Understanding air leaks and their impact on your airtightness
Where air passes through most often: hatches, sockets, ducts and electrical boxes
On a renovation, leaks often hide in the building's weak points. The loft hatch, sockets and switches on a wall bordering an unheated space, duct penetrations (mechanical ventilation, plumbing), and unsealed electrical boxes let air escape. Without treatment, your airtightness becomes a sieve, even with good insulation.
Consequences for comfort, the bill and insulation performance
Cold air coming in and warm air going out create draughts and uncomfortable zones. As a result, the heating runs harder, the bill climbs, and the insulation performs poorly. With degraded airtightness, you also lose acoustic comfort, and the risk of condensation increases around poorly sealed penetrations.
Airtightness and ventilation: finding the right balance on site
Making the envelope more airtight doesn't mean blocking fresh air. On site, the aim is continuous airtightness, then securing ventilation with correctly sized mechanical ventilation and planned air inlets. The objective is simple: keep the heat inside, and let moisture out through the right paths.
Detecting air leaks: simple methods and suitable tools in 2026
Visual inspection and field tests: smoke pencils, paper sheet, anemometer and thermal camera
Before bringing out the heavy artillery, start simple. Spot light gaps around window frames, crushed seals, network penetrations. To confirm, use a smoke generator or smoke pencil, then the paper-sheet test on a door or a hatch. An anemometer helps quantify a draught. In 2026, compact thermal cameras (often on a smartphone) remain very effective, provided there's a temperature difference and little wind. To go further on this topic, you can rely on infrared thermography.
Blower-door test: when to schedule it and how to use the results
Plan the blower-door test when the airtightness layer is continuous but before finishes go on. You locate leaks (smoke, camera) under negative pressure and prioritise the fixes. Use the indicators (n50, Q4Pa-surf) to compare before and after, and secure the sensitive points.
Mapping your risk zones: a detection checklist before closing up the linings
- Wall-floor, wall-roof and partition-wall junctions, loft hatches.
- Window reveals, thresholds, roller-shutter boxes.
- Sockets, boxes, spotlights, mechanical-ventilation ducts, drains, heat-pump penetrations.
- Membrane overlaps, tapes, sealant, bearings.
Access hatch: effective solutions for lasting airtightness
Choosing and fitting the right seal: compression, continuity and maintenance
Aim for a continuous compression seal (high-density foam or EPDM). It must go all the way around the hatch, without rough cuts at the corners. The closing mechanism must ensure even pressure. Without compression, airtightness degrades quickly. Plan an annual check. A crushed, detached or dusty seal should be replaced before it starts leaking air.
Creating an airtight frame: adhesive tapes, sealant and connections with no weak point
Take care over the airtight frame between the fixed frame and the vapour barrier or airtightness membrane. Use adhesive tapes designed for airtightness, on a clean, dry substrate. At difficult connections, a suitable sealant (often acrylic) secures continuity. The objective is simple: zero gaps, zero breaks.
Common mistakes: warped hatch, uneven bearing, interrupted insulation
Leaks often come from a warped hatch, an uneven bearing surface, or insulation that stops abruptly at the edge of the opening. The result: an air bridge and sometimes a cold spot. Correct the installation plan, stiffen the hatch if needed, and insulate the panel too, with continuity as close as possible to the insulation of the loft.
Sockets, switches and boxes: treating air leaks without improvisation
Sealed boxes and membranes: good installation and connection practices
Sockets and switches often pass through the airtightness layer. In renovation, favour sealed boxes (or covers) compatible with the lining. Cleanly connect the membrane or vapour check to the box with tape designed for this use, without folds or tears. A continuous perimeter seal makes the difference. To go further on this point, see our article on sealed boxes.
Managing cable penetrations: cable glands, sealant and sleeves
Cables are little "air chimneys". Use integrated cable grommets or cable glands. Failing that, seal with an acrylic sealant or a bonding adhesive suited to the substrates. A properly tightened sleeve avoids damaging the insulation and keeps a clean penetration.
The case of old walls and linings: adapting airtightness without trapping moisture
On old walls, the goal is to reduce leaks without blocking vapour transfer. Favour a vapour-variable check membrane on the interior side and take care over the connections at the boxes. Avoid solutions that are too airtight on the cold side. Keep the ventilation working — it's your safety net.
Duct penetrations and other pass-throughs: ensuring continuity of the building's airtightness
Sealing around ducts: sleeves, collars and suitable sealants
Every penetration is a potential small leak. To maintain good airtightness, treat it as close to the substrate as possible. On membrane or vapour barrier, favour compatible sleeves and collars, then a dedicated tape. In masonry, combine backfilling (mortar or foam depending on the case) with a sealant suited to movement and moisture.
Sensitive points: mechanical ventilation, hydraulic networks, ducts and floor recesses
Mechanical-ventilation penetrations, hydraulic networks, ducts and floor recesses concentrate defects. Plan penetrations ahead of time, avoid "loose" duct runs, and group penetrations together. Wherever there's movement or heat, choose solutions designed for those constraints, not a "universal" product.
Quality check before handover: quick fixes and proof of good airtightness
Before handover, do a complete walk-through with a visual check and a simple test (smoke, anemometer, or a pre-test with the blower door if planned). Fix issues right away. Keep dated photos and the technical data sheets of the products installed — that's your proof of continuity.
Key figures
10–20%
Duct penetrations
10–15%
Electrical sockets
15–25% of leaks
Loft hatch
Frequently asked questions
On many jobs, you can aim for at least Q4Pa-surf ≤ 0.8 m³/(h·m²) in a detached house (and ≤ 1.0 in a multi-unit building), thresholds commonly used in high-performance renovation. Run an intermediate test before boards/facings go up so you can correct issues at lower cost, then a final test at the end of the job.
Louis Airy
COO of Argile
