A range, and what lies on either side
Relative humidity expresses how much water vapour the air holds, relative to what that air could hold at its temperature. The same quantity of vapour gives a high relative humidity in a cold room and a low one in a warm room, which accounts for most of the confusion on the subject.
The commonly accepted range runs from 40 to 60 per cent. It is not arbitrary.
Below 30 per cent, nasal and ocular membranes dry out, respiratory discomfort increases in sensitive people, and static electricity becomes noticeable. This is the situation of an overheated home in winter, or of a continuously air-conditioned flat.
Sustained above 60 per cent, two populations settle in. Dust mites first, whose development depends directly on ambient humidity: they thrive in bedding and textiles, and their droppings are among the most widespread domestic allergens. Mould next, which needs neither a leak nor a flood, only a surface whose temperature falls below the dew point.
The dew point, or why mould always picks the same corner
Air at 20 degrees and 65 per cent relative humidity reaches saturation when it meets a surface at 13 degrees. At that spot, and only there, vapour turns back into liquid. It is the corner of the north wall, the window reveal, the uninsulated lintel, the balcony slab that runs through the façade: thermal bridges.
This explains a constant observation: mould always appears in the same place, while the air in the room seems normal. The air is not what is at fault. The meeting between that air and a surface that is too cold is. Treating the stain without treating the surface guarantees its return, in the same season, to the same millimetre.
What the literature says
The World Health Organization synthesis on dampness and mould in buildings concludes that there is an association between visible damp or mould and increased respiratory symptoms, cough, wheeze and asthma exacerbations. The association is consistent across many studies and populations.
Two honest qualifications go with that finding. The exact mechanisms remain debated: spores, fragments, microbial volatile organic compounds, endotoxins, and the dust mites that accompany humidity all coexist, and attributing the effect to one rather than another is difficult. And the relationship between a humidity reading and an individual symptom is not established: a damp home is known to expose more, but no one can tell a given person what their reading is doing to them.
The operational recommendation does not vary: prevent or remove the moisture, rather than treat its consequences.
Which end to take a mould problem from
Bleach discolours the stain and leaves the mycelium in place once it has penetrated a porous material. It also adds water. So it is never the first step, and rarely the right one.
The useful sequence is this.
Find the source of water. Penetration through façade or roof, rising damp, a leaking pipe, condensation on a thermal bridge, or simply an excess of vapour that is not being extracted. These five origins are not corrected in the same way, and are told apart by location, seasonality and surface temperature readings.
Remove that source. Repairing the waterproofing, treating the thermal bridge, fixing the leak, or bringing into service an extractor genuinely ducted to the outside. A recirculating hood removes not one gram of vapour.
Extract the vapour produced. A shower releases half a litre of water, cooking as much, drying laundry indoors can exceed two litres. Extraction in wet rooms, used during and after use, handles most of it.
Then make good the affected materials, replacing what is porous and lastingly contaminated rather than cleaning it.
What we do
We record relative humidity and temperature over several days, in living rooms and in wet rooms, we take surface temperatures wherever the geometry of the building suggests a thermal bridge, and we reconstruct the history of water damage. The report separates what belongs to use, what belongs to ventilation and what belongs to the fabric, because the three call for neither the same budget nor the same trade. Lif8 measures places: we are not healthcare professionals and make no medical diagnosis.
