No public French source states that a loft hatch accounts for so many percent of a house's air leakage, and the breakdowns found online refer to no measurement at all. What the Cerema does document is a typology of four families of weak points, with no weighting between them, and the finding that it is virtually impossible to judge a building's airtightness by eye because leaks are diffuse. The solid figures lie elsewhere: the 0.60 and 1.00 m³/(h·m²) thresholds of the order of 4 August 2021, the equivalent leakage area of a dwelling, and the share of heating lost to infiltration, of the order of 10% with extract-only ventilation and up to 25% with heat recovery.
Understanding air leaks and their impact on your airtightness
Where air passes through most often: the four families of weak points
From measurement campaigns, the Cerema has established a typology of envelope weak points. It is qualitative: it says where to look, not how much each one weighs.
| Family of weak points | What it covers in practice |
|---|---|
| Facade and floor junctions | wall on ground-bearing slab, wall-to-floor junction in the main field |
| External joinery | threshold of the entrance door, threshold of a French window, wall-to-window junction at the lintel |
| Electrical fittings | switches and sockets fitted on a wall giving onto the outside |
| Hatches and elements crossing the envelope | loft access hatch, service-shaft access hatch |
That list is what serves as the survey template on a technical visit. Adding a percentage breakdown to it would bring nothing: no public campaign supplies one, and leakage depends too heavily on the construction method for an average to mean anything on a given job. See also our article on the loft hatch, which covers the treatment of the point most often reworked after the fact.
What permeability actually costs, in leakage area and in heating
The telling figure is not a percentage per weak point, it is the equivalent leakage area: the size of a single hole that would pass as much air as the whole envelope. The Cerema calculates it for a house of 110 m² floor area, 275 m³, with 196 m² of cold envelope.
| Q4Pa-surf measured | Equivalent leakage area | Infiltration share of total air renewal |
|---|---|---|
| 0.16 m³/(h·m²), passive level | 56 cm² | 8% |
| 0.80 m³/(h·m²), median of the stock | 282 cm² | 25% |
| 1.30 m³/(h·m²) | 458 cm² | 44% |
| 1.80 m³/(h·m²) | 654 cm² | 67% |
At the median of the stock, that is the equivalent of half a sheet of A4 permanently open in the envelope, and a quarter of the renewed air bypassing the ventilation system. On energy, the Cerema puts the weight of permeability at roughly 10% of heating demand with extract-only ventilation, and up to 25% with heat recovery, since every cubic metre entering through a leak escapes the exchanger.
Airtightness and ventilation: two packages that stand together
Making an envelope airtight only makes sense if fresh air arrives by the intended route. The two packages therefore sell together: an envelope reworked without properly sized ventilation turns a dwelling ventilated by its own defects into an under-ventilated one, and you carry that defect. Conversely, the table above shows why a heat-recovery system fitted onto a mediocre envelope never delivers the efficiency it promises.
Detecting air leaks: simple methods and suitable tools in 2026
Visual inspection and field tests: smoke pencils, paper sheet, anemometer and thermal camera
Survey comes before instruments. Light gaps around joinery, crushed seals, untreated service penetrations. To confirm, a smoke generator or smoke pencil, the paper-sheet test on a hatch or a door, an anemometer to put a figure on a draught. A compact thermal camera stays effective provided there is enough temperature difference and little wind. Worth restating: these methods locate, they do not quantify. To go further, see infrared thermography.
Blower-door test: when to schedule it and how to use the results
Schedule the intermediate test when the airtightness layer is continuous but before the finishes: it is the last moment when a fix is still reachable and cheap. Under negative pressure you locate the leaks with smoke or a camera and prioritise the corrections. The final test gives the value you can be held to. Reading the indicators, n50 and Q4Pa-surf, and converting between them is covered in our article on Q4PaConv and building air permeability.
Mapping your risk zones: a detection checklist before closing up the linings
- Wall-to-floor, wall-to-roof and party-wall junctions, loft hatches.
- Window reveals, thresholds, entrance-door thresholds, roller-shutter boxes.
- Sockets, boxes, spotlights, ventilation ducts, drains, service penetrations.
- Membrane overlaps, tapes, sealant, bearings.
Access hatch: effective solutions for lasting airtightness
Choosing and fitting the right seal: compression, continuity and maintenance
A continuous compression seal, high-density foam or EPDM, all the way around the hatch, with no rough cuts at the corners. The closing mechanism has to deliver even pressure along the whole seal, otherwise airtightness degrades within the first opening cycles. Put the seal replacement into the maintenance instructions handed to the client: a crushed or detached seal gets changed, it does not get rescued with tape.
Creating an airtight frame: adhesive tapes, sealant and connections with no weak point
The sensitive point is not the hatch, it is the junction between its fixed frame and the vapour barrier or airtightness membrane. Adhesive tapes designed for airtightness, on a clean dry substrate, and a suitable sealant at awkward connections. Treat that junction before the trim goes on, because afterwards the fix means taking the finish back off.
Common mistakes: warped hatch, uneven bearing, interrupted insulation
A warped hatch, an uneven bearing surface, insulation stopping dead at the edge of the opening: all three turn up on the same job and produce both a leak and a cold spot. Correct the installation plan, stiffen the panel where needed, and insulate it to the same level as the rest of the ceiling, 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
Electrical fittings cross the airtightness layer, and they are the most numerous family of weak points in a dwelling. Favour sealed boxes or covers compatible with the lining, and connect the membrane or vapour check to the box with the intended tape, with no folds or tears. The count is done on the take-off: ten fittings on external walls means ten items to price, boxes and tapes listed one by one on the quote. The detail of the products is in our article on sealed boxes.
Managing cable penetrations: cable glands, sealant and sleeves
An untreated cable in a conduit is a chimney. Use cable grommets or glands built into the box. Failing that, acrylic sealant or a bonding adhesive compatible with the substrates, applied onto a prepared surface. A sleeve tightened at the right diameter avoids opening the insulation more than necessary and leaves a penetration that can be reworked.
The case of old walls and linings: adapting airtightness without trapping moisture
On an old wall, the goal is to reduce leakage without blocking vapour transfer. A vapour-variable check membrane on the interior side, careful connections at the boxes, and nothing airtight on the cold side. Ventilation remains the safety net, which is one more reason to sell both packages together rather than sealing on its own.
Duct penetrations and other pass-throughs: ensuring continuity of the building's airtightness
Sealing around ducts: sleeves, collars and suitable sealants
Treat as close to the substrate as possible. On a membrane or vapour barrier, sleeves and collars compatible with the system, then a dedicated tape. In masonry, backfilling with mortar or foam depending on the case, completed with a sealant suited to movement and moisture. Sealant alone on a wide opening does not hold, it cracks on the first thermal cycle.
Sensitive points: ventilation, hydraulic networks, ducts and floor recesses
Ventilation penetrations, hydraulic networks and floor recesses concentrate the defects because they are cut by different trades at different times. Plan the openings on the drawing, group them, and assign the treatment to an identified package. Wherever there is movement or heat, use a product designed for that constraint, not a universal sealant.
Quality check before handover: quick fixes and proof of good airtightness
Before handover, walk the job with a visual check and a simple test, smoke or anemometer, or a pre-test with the blower door if the contract provides for one. Correct immediately, while the substrates are still reachable. Keep dated photos of the points treated and the datasheets of the products fitted: that is your proof of continuity, and it is what will be asked for if the value measured at the final test does not pass.
