there does not mean it came up the plughole.
Only as far as conditions allow. Common silverfish are limited by humidity, so they tend to stay within the damp zone rather than colonising a dry, well-ventilated living room. If you are finding them in rooms that should be dry, that is a meaningful signal. It usually means either the humid area is larger than you thought, or there is a moisture source in the newly affected room, or that these are grey (long-tailed) silverfish, a related species that tolerates normal indoor humidity and is now common in UK homes. The two look similar, so it is worth having them identified, because drying a room out does not deal with the grey one.