Persistent Odors in Your House with Normal CO2 and Humidity Readings

You bought a sensor. You logged readings for a week. Humidity sits between 45 and 55 %, CO2 never climbs past 900 ppm. Every number says the air in your house is fine. The smell says otherwise, and your guests notice it before they take their coats off.

Those readings are not wrong. They are simply answering a different question from the one you are asking.

What CO2 actually tells you

Carbon dioxide is an occupancy indicator. It reflects how many people are breathing in a given volume relative to how much fresh air comes in. A low reading means the air is renewed fast enough to clear what you exhale. It says nothing about chemical contamination.

Odor molecules behave differently. They are heavier, they pool near floor level, they settle into porous surfaces and release again later. A room can post an excellent CO2 figure while trapping a loaded layer of air in its lower third that general air movement never reaches. A sensor sitting on a shelf at chest height measures something your nose does not experience.

CO2 also averages. A source that releases in bursts — driven by barometric pressure swings or by the use of an appliance — vanishes completely inside a daily mean.

What humidity readings cannot see

Relative humidity describes air. It does not describe materials. A slab, a screed, a parquet underlay or a plasterboard lining can hold significant residual moisture and degrade chemically while the surrounding air reads perfectly comfortable.

This is the standard picture after a poorly dried water escape, a floor laid over a support that was still wet, or an old rising-damp problem treated only at the surface. The event is over, the hygrometer is back to normal, and the material keeps releasing sulphur compounds or aldehydes at concentrations far below anything an ambient device can register — yet well above the threshold your nose detects.

The sources neither reading covers

Volatile organic compounds. Adhesives, resins, varnishes, foams and board binders release for months, sometimes years. Concentrations are measured in micrograms per cubic metre and require a dedicated sampling method.

Soil gases under a ground-bearing slab. Many houses on the coast sit on a slab poured straight onto the ground. The original backfill — often undocumented construction spoil — and historic chemical soil treatments against termites remain active for decades. Those gases migrate through shrinkage cracks, movement joints, plumbing penetrations and cable ducts. Their flow rises and falls with atmospheric pressure and rainfall, which is why the smell seems to come and go for no reason.

Drainage back-flow. Every waste outlet is sealed by a water trap. When that water evaporates, the sewer connects directly to your room. Rarely used showers, garage floor gullies, disconnected appliance outlets and guest-room basins are the usual culprits. A blocked or missing soil vent pipe at roof level produces the same result differently: traps are siphoned dry by pressure drop each time another fitting discharges.

Localised ventilation failure. A crushed duct, a painted-over extract grille, a sleeve disconnected above a suspended ceiling — each creates a zone under permanent negative pressure that draws air from adjoining spaces such as a garage, a cellar or a crawl space instead of exhausting its own. Whole-house CO2 stays good because the rest of the property ventilates normally.

The method: build hypotheses first, then measure

Adding more sensors solves nothing. A useful investigation works the other way round. A small number of hypotheses is built from the building itself and from how the odor behaves, then the measurements are chosen specifically to separate them.

Three questions shape that stage. When is the smell strongest, and at what height? What makes it change — rain, wind, heating, a door being opened, a tap being run? And what is the building’s history: ground conditions, construction date, successive works, past water damage, window replacement?

The answers already point somewhere. An odor that tracks barometric pressure points to the ground. An odor that tracks water use points to the drainage system. A constant odor, stronger with windows closed, points to materials or to a localised ventilation defect. Measurements then confirm or rule out, each one placed where and when it can decide something.

For buyers, this matters before you commit. A persistent smell with clean air readings is not cosmetic — it usually names a structural or servicing issue that will need addressing, and it belongs in your negotiation rather than in your first winter.

Getting a definite answer

We carry out this investigation on site, with targeted measurements chosen according to the hypotheses that fit your property: VOC screening, soil gas detection, inspection of every water trap and of the soil vent, examination of the floor structure, hatches and crawl spaces, and airflow checks room by room. You receive a written report within 48 hours naming the source and setting out what to do next. We sell no remedial works of any kind, and the quotation is free ☎ 04 22 46 06 04.

A baby at home: why the odor source must be identified fast

When a young child lives in the home, the question changes: infants breathe closer to the floor — exactly where soil gases and slab emissions concentrate — and their respiratory system is still developing. Parents often wonder if the smell is « linked to the location »: the plot’s soil, its history, old treatments. The answer can be measured: an expert assessment determines whether the emissions come from the plot’s ground (location-dependent), from building materials, or from the plumbing — three causes with very different fixes. Either way, the child’s bedroom is the first room analysed.

📅 Mis à jour le 28/09/2026 — Cassini Expertise, expert bâtiment indépendant (06·83·13·40).