Indoor humidity: what the percentage in your flat is telling you

Never write down a reading without the temperature beside it

Warm air can carry more water vapour than cold air, and the gap widens the warmer it gets. Relative humidity — humedad relativa in Spanish, humitat relativa in Catalan — compares two quantities: how much vapour the air is actually carrying, and the most it could carry at the temperature it happens to be. The reading is the first divided by the second, as a percentage.

That makes it a ratio with a moving bottom half. The amount of water in a room can stay exactly the same all evening while the percentage climbs, purely because the flat is cooling down. Nothing was added. The air's capacity shrank, so the same water now fills more of it.

Which is why a bare figure is not yet information. Seventy per cent in a living room heated all evening and seventy per cent in an unheated back bedroom describe two noticeably different amounts of water in the air. Almost every domestic hygrometer — higrómetro, higròmetre — displays temperature alongside humidity. Both numbers belong in the log. On its own, neither one can be read.

The same air, measured twice

Air moves between rooms more freely than a closed door suggests: under the leaf, through transfer grilles, along the hallway. Much of the time the rooms of a flat are working with broadly the same air. What separates them is temperature — and temperature is the half of the ratio that decides what the meter says.

So the living room, warm since six o'clock, reads modestly. The bedroom at the end of the corridor, door shut and no heating on, reads far higher at the very same moment. The instinctive conclusion is that the bedroom is producing moisture. Often it is doing nothing of the kind. It is showing you the same air, colder.

The process does not stop at the scale of a room. The coldest air anywhere in a room is the thin film touching its coldest surface: a window reveal, the corner where two external walls meet, the plaster behind a wardrobe. That film is colder than the room and therefore at a higher relative humidity than anything the meter on the shelf will ever display. Where it reaches saturation, the surplus is set down as liquid water — Spanish technical reports call the temperature at which that happens the punto de rocío, punt de rosada in Catalan, the dew point. The mechanism and its remedies are a subject of their own. What matters for reading a meter is the gap it implies: the room average is always the optimistic number.

Two ways the number goes up, and only one is about water

A rising reading has one of two causes, and telling them apart decides what you do next.

Water was added. A shower, pans boiling uncovered, laundry on a rack indoors, more people in the flat than usual. These move the number quickly, and the rise begins in the room where it happened.

The air got colder. The heating went off, a room stayed shut all day, the small hours arrived. Nothing was added anywhere. Capacity fell and the ratio rose. It is why a bedroom's reading tends to peak just before waking: the occupants have been adding vapour for hours, and the outdoor temperature has reached its lowest point of the night at the same time.

The mirror image of this is the trap. Turning the heating up lowers the percentage without removing one drop of water from the flat. The meter improves, the moisture is untouched, and the surfaces in the rooms nobody is heating stay exactly as cold and exactly as exposed as they were. A flat can look drier on every meter in it and behave no differently at all.

Only two things genuinely take water out of indoor air. Renewing the air removes the moist air and replaces it — that is what ventilation does, whether by opening windows or mechanically. Condensing it out removes water in one room while a machine runs, which is a different tool for a different job. Everything else moves the reading by moving the temperature.

Outdoor air on this coast is not the dry air the advice assumes

Guidance written for continental winters assumes outdoor air that is cold and genuinely dry. Catalonia has a Mediterranean coastal climate — mild, humid winters and hot, humid summers — which changes what opening a window does to your readings, and the reasoning is worth having rather than taking on trust.

In winter, airing still dries the flat even when the outdoor humidity reading looks discouraging. Air on a damp January morning may itself sit at a high relative humidity, but it is cold, and cold air near saturation still holds only a small absolute quantity of water. Bring it inside, let it warm up, and its capacity rises, so its relative humidity falls and it can take on moisture from the rooms it passes through. What arrives is not dry air; it is thirsty air. On a mild, still, wet day the temperature difference is small, the exchange is slow, and the same open window achieves visibly less.

In summer the arithmetic inverts. Hot, humid air carries a large absolute quantity of water, and letting it into a cooler flat in the middle of an August afternoon can leave every reading higher than before it was opened. The night and the early morning are the useful hours, and a few days of logging will identify your own better than any rule imported from a colder country.

Summer brings one more way a meter can mislead. An ordinary domestic split air conditioner takes in the air already in the room, cools it and returns it to that same room; what travels to the outdoor unit is refrigerant, not your air. It will lower the humidity reading, because cooling air past its dew point wrings water out of it at the indoor coil. A low percentage with the split running is not evidence that the flat is being ventilated. It is evidence that the flat is being cooled.

