Crawl Spaces and Service Ducts: The Invisible Highways Pests Use Before Winter 2026

Introduction: the infestation that doesn't come through the door

Every autumn, pest control companies hear the same sentence: "I don't understand, everything is shut tight at my place." That's usually true of the doors and windows. And that's precisely why the problem isn't solved.

A rodent or a German cockroach doesn't use human circulation routes. It uses the service routes: the crawl space beneath the ground floor slab, the electrical conduits running from the floor up to the panel, the wall penetrations for water supply lines, telecom ducting, extractor hood vents, riser shafts in apartment buildings. This network is continuous, warm, dark, and it connects the ground outside to your kitchen without ever crossing a lock.

The Ipsos survey reported in 2022 and updated in 2026 reminds us that roughly 62% of French people have experienced a pest infestation in the past five years. In a large share of residential cases, the entry point identified by technicians is neither a door nor a window, but a sealing defect at the ground floor slab or at a utility penetration.

This article doesn't rehash our content on rodents in attics, apartment building basements, or mice in winter. It tackles one specific, technical angle: the building's lower layer and its utility runs, what to inspect there, and how to close these passages before the first cold spells of winter 2026-2027.

Understanding the crawl space: why it's a perfect habitat

A crawl space is the open void, generally between 20 cm and 1.20 m high, left between the natural ground and the lower floor slab of a building. It serves to isolate the structure from ground moisture and to route utilities. Ventilating it is mandatory: air vents pass right through the base wall.

From the point of view of a field mouse, a brown rat, or a house mouse, this space ticks every box of an ideal shelter:

• buffered temperature: 8 to 14 °C in winter, versus sometimes –5 °C outside;
• total darkness and no predators;
• unlimited nesting material if insulation, cardboard, or rubble has been left lying around;
• utilities to follow: every pipe is a signposted corridor leading inside;
• no human oversight: a crawl space is, on average, only visited when something goes wrong.

The catch is that code-required crawl space ventilation is essential — you never block it. The whole challenge is to make it passable for air and impassable for a rodent.

The three defects found in 80% of cases

1. Air vents with no grille, or fitted with a moulded plastic grille that's cracked or has come off its hinges. A mouse gets through a 6 mm opening, a rat through 20 mm.
2. Access hatch made of chipboard, fitted without a seal, gnawed at one corner.
3. Unsealed utility penetrations: the mason leaves a 2 to 5 cm gap around the water pipe or electrical conduit, and it's never closed up afterwards.

Service ducts: the forgotten vertical network

In a dwelling, a residential service duct (GTL in France) or a building's service riser links several levels. In a single-family home, the flexible conduits running from the crawl space up to the electrical boxes create the same chimney effect.

What this produces in practice:

In multi-unit buildings, the phenomenon is well documented: a German cockroach infestation in a single kitchen spreads along the vertical plumbing ducts to the units above and below. That's why treating apartment by apartment almost always fails, and why an intervention has to be designed around the entire riser. We break down who is responsible for what in our article on pests in co-owned and rented housing.

The case of wastewater pipes

The brown rat (Rattus norvegicus) is both a swimmer and a climber. It travels up sewer networks and emerges through:

• a dried-out floor drain in a cellar or garage (a dry trap no longer acts as a barrier);
• a connection chamber with a cracked cover;
• an unlocked inspection chamber;
• more rarely, a toilet bowl via the soil stack in the event of a major defect.

The countermeasure is well known to sewerage services: the rodent-proof non-return valve fitted on the connection, and keeping floor drain traps permanently filled with water (half a litre every two weeks is enough in a rarely used garage).

Diagnosis: the inspection round to carry out in October

Allow an hour and a half for a single-family home. Now is the right time: before the cold pushes populations indoors, and while there's still enough daylight late in the day.

Step 1 — Walking the exterior base wall

Move slowly, 50 cm from the wall, eyes on the ground. Look for:

• all the air vents: count them, photograph them, note the condition of the grille;
• burrow holes: 4 to 8 cm across, often against a foundation, with a small spill of soil in front;
• blackish greasy streaks: these are signs of repeated passage (the "smear mark");
• utility penetrations: water meter, electrical conduit, gas supply, fibre cable.

A powerful focused-beam torch radically changes the quality of the inspection: rub marks only show up under raking light.

Step 2 — Inside the crawl space

If the height allows, and only after ventilating through the hatch for 20 minutes (risk of confined air and, in some regions, of radon):

• look for droppings: 4-6 mm for a mouse, 1.5-2 cm for a rat;
• spot the nests: balls of shredded glass wool, paper, chewed plastic;
• check every pipe penetration heading upwards.

If access is too low or too uncomfortable, an endoscopic inspection camera with a remote screen lets you explore the corners from the hatch without entering the space. It's also the tool that settles the question of "thermal expansion noise" versus "biological activity".

