Energy retrofits: deal with rodents before you insulate

The file always lands too late. A 54-unit building in Val-de-Marne, external wall insulation (EWI) voted through at the June general meeting, scaffolding up by late September, cladding and 16 cm of mineral wool installed before the first frosts. In February, three top-floor flats report scratching noises inside the walls at night. The renovation contractor points to the managing agent, the managing agent points to pest control, and the pest controller explains that the insulation assembly will have to be opened up in several places. The remedial quote exceeds the cost of the rodent control campaign that should have been carried out six months earlier, and the defects liability warranty covers none of it, because the installation itself is compliant.

This scenario is multiplying across the Paris region as whole-building retrofits follow one another under MaPrimeRénov' Copropriété and the grants from the Agence nationale de l'habitat (Anah). We insulate, we lag the pipework, we replace the windows, we close up the roller shutter boxes — and in the process we hand rats and mice kilometres of warm, invisible, inaccessible tunnels.

The issue is neither ecological nor anecdotal: it is a matter of site sequencing. A rodent control campaign and proofing work carried out before installation cost a fraction of the same operation afterwards. But someone on the client side has to have written it into the programme.

Residential building façades with fire escapes and windows, seen from below in an urban setting

Why energy retrofits accelerate infestations

Insulation is a habitat, not a barrier

Mineral wools (glass wool, rock wool), bio-based insulants (cellulose wadding, wood fibre) and expanded polystyrene boards provide no food for rodents, but they offer the three conditions rodents look for: a stable temperature, a soft medium in which to dig a nest, and complete protection from predators and light. A house mouse (Mus musculus) needs a gap of just 6 to 7 mm; a brown rat (Rattus norvegicus) gnaws through polystyrene, a breather membrane, a silicone seal or a PVC conduit without difficulty.

In an EWI system, the air cavity between the structural wall and the cladding, the rail sections, the slab edges and the ventilation outlets form a continuous vertical network from the basement to the roof space. In a cold-roof installation, blown insulation becomes an open-air burrow. Thermal engineers know the outcome well: tunnels compact the insulation, creating thermal bridges, while urine soiling gradually saturates the material — a subject that overlaps with rats and mice in roof spaces and lofts.

Basement pipe lagging: the classic trap

Lagging heating and domestic hot water pipework in the basement is one of the most heavily subsidised measures, because it is cost-effective and straightforward. It involves fitting insulating shells along the full length of the pipes, often at ceiling level in cellars and car parks. The result is a horizontal, warm, elevated motorway, out of reach of maintenance staff and perfect for moving from one stairwell to the next. Fixing clips and aluminium films are pierced within a few nights.

The same logic applies to refurbished service ducts, renovated lift shafts and re-partitioned plant rooms. Every metre of new pipework left unsealed where it passes through a wall is an open door. The meter cupboard is the most common illustration of this.

The works themselves displace populations

Construction disturbs outdoor burrows, building perimeters, landscaped areas dug up for services, and open cellar stairwells. The rodents do not disappear: they move into the building fabric, typically within two to four weeks of the start of earthworks — a mechanism already described for road and façade renovation sites. Add to that, for the whole duration of the works, the site welfare facilities, the workers' food waste and uncovered skips.

The preliminary pest survey: what it must contain

Before voting the works through, or at the latest before the order to proceed, the client would do well to commission a pest status survey from a company holding the Certibiocide certification and, ideally, CEPA CEN 16636 certification. This document is nothing like a sales quote: it is a technical item appended to the works file.

It must include:

• A building plan showing the evidence found: droppings, runways, smear marks, gnawing, burrows, footprints in dust, urine odour. One pinpointed sign is worth more than a blanket statement of "rodents present".
• Species identification: the strategy differs between the house mouse (many bait points, low water consumption, a territory of a few metres) and the brown rat (outdoor burrows, neophobia around food, a territory of 50 m and more).
• An inventory of the entry points to be dealt with by the works: pipe penetrations, low- and high-level ventilation grilles, cellar door thresholds, gaps under car park gates, refuse chute hatches, drainage inspection chambers.
• A list of food sources: bin store, composter, animal feeding, stock held by ground-floor shops.
• Dated photographs, useful in the event of a later dispute with the contractor or the structural damage insurer.

For surveys in crawl spaces, roof voids or behind dry lining, technicians routinely use an endoscopic inspection camera a few millimetres in diameter, which avoids opening up blind. It is also the tool that makes it possible to document a service penetration before sealing it for good.

A simple rule: every void that will become inaccessible after the works must be inspected, cleaned and photographed before it is closed. Whatever is not seen before installation will not be seen again for twenty years.

The schedule to follow

The most widespread mistake is to plan "a rodent treatment" for the month work starts, as though it were a cleaning service. A rodent treatment is not measured in visits but in duration: the drop in activity is assessed over several weeks.

The decontamination phase is the one most often forgotten. Fitting new cladding over existing insulation laden with droppings means sealing in a source of odours, allergens and contamination. The protocol for cleaning, pre-wetting the soiling and disinfecting is set out in our guide to disinfection after a rodent infestation.