How to take readings that are worth something

One reading, in one room, on one afternoon, captures a single temperature, a single moment in the household's day and a single state of the weather. It is the pattern that has diagnostic value, and a pattern takes a little method.

• Measure more than one room. At minimum, the room that feels worst and the room you heat most. The difference between the two is the most informative figure you will obtain.
• Measure across days, not hours. A week that takes in a cold spell, a mild spell, a wash and a few showers.
• Include the coldest room, even if nobody uses it. A closed spare bedroom, or a room whose only window gives onto an interior light well — the patio de luces, pati de llum, the narrow internal shaft that several flats open onto, where the air is stiller and often damper than street air. Rooms that are neither heated nor opened hold high readings while producing almost no moisture of their own.
• Capture the night. A bedroom at seven in the morning, after eight hours with the door closed, is the measurement that explains a running window; the same room at midday with the door open explains nothing. A meter with a minimum and maximum memory records the extremes so nobody has to set an alarm.
• Mind where the meter sits. Not taped to an external wall, not above a radiator, not in direct sun, not on the floor, and not in the bathroom during a shower — there it will read close to saturated and tell you only that you have had a shower. Roughly chest height, away from doorways and vents, and give it an hour to settle after being moved.
• Compare with the same meter. Inexpensive hygrometers are indicative, and two of them side by side will often disagree. That matters far less than it sounds, provided the comparison uses the same instrument in the same positions at the same times. Differences are reliable in a way absolute values are not.

Keeping a log by hand is, in effect, what a humidity-controlled extract does automatically. On Spanish datasheets these appear as higrorregulable units, whose airflow responds to the moisture in the air passing them — a useful term to recognise, and a useful reminder that a sensor only ever knows about the air that reaches it.

Why there is no single correct percentage

A target figure is easy to want and hard to justify, and it is worth being explicit about why this page does not hand one over.

Spain's building code approaches indoor air from the opposite direction. DB-HS3, Calidad del aire interior, the section of the Código Técnico de la Edificación covering indoor air quality, sets out how a dwelling's ventilation is to be provided: outdoor air admitted in the dry rooms — bedrooms, living rooms — passed through the flat, and extracted from the wet rooms, bathroom and kitchen, to outside, with a specific extraction provision in the kitchen for cooking fumes on top of the general ventilation. It regulates the provision, not a percentage for the household to maintain. Nothing your meter displays is a compliance figure.

The World Health Organization's guidelines on dampness and mould support a cautious link between persistent indoor dampness and mould and respiratory symptoms. That is a statement about dampness that persists, not about a number crossing a line for an hour on a cold Tuesday.

And the ratio resists a single target on its own terms: the same percentage in a warm room and a cold one is not the same amount of water, so a whole-flat figure is the wrong shape of goal from the start.

Four behaviours are worth more than a target, and a week of readings will show all four:

• The spread between rooms, measured at the same moment. The widest gap points at where to look, and it often points at a temperature difference rather than a water one.
• Recovery time. How long after a shower, a pan of pasta or ten minutes of airing before the figure comes back towards where it started. A bathroom whose reading stays up long after the door is opened is telling you about the air path out of that room, not about how much water you used.
• Whether it comes down at all. A flat sitting at much the same figure day and night, in cold weather and mild, is a flat that is not being renewed.
• The direction across a season, rather than the value on any one morning.

What the meter will never show you

A hygrometer measures air. Damp appears on surfaces. That gap is the instrument's permanent limitation, and it accounts for a case that looks like a contradiction: a room whose readings appear entirely unremarkable, with a dark patch in one corner. The corner is not contradicting the meter. It is colder than the room, and the air against it stands at a far higher relative humidity than the air in the middle where the meter sits. If the question is about that corner, the instrument that answers it is a surface thermometer — an infrared one, pointed at the wall — not a hygrometer.

Two further limits are worth stating plainly. A humidity reading cannot identify what kind of damp you have: condensation, a leak from a pipe or from the flat above, rising damp and rain penetration are different mechanisms with different remedies, and they are not separated reliably without an on-site inspection. And readings that come down lower the risk of condensation without guaranteeing that mould will not return.

What a log is genuinely good for is precision. "The flat is damp" gives anyone assessing it nothing to work with. Room, time, humidity, temperature and what the household was doing, over seven days, describes something specific: which rooms, under which conditions, and whether the pattern shifts when one thing changes. That is a question with an answer.