Step 3 — Inside the home, at floor level

On your knees, torch in hand, inspect:

• behind the dishwasher and the washing machine (supply and drain penetrations);
• under the sink, where the trap enters the wall;
• the corner skirting board in wet rooms;
• the base of the electrical panel and every low junction box.

The blotting paper test remains formidable: dust a fine layer of flour or talc over 20 cm in front of a suspect passage, and come back 48 hours later. The footprints and the tail drag mark leave no room for doubt.

Sealing up: what genuinely holds and what doesn't

This is where most DIY work fails. A rat's incisors exert a pressure in the order of several tens of megapascals and grow continuously: the animal has to gnaw. A soft material is an invitation.

Ineffective materials

Materials that hold up

• Stainless steel wool (or a stainless mesh plug) pushed into the gap, then locked in place with mortar. The metal fibre injures the gums and stops gnawing. The go-to choice for all pipe penetrations. A rodent-proof steel wool roll can be cut with scissors and packed in with a spatula.
• Fast-setting repair mortar, packed in once the stainless wool is in place. This is the most durable combination on a masonry base wall.
• Metal grille with ≤ 5 mm mesh, in galvanised or stainless steel, screwed on rather than simply glued, over every air vent. A fine-mesh stainless air vent grille maintains the code-required ventilation flow while blocking mice and crawling insects.
• Folded sheet metal or metal trim on the corners of garage doors and wooden hatches, where gnawing always starts at an edge.
• Metal brush door sweeps for service doors and garage doors: the gap under an up-and-over garage door often exceeds 15 mm — effectively a wide-open door for a rat.

Operational rule: never seal a passage without first checking that no animal is trapped inside. A rodent shut in behind a partition dies on the spot and produces a smell nuisance lasting several weeks, plus flies from decomposition. The correct order is: trapping/treatment first, sealing afterwards.

The October-to-December schedule

First half of October Full exterior and interior inspection. A quantified inventory of the points to deal with. Clearing the base of the façade: firewood 50 cm from the wall and on a pallet, pots and rubble removed, climbing vegetation cut back 1 m from the base wall.

Second half of October Setting up monitoring: unbaited mechanical traps in the crawl space and the garage to gauge activity, filling floor drain traps, checking inspection chamber covers.

November Sealing work, from the largest passage to the smallest. Closing one 30 mm opening has more effect than ten 3 mm cracks. Torch-check after each intervention.

December Follow-up visit. Repeat the talc test at historically active points. Check that no new grille has been loosened by frost or damp.

When the situation goes beyond DIY

A few signals call for a Certibiocide-certified professional rather than pressing on alone:

• signs of rats (droppings > 1.5 cm, active burrows): brown rat populations are managed at network level, not at dwelling level;
• German cockroaches seen during the day: this signals an already dense population, and bait gel treatment has to cover the building's entire riser;
• gnawed electrical wiring: fire risk, to be checked by an electrician;
• inaccessible or flooded crawl space;
• multi-unit building: sealing the riser shafts and treating the columns falls to the building manager, in the common areas.

For mixed situations — an individual treating their own home while the building remains the source — our services page details the scope of a professional intervention, and the shop brings together the products we've tested for prevention. Our price benchmarks are gathered in the article on the cost of a pest control intervention.

Three classic mistakes to avoid

Blocking the crawl space vents. This is the most common and most damaging reaction: condensation, mould, deterioration of the lower floor slab, and on some sites a build-up of radon. You screen it, you don't block it.

Scattering loose rodenticide pellets in the crawl space. Under the European biocides regulation and ANSES opinions, use of anticoagulants by private individuals is strictly regulated: bait in secured stations only, out of reach of children and non-target animals, with removal of carcasses. Loose bait in an unmonitored space stacks up non-compliances and the risk of secondary poisoning of birds of prey and cats.

Believing ultrasonic repellents are a stand-alone solution. ANSES has pointed out the lack of evidence of lasting effectiveness for these devices: rodents habituate within a few days. They are in no way a substitute for physical sealing.

Key takeaways

• The entry point of a winter infestation is almost always a utility penetration at the lower floor slab, not a visible opening.
• A mouse gets through 6 mm, a rat through 20 mm: the scale of the inspection is the millimetre, not the room.
• The only materials that hold are metal: stainless steel wool, fine-mesh grille, packed mortar, metal trim.
• Always ventilate the crawl space — you screen it, you don't seal it.
• Order matters: eliminate first, seal afterwards.

The infestation figures reported by Ipsos and the national press in 2026 describe a mass phenomenon, but the mechanics remain individual and very physical. A house whose ten to fifteen utility penetrations are properly sealed gets through the winter without visitors, while the identical house next door takes in a colony at the first frost. The difference comes down to an hour and a half of inspection in October and a few metres of stainless mesh.

Sources

• ANSES — opinions and studies on rodenticide biocidal products and electronic repellent devices
• Ipsos — survey on French people's exposure to pest infestations
• ADEME / CSTB — recommendations on ventilation of crawl spaces and lower floor slabs
• Service-public.fr — landlord and building manager obligations regarding housing sanitation