A reminder of the health rationale: leptospirosis is transmitted through rodent urine, and Santé publique France records several hundred cases a year in mainland France. The INRS classifies work carried out in the presence of rodent droppings among the situations requiring respiratory protection. A disposable FFP3 mask and long-cuffed gloves are not overkill: they appear in occupational health recommendations for any work in a soiled roof void or crawl space.

The clauses to include in the works contract

This is where 80% of the outcome is decided. Paris-region energy retrofit specifications are drafted by thermal design consultants, who think in terms of thermal resistance and airtightness, rarely in terms of rodent-tightness. A few lines are enough to change the picture.

Structural works / EWI package

• Sealing of all wall penetrations with filled mortar or expanding foam reinforced with metal mesh: polyurethane foam on its own is gnawed through within a few nights.
• Closure of the low- and high-level air inlets of the insulation assembly with a stainless steel grille or mesh with a maximum 5 mm aperture, mechanically fixed, never simply glued.
• Continuous metal closure flashing at the base of the EWI, 20 cm above finished floor level, to block access to the air cavity.
• Protection of external polystyrene corners at ground level with a rigid profile over the first two metres.

Windows and shading package

• Replacement of the brush seals on thresholds and the bottoms of plant room and cellar doors; a cellar door leaving a 12 mm gap will not stop a brown rat.
• Roller shutter boxes: sealing of the side cheeks and strap outlets, classic entry points from an insulated façade.

Plumbing / heating package

• With pipe lagging: metal collars where services pass through walls and floors, and finishing tapes fixed with stainless steel clips rather than plain aluminium tape.
• Maintenance of a minimum 5 cm clearance between lagged pipework and walls, to allow visual inspection.

Administrative clauses

• Obligation on the contractor to sheet over skips, store meals in sealed containers and clean the site welfare facilities daily, with a contractual penalty for breaches.
• Hold point before voids are closed: cladding installation cannot begin until the pest control inspection record has been signed off.
• Pest control services identified as a separate package or as a priced option, with an intervention report for each visit. Drafting this section follows the same logic as that described in our guide to the specification for a pest control contract.

Who decides, who pays, how to get it voted through

In a homeowners' association, the preliminary pest survey and the rodent control campaign fall under the maintenance of common parts: a decision by the majority set out in article 24 of the Act of 10 July 1965 where it is a maintenance service, and inclusion in the works vote itself where proofing becomes a permanent construction item (grilles, flashings, permanent sealing). In practice, the most effective approach is to include the line "preliminary rodent control and proofing" in the same overall quote as the works, to avoid a second trip to the general meeting and to make it eligible for the same collective funding. The procedure for the vote is set out in our article on how to get a rodent control contract approved at a general meeting.

For social or institutional landlords, the expenditure naturally falls within the refurbishment budget. The point to watch lies elsewhere: pest control services are often covered by a call-off contract separate from the works contract, with two different project managers who never speak to each other. A simple monthly coordination report, with the pest control contractor invited, resolves most of the problem.

A reminder of the legal framework: the departmental sanitary regulations require the owner or manager to keep buildings and their outbuildings free of rodents, and the decree of 30 January 2002 on decent housing lists the absence of infestation among the required characteristics. An energy retrofit that makes an infestation worse does not remove that obligation: it exposes it.

The six checkpoints at handover

Before signing off completion, a property manager can check the essentials in person, armed with a powerful focused-beam torch and a tape measure.

1. Base of the EWI: run your hand along the foot of the façade. Any open air cavity, any perforation of the cladding less than 60 cm above the ground is a defect to be put right.
2. Ventilation grilles: check the aperture (5 mm max) and pull gently on them. A grille clipped in without mechanical fixing will not hold.
3. Service penetrations in the basement: look for gaps around the new pipes. A 2 cm gap around a pipe is a mouse entry point.
4. Cellar and plant room doors: check the bottoms of the doors and the new brush seals.
5. Hatches and chambers: check that refuse chute hatches and privately owned drainage inspection covers are locked or heavy enough.
6. Monitoring stations: the boxes must be numbered, fixed in place and match the plan in the report. A station left loose on the floor, unidentified, proves nothing — as explained in our guide to checking an intervention report.

For common parts and plant rooms, a rodent proofing kit (stainless steel mesh, metal wool, repair mortar) allows the caretaker or maintenance staff to deal immediately with small gaps noticed after the works, without waiting for the contractor to return. For bin stores and cellars where bait is not advisable, spring-triggered steel traps set along runways, with a monitoring log, usefully complement the professional arrangements.

Key takeaways

From a rodent's point of view, an energy retrofit is the creation of a brand-new, heated, enclosed habitat. The thermal benefit is never in question; it is the sequencing that makes the difference between a building cleared for twenty years and a building that seals its infestation behind new cladding.

Three decisions are all it takes, and all of them are made before the first scaffold goes up: a pest survey at the outline design stage, a rodent control campaign that finishes at least four weeks before installation, and four or five rodent-proofing clauses written into the specification with a contractual hold point. If you are preparing a works programme in the Paris region, request a quote for the pest control element at the same time as the thermal tenders, and have the specification reviewed before publication: corrections cost a few lines, omissions cost you an opened-up façade. Estate managers can also rely on an annual contract to cover the post-works monitoring period, which is the most decisive of all.