Introduction: the scratching that starts two weeks after the heating
The scenario comes round every year with calendar-like regularity. Mid-October, the night-time temperature drops below 8 °C and you fire up the boiler or the electric heaters. All is well. Then, around the first week of November, a noise begins: a dry scratching above the bathroom's suspended ceiling, or a rubbing sound inside the wall behind the living-room radiator. Always between 10 p.m. and 2 a.m. Never when you stop to listen.
The most common reaction is to assume that "something got in through the roof". Sometimes that's true, but above all it's incomplete. What has changed in the home isn't an opening: it's the thermal regime. By switching the heating on, you turn cold, inert spaces — service ducts, crawl spaces, unconverted lofts, ventilation risers — into warm, ventilated and remarkably discreet corridors. For a rodent, a cockroach or an overwintering fly, that is precisely the migration signal they've been waiting for.
This site has already covered mice in winter, cellars in apartment blocks and lofts taken over by stone martens. This article takes a different and rarely documented angle: a building's service networks as a vector of seasonal infestation. In other words, what happens inside the walls when the temperature rises by 6 °C.
Why heat triggers movement
The 10 °C threshold, the pivot of the calendar
Most domestic pests in France behave in ways that are strongly dependent on ambient temperature. The house mouse (Mus musculus) tolerates cold but spends a great deal of energy on thermoregulation: below 10 °C its metabolic cost rises sharply, and it favours cavities where the air stagnates above 15 °C. The brown rat (Rattus norvegicus) follows the same logic on a larger scale: it moves up from sewer networks into crawl spaces and the ducts of rising service columns.
Among insects the effect is even more marked. The German cockroach (Blattella germanica) only breeds properly between 25 and 32 °C; below 15 °C its development all but stops. A soil-stack duct warmed by a domestic hot water pipe offers it, in the depths of winter, a microclimate found nowhere else in the building. That is why reports of cockroaches in apartment blocks don't fall in winter: they simply move vertically.
Heating creates air movement, not just heat
A heated home is slightly depressurised relative to the outside and to unheated spaces. The extract ventilation pulls air out, the loft stays cold, and air flows from cold zones towards warm ones through every available crack. This draught carries food and damp odours right up into lofts and service ducts. For a rodent navigating by whisker and nose, it's a treasure map.
The principle to remember: you don't defend a home against pests, you defend a volume of warm air against the cold volumes surrounding it. The weak points are always at the interfaces.
The five critical interfaces in a heated home
1. Wall penetrations behind radiators
Every wet radiator is fed by two pipes passing through a floor slab or a partition wall. In buildings put up between the 1960s and the 1990s, these holes are rarely made good properly: you find an annular gap of 5 to 20 mm around the pipe, often hidden by a decorative plastic escutcheon. That gap is a direct passageway between the home and the service duct.
A mouse gets through a 6 mm opening, a juvenile rat through 20 mm, an adult cockroach through 2 mm. Decorative escutcheons seal nothing: they simply rest against the wall with no gasket.
The useful move is to remove the escutcheon, clean out the hole and fill it. Stainless steel wool for rodent-proofing remains the go-to material for this job: it resists gnawing, doesn't rust in a damp duct, and packs into irregular shapes. You then cover it with acrylic sealant or a patching mortar. Expanding polyurethane foam on its own should be avoided: rodents chew straight through it in a single night.
2. Cable and pipe penetrations in the kitchen
Under the sink, behind the dishwasher, at the back of a built-in oven: the holes made for water supply, waste pipes and electrics are the number one entry point for cockroaches in apartment blocks. The duct there is warm, damp, never cleaned, and connects to neighbouring flats.
Inspecting means taking apart the under-sink unit and shining a light into the corners. A narrow-beam torch, or even a small endoscopic inspection camera, lets you see what's going on behind a sealed unit without dismantling it — the tool has become far more affordable and reveals in ten minutes what you've been missing for years.
What to look for:
- isolated brown specks (cockroach droppings, the size of a grain of ground pepper);
- empty oothecae (light brown, purse-shaped 8 mm capsules);
- the distinctive sweetish smell of a long-established infestation;
- blackish smear marks along a pipe (rodents).
3. Ventilation vents and risers
Mechanical extract ventilation poses a specific problem. The shared extract riser passes through every floor and exits at roof level. A damaged roof-terminal mesh — a common occurrence after a few winters — opens a direct route for birds, wasps in summer, and rodents in autumn.
Typical symptom: an animal smell or a localised rubbing sound in the bathroom and WC extract vents, while nothing is visible inside the home. Conversely, a clogged extract vent and weakened airflow encourage damp in wet rooms, and therefore silverfish and booklice.
Two points to watch:
| Component | Expected check | Frequency |
|---|---|---|
| Roof ventilation terminal | Mesh intact, apertures ≤ 6 mm | Annual (autumn) |
| Extract vents | Remove, wash, refit | Twice a year |
| Flexible ducting in lofts | No perforations, connections tight | Annual |
| Extract fan unit | Cleaning, free-running impeller | Every 3 years |
Cleaning the vents requires no special equipment — hot soapy water and thorough drying — but roof access is a job for a professional or the managing agent.
4. Unconverted lofts and glass wool insulation
Lofts are the space most sought after by rodents in winter: the insulation provides both the heat rising from the home and an ideal nesting material. A mouse nest in blown glass wool is almost impossible to spot from the hatch: you have to move along the joists with a head torch and an FFP2 mask.
What you're looking for:
- tunnels burrowed through the insulation, 4 to 6 cm across;
- accumulations of droppings along the load-bearing walls;
- stripped electrical cables — this is the real risk, not an anecdotal one;
- urine stains that fluoresce under UV light.
The fire risk alone justifies an annual inspection. The INRS and insurers regularly point out that gnawed electrical conduit is among the identified causes of loft fires. A mouse gnaws constantly: its incisors grow by around 0.3 mm a day.
5. Crawl spaces and ducts at the base of a building
In a house built over a crawl space, the low-level ventilation (air bricks in the façade) must stay open to prevent damp, while being fitted with 5 mm stainless steel mesh. The original plastic grilles give way within a few seasons. In an apartment block, the plant room at the foot of the riser — boiler room, adjoining bin store, lift shaft — concentrates heat, waste and traffic: it's the classic starting point for rats colonising upwards.
