"I sealed the hole behind the washing machine with expanding foam one Sunday. By the following Wednesday, there was a neat tunnel right through the middle, and crumbs of yellow foam all over the tiles." This homeowner from Créteil hadn't done anything stupid: he had done what nine out of ten homeowners do, with the most prominent product on the DIY store shelf. And he had just handed the mice a soft, easy-to-gnaw material that, on top of everything, hid the passage from view.
Sealing off access points — what professionals call proofing — is the only rodent control measure whose effect doesn't wear off. Bait gets eaten, a trap fills up, an outdoor population renews itself. A properly sealed hole, on the other hand, stays sealed. But you still need to know what stands up to a rat's incisor, which scratches glass and bites into aluminium.
This blog has already covered insulated attics, broken drain pipes, trapping, and the rules governing rodenticides. This article addresses a distinct and purely technical question: which material goes into which hole, in what order, and which mistakes doom the job from the outset.
The two figures that decide everything: 6 mm and 20 mm
Before choosing a material, you need to know what you're trying to close off.
An adult house mouse can squeeze through an opening of 6 to 7 mm, roughly the thickness of a pencil. Its skull is compressible and its rib cage flexible: if the head fits, the body follows. It climbs rough vertical surfaces (render, bare breeze block, brick), jumps about 25 cm high and can drop a full storey without injury.
A brown rat (Rattus norvegicus), the species found in the cellars and sewer networks of the Val-de-Marne, needs 20 to 25 mm. It burrows into loose soil down to 60 cm deep, swims, and actively enlarges an opening that's too small by gnawing at the edges.
The practical consequence: any opening wider than 5 mm must be treated as an open door. And there's no point sealing eight access points out of ten. The rodent only needs the ninth.
Field rule: if a one-cent coin (16.25 mm) fits into a gap, a young rat will get through. If the tip of a Bic pen fits, a mouse will get through.
Ranking the materials, from worst to best
What doesn't hold (and what you see everywhere)
Expanding polyurethane foam on its own. It's soft, porous, and a mouse will tunnel through it in a single night. Worse still: some foams trap moisture against the substrate. There are so-called "rodent-proof" foams containing a repellent additive or mineral filler; in practice, their mechanical resistance remains poor. Foam is only acceptable as an airtightness complement, behind a hard barrier.
Silicone and acrylic sealant from a cartridge. Excellent against draughts, useless against teeth. A mouse will peel them off by biting the edge.
Adhesive tape, cardboard, rags, plastic bags. These aren't seals, they're free nesting material.
Plastic ventilation grilles. The ones supplied as standard on cellar or crawl space vents give way within days. A rat will punch through them at the corner.
What holds
Loose stainless steel wool. This is the trade's basic consumable. Packed tightly into a gap or around a pipe, it's abrasive and impossible to gnaw through: the rodent injures its gums and gives up. Mind the grade: ordinary steel wool rusts within a few months in a cellar or outdoors and eventually falls apart. Stainless steel wool for rodents, sold by the roll, can be cut with tin snips and packed in with a flat-head screwdriver.
Fine-mesh metal wire mesh, known as "rodent-proof mesh". For openings that must remain ventilated: cellar air vents, crawl space outlets, roof vents, the bottom edge of cladding. You need a mesh of 5 mm maximum and a wire of at least 0.7 mm, in galvanised or stainless steel. Chicken wire (13 mm mesh, soft wire) is useless: it keeps out birds, not mice. One roll of 5 mm stainless steel mesh covers most of a single-family home's needs.
Mortar, rapid-set cement and filled repair fillers. For masonry holes, chases and duct sleeve sealing. A rapid-set cement lets you close off a void in twenty minutes, provided you reinforce it: first pack steel wool deep inside, then seal over the top. Mortar alone over a deep void will crack, and the rodent will attack the crack.
Perforated sheet metal and metal plates. To protect a gnawed wooden door bottom, a cladding corner, or a roller shutter box. A 0.8 mm plate screwed over the damaged area solves the problem for good.
Metal brush door sweeps. In single-family homes, the door threshold is the number one real entry point. A metal-brush door bottom seal leaves less than 3 mm clearance and withstands daily friction, unlike foam draught excluders.
Surveying: finding the holes before sealing them
Sealing at random wastes a weekend. The method is always the same: walk the perimeter of the house at ground level, then do the same circuit indoors.
Outside, starting from a corner and moving slowly:
- the junction between wall and slab, especially at corners and beneath downpipes;
- cable and pipe penetrations: water supply, electrical conduit, telecoms cable, air conditioning line, boiler condensate drain — this is point number 1;
- crawl space and cellar air vents, often broken or rusted;
- water meter chambers and service connection boxes, whose covers no longer seat properly;
- garage door thresholds, particularly the corners where the rubber seal is crushed;
- the bottom of timber or composite cladding, and the returns of external insulation panels;
- fence post holes and abandoned conduits left unsealed.
Inside:
- behind and under kitchen units, around the sink trap and the dishwasher supply;
- the pipe penetration behind the washing machine and the hot water tank;
- vertical duct boxing, cupboard risers, ventilation duct entries;
- the attic hatch, flush-mounted electrical boxes on external walls, recessed ceiling spotlights below the attic;
- the roller shutter box, often directly connected to the outside.
Two simple aids. A UV lamp passage detector isn't magic, but rodent urine fluoresces, which reveals streaks running down a wall. And a rechargeable head torch radically improves the quality of a crawl space or attic inspection: both hands stay free to push insulation aside.
The talc test: lay a thin layer of flour or talcum powder in front of a suspect opening in the evening. Footprints and a tail drag mark the next morning confirm the passage. It's the most reliable way to tell an active hole from a cosmetic one, and therefore to set priorities.
