Dampness and water damage

Why it matters indoors

Dampness is not itself something you breathe. It is the condition that lets things grow which you then breathe: mould, bacteria, endotoxin and dust mites all need water. It also changes the building materials themselves — wet gypsum, chipboard and adhesives release compounds they would not release dry, which is why a damp room can smell of chemistry rather than of mould.

Why studies measure dampness rather than mould

Visible growth is only the part that reached a surface. Most growth in a damp construction is inside the wall, under the floor or behind the cladding, and sampling air for spores catches a moment rather than an exposure. Health studies therefore record observable dampness — stains, condensation, a history of water damage, reported mould odour — and relate that to symptoms. Assessment tools built for this purpose score rooms by observation for exactly this reason. The consequence for reading the evidence: a study about dampness is a study about the whole ecosystem that moisture creates, not about one organism.

Where it comes from

Four routes, and they need different repairs: liquid water from leaks and floods; rising damp from the ground; condensation where warm humid air meets a cold surface, typically window reveals and thermal bridges; and moisture produced inside — cooking, showering, drying laundry, breathing — that ventilation never removed. Only the last one is solved by ventilating; the first three are building repairs, and running a dehumidifier against a leak treats the symptom.

What to measure

Relative humidity in the room tells you whether the air can dry a surface, not whether the surface is wet. Surface temperature is the missing half: condensation happens where the surface is below the dew point of the room air, which is why a cold corner in an otherwise dry flat grows mould. Measuring both, in the coldest corner rather than in the middle of the room, is what makes the difference between a guess and a diagnosis.