Introduction: these creatures aren't the problem, they're the diagnosis
In late September, after three days of steady rain, the same scenario plays out in thousands of homes: a dozen grey woodlice curled into balls under the washing machine, a black millipede tracking along the bathroom skirting board, and sometimes that fast, many-legged silhouette that crosses the ceiling in a single second — the house centipede, undisputed champion of domestic panic.
The instinctive reaction is to spray an insecticide and reach for the vacuum cleaner. A week later, the same creatures are back, in the same spot. That isn't a product failure: it's that the product isn't treating the right problem. These three species share one decisive physiological trait: they cannot retain water. Their cuticle is permeable, and they dry out within a few hours in dry air. Their presence in your home is therefore no accident — it's a walking humidity reading.
Put another way: a home that permanently hosts woodlice indoors has a stray water source somewhere — rising damp, a cracked shower seal, a blocked vent, soil in direct contact with a buried wall. This guide sets out how to read those clues the way a technician would, then act in the right order: identify, dry out, seal, and only as a last resort treat.
Three very different tenants under your skirting boards
They tend to be lumped together under the convenient label of "damp bugs", but their lifestyles differ, and that changes how you should handle them.
| Species | Appearance | Diet | Typical origin | Actual risk |
|---|
| Woodlouse (Porcellio, Armadillidium) | 8–18 mm, grey-brown, segmented shell, rolls into a ball | Detritivore: damp wood, decomposing leaves | Soil, mulch, woodpile stacked against the wall | None (a land crustacean; it doesn't sting) |
| Millipede (Julidae) | 2–4 cm, black cylindrical body, coils into a spiral | Decomposing plant matter | Garden soil, crawl space, poorly insulated slab | None; gives off a smelly secretion if crushed |
| House centipede (Scutigera coleoptrata) | 2–3 cm body, 15 pairs of long legs, very fast | Predator: spiders, juvenile cockroaches, silverfish | Cracks, cellars, service ducts | Bites are very rare and harmless |
The woodlouse isn't an insect: it's an isopod crustacean, a land-dwelling cousin of shrimp, breathing through pseudo-tracheae that require constant humidity. It doesn't sting or bite, transmits no disease and doesn't attack sound timber — it eats wood already degraded by fungi. Finding woodlice in an interior skirting board is therefore a serious signal: there is probably damp, possibly rotting, wood nearby.
The millipede (the julid, not to be confused with the centipede) is a true soil animal. It comes indoors by mistake, through negative phototropism, when rain saturates the garden soil and drives it out of its burrows. Indoors it usually dies within 24 to 48 hours, dried out.
The house centipede deserves an important nuance: it's an ally. It actively hunts silverfish, young cockroaches, spiders and gnats. Its presence indicates two things: humidity, and a prey population large enough to feed it. Eliminating it without addressing the rest means removing a natural regulator. Many professionals say so bluntly: better one house centipede than a colony of silverfish.
Rule of thumb: a stray woodlouse at the foot of a French window in October is unremarkable. Twenty woodlice under a bathroom cabinet, every week, point to a building defect that needs identifying.
Why September to November concentrates the reports
The timing is anything but random. Three factors overlap in autumn.
1. Saturated soil. After the late-season rains, water fills the gaps in the soil. Millipede burrows and woodlouse shelters become uninhabitable: the animals come to the surface and look for dry cover — under a tarpaulin, a plant pot, a terrace step… or on the other side of your garage door threshold.
2. Falling night-time temperatures. As soon as the thermometer drops below 10–12 °C at night, crawl spaces, basements and garages, which stay mild, become thermally attractive refuges.
3. The heating goes back on. Heating creates a differential between indoor air laden with vapour (showers, cooking, drying laundry) and cold walls. The vapour condenses at the weak points: north-facing wall corners, the backs of cupboards, window surrounds. That condensation sustains exactly the microclimate these species are looking for.
France's ecological transition agency (ADEME) points out that a household of four routinely releases 10 to 15 litres of water vapour a day into the home. Without effective air renewal, that water stays in the walls and the furniture. The woodlice are simply taking advantage of an environment that suits them.
The DIY diagnosis: follow the water, not the animal
Before any treatment, a methodical inspection resolves 80% of situations. It takes an hour.
Step 1 — Measure the actual humidity
Impressions are misleading: a room can feel "cool" rather than damp and still read 78% relative humidity. A small digital thermo-hygrometer left for a few days in each vulnerable room (bathroom, utility room, cellar, bedroom on a north-facing wall) gives you a precise map of the home. The benchmarks that matter:
- 40–60% RH: comfort zone, no moisture-loving species settles in for long.
- 60–70% RH: warning zone, silverfish and mould may appear.
- > 70% RH: ideal conditions for woodlice, millipedes, house centipedes and wood-decay fungi.
Step 2 — Look for entry points at floor level
These animals crawl: they get in from below. Take a torch and inspect the perimeter of the home on your knees:
- the bottom of garage and cellar doors (a 5 mm gap is more than enough);
- French window thresholds and sliding door tracks;
- pipe and cable penetrations through buried walls;
- crawl space vents blocked by leaves;
- slab cracks, particularly along load-bearing partition walls.
Step 3 — Inspect the outdoor perimeter
The 50 centimetres running along the façade are decisive. That's almost always where the source lies: thick mulch against the wall, a planter sitting on the ground, a woodpile leaning on the house, an overflowing gutter watering the base of the wall, soil built up above the level of the waterproof render. Each of these is a woodlouse nursery thirty centimetres from your door.
Drying out: the action that actually solves the problem
Depriving these species of water is more effective than any insecticide, and the effect lasts.
