Damp at home: diagnosis before the product

“Damp in the house” is not one diagnosis. The same black stain can be glass below dew point, a windowless bath, indoor drying, a thermal bridge, a leaking hopper or rising damp. Buying before you tell those apart is the most reliable way to pay twice.

Four families of damp

It helps to sort what you see into four families. They can coexist in one flat.

1. Excess indoor vapour: showers, cooking, breathing, drying racks. Indoor RH is high. Glass wets. Mould finds cold corners and bathrooms.
2. Condensation on cold surfaces even when RH is not extreme: single glazing, reveals, shutters boxes, north corners. The issue is surface temperature.
3. Rising damp: water from below, capillary action, stains that start at the skirting and climb, often with salts. They linger in summer.
4. Rain or a leak: facade, roof, pipe, sill. Local stains, sometimes after a storm.

Look before you measure

Walk the dwelling first. Note room, wall, height from the floor, season, and whether rain days matter. A dated photo beats a feeling.

• Wet glass and sills in the morning, especially in winter: condensation / vapour track.
• Mould behind a wardrobe, high in corners, on a cold external wall: condensation on a thermal bridge.
• A continuous low band, flaking paint, salts, cellar smell in July: rising damp or earth-retaining wall.
• A blotch that grows after rain: leak track.

Measure two numbers, not one

Relative humidity alone is incomplete, because it is relative to temperature. 60% at 21 °C and 60% at 16 °C do not load cold surfaces the same way. You want air temperature and RH, and if possible the coldest surface temperature.

A comfort band often cited in design literature (not an Italian legal obligation) sits roughly between 40% and 60% RH. Above that, thermal bridges condense more readily. Below, air feels dry. These are bearings, not magic lines.

The 80% surface threshold

EN ISO 13788, used in hygrothermal checks of building parts, treats 80% surface relative humidity as a conventional mould-risk threshold. It does not mean “mould is present”. It means that if the condition lasts, the surface climate favours growth.

When not to buy a dehumidifier yet

• You have never measured RH: log a few days, at different hours.
• Stains start low and climb: call a specialist, or at least compare material moisture low versus high on the wall.
• There is an obvious leak: a compressor is not a plumber.
• The problem is only the bathroom after showers: a humidistat extractor is often more rational.

The order we keep

1. Cut water at the source (shower, pans, laundry) and air when outdoor air is drier.
2. Measure for a week.
3. Ease obvious thermal bridges (wardrobe off the external wall, curtain not blanketing the radiator under the window).
4. Only if RH stays high and the cause is the air: a sized dehumidifier, or mechanical ventilation if the envelope justifies it.
5. Remove visible mould after the climate improves, not before you close the vapour tap.