Ventilate properly
Mechanical ventilation is the first thing to check. A clogged extract vent loses a large share of its airflow. Remove the bathroom and kitchen vents, wash them in soapy water, and check that a sheet of paper stays stuck to them when the system is running. If it falls, the system is no longer pulling.
In a home without mechanical ventilation, manual airing remains the rule: 5 to 10 minutes, windows wide open, twice a day, including in winter. Contrary to a stubborn myth, this doesn't chill the walls, which keep their thermal inertia; you're only removing the vapour.
Dehumidify spaces that can't be ventilated
A buried cellar, a semi-windowless basement or a utility room without an opening window can't be fixed by airing. An electric condensation dehumidifier sized for the volume brings humidity below 55% within a few days, and the effect on woodlouse and silverfish populations is spectacular: deprived of water, they leave the area or die. For small cupboards and closed storage, refillable moisture absorbers are enough, provided you change the tablets regularly.
Treat the structural cause
If humidity stays high despite adequate ventilation, you're dealing with a building defect: rising damp, a missing damp-proof course, lateral infiltration, a leaking pipe. A damp survey by a building professional is then money well spent — well ahead of any chemical treatment. In apartment blocks, such defects often fall under the common parts: see our article on how pest-related costs are split between landlord, tenant and managing agent.
Blocking the entrances: the sealing work
Once the air has been improved, the doors still need closing.
- Door bottoms: a brush seal or a double-lipped draught excluder on the garage and cellar doors removes the main entry corridor. It's the best value-for-money measure there is.
- Cracks and duct penetrations: acrylic sealant indoors, polyurethane sealant for areas exposed to water. Never block a crawl space vent: it must stay open but protected.
- Grilles and air vents: a fine-mesh stainless steel pest-proof grille lets air through and stops crawling pests; on basement air bricks, it doubles as rodent protection.
- Patio door thresholds: clean the tracks, clear the drainage holes, replace compressed seals.
Outside, clear a 30 to 40 cm strip at the base of the walls: free-draining gravel rather than organic mulch, pots raised on feet, firewood stored away from the house and off the ground, gutters and drainage chambers cleared before winter.
Treatments: what helps and what doesn't
Immediate measures
For visible individuals, the vacuum cleaner remains the go-to tool: fast, chemical-free, and it also removes the eggs and debris that feed detritivores. Empty the bag or canister outdoors.
For house centipedes, the simplest method is to catch them with a glass and a sheet of paper and release them outside. Remember: they eat your silverfish.
Mechanical and mineral barriers
Diatomaceous earth (fossil silica powder, non-food grade for insecticidal use) works by abrading the cuticle and causing dehydration. It's effective against woodlice and silverfish, provided it is applied in a very thin layer, in dry, inaccessible areas (under low furniture, in skirting board voids, along cellar walls). Once wet, it stops working entirely. Wear a mask when applying it: ANSES points out that inhaling fine dust irritates the airways.
Flat glue traps placed along skirting boards and behind appliances solve nothing on their own, but they are an excellent monitoring tool: they show where the animals travel and whether the population is falling after your work.
What to avoid
- Insecticide "foggers" for a moisture problem: they kill the individuals present, not the cause, and needlessly expose the household.
- Heavy outdoor spray treatments, which hit beneficial wildlife (ground beetles, spiders, pollinators) with no lasting effect, since rain washes the products away.
- Ultrasonic repellents, whose effectiveness against crawling arthropods has never been demonstrated by any independent study.
- Using unapproved or old products: France's consumer protection authority (DGCCRF) regularly points out that only biocidal products holding a valid authorisation may be used domestically.
When to genuinely worry: the signals not to miss
These three species are harmless. But they sometimes accompany problems that are genuinely expensive.
Woodlice in numbers in a living space. If they come with a musty smell, a floor that "gives" underfoot or soft skirting boards, look for wood-decay fungi (dry rot, cellar fungus). Dry rot is a property law issue: in departments covered by a prefectural order, buyers and tenants must be informed at the time of a sale or letting.
Coexistence with large numbers of silverfish. This is the sign of long-standing humidity, often compounded by a general ventilation problem.
Cockroaches appearing at the same time. Beware of a crossed diagnosis: German cockroaches seek out the warmth and humidity of service ducts. If, alongside the woodlice, you find black peppercorn-like droppings and a sweetish odour, the problem changes in nature and calls for professional treatment — see our pages dedicated to cockroaches and to our call-out services.
A persistent sewage smell. A dried-out trap (utility room, cellar, garage) lets air rise from the drains, a source of humidity and a possible route for other pests. Pour a litre of water mixed with a little vegetable oil into rarely used traps to maintain the water seal.
The five-step protocol
- Measure the humidity in each vulnerable room over a week.
- Dry out: clean the ventilation vents, air twice a day, install a dehumidifier in windowless spaces.
- Clear the outdoor perimeter: mulch, wood, pots, gutters.
- Seal: door bottoms, cracks, duct penetrations, grilles.
- Monitor: control traps along the skirting boards, weekly counts. A population divided by ten in three weeks confirms the cause has been dealt with.
If, despite these five steps, the sightings persist in the same place, the water source is structural rather than behavioural: what you need to order is a damp survey, not an insecticide.
Conclusion: the best weapon against damp isn't an insecticide
Woodlice, millipedes and house centipedes destroy nothing, don't sting and transmit no disease. Yet they carry real information: they mark the points where your home lets water in or holds on to it. Responding with chemicals means removing the witness without fixing the leak.
By addressing humidity, ventilation and low-level entry points, you resolve not only these autumn appearances but also part of the conditions that favour silverfish, mould and, more broadly, cockroaches. It's the same job — and it's the only one that holds up year after year.
