Hygrometer
The range, and why it is not the interesting part
30–50% relative humidity is the band recommended for occupied rooms, and it is not controversial: below 30% the air pulls moisture out of skin and airways, and sustained readings above 60% let mould and dust mites establish.
Knowing the target is easy. The useful question is the one underneath it: where is my room actually sitting? That has an answer you can reach without buying anything, because indoor humidity is not arbitrary. It is the outdoor air, warmed, plus whatever the people in the building add to it.
The floor the room cannot go below
Air that comes in from outside brings its water with it and nothing else. Heating that air does not add a drop — it only increases how much the air could hold, which is the denominator of relative humidity. So the percentage collapses.
This is why the cold months feel dry, and it is worth seeing as numbers.
| Outdoor air | Water it carries | Same air at 20°C indoors |
|---|---|---|
| −10°C / 85% | 2.0 g/m³ | 12% |
| −5°C / 85% | 2.9 g/m³ | 17% |
| 0°C / 80% | 3.9 g/m³ | 22% |
| 5°C / 80% | 5.4 g/m³ | 32% |
| 10°C / 75% | 7.0 g/m³ | 41% |
Read the last column as a floor. A room ventilated hard enough to be entirely outdoor air, with nobody in it, would sit there. Nothing you do with the heating moves it — the heating is not what dried the air.
What the occupants add
Real rooms sit above the floor, because people breathe, cook, wash and dry clothes. EN ISO 13788 puts a figure on it, as a moisture excess over the outdoor air, banded by how the room is used:
| The room | Water added | At 0°C / 80% outside |
|---|---|---|
| Storage, unheated | +2 g/m³ | 34% |
| Office, lightly used | +4 g/m³ | 46% |
| Normal dwelling | +6 g/m³ | 57% |
| Kitchen, bathroom | +8 g/m³ | 69% |
| Laundry, pool | +10 g/m³ | 80% |
Two things follow immediately. A normally occupied home on a cold day is not dry — it is around the middle of the comfort band, and the dryness people complain about belongs to well-ventilated or lightly used rooms. And a bathroom on the same day is already past the threshold where mould becomes possible, which is why that is the room it appears in.
Putting the two together
The estimate is the sum: take the water in the outdoor air, add what a room of that kind contributes, and re-express it at room temperature. That is the whole method.
It gives a range rather than a number, and the range is not decoration. The honest uncertainty is which class your room belongs to — a "normal dwelling" that hosts a family of five and dries laundry indoors behaves like a kitchen. Reading the neighbouring classes as the bounds is more truthful than inventing a tolerance.
The excess is also held at its maximum in the cold and tapers to zero at 20°C outdoors, which matches how buildings are actually run: sealed and heated in winter, open in summer.
Where it stops working
Above about 20°C outdoors, stop using it. With nothing added by the occupants, every room class collapses onto the same figure — the outdoor air, re-expressed at room temperature. The arithmetic still produces a number and the number is worthless.
It is also an estimate of a kind of room, never of your room. It cannot tell you whether you are at 55% or 65%, and several of the thresholds that matter — mould, a humidor, a guitar — sit inside gaps that narrow. When a threshold is the question, measure it or improvise an instrument.
When a sensor is worth it
Get one when you are going to act on a threshold rather than on a trend: you are drying out a damp room and want to know whether it is working, you keep instruments or cigars, or mould keeps returning and you need to know whether the room or the surface is the problem.
For everything else — is the house drier than usual this week, will opening the window help, is the bathroom the room at risk — the estimate and the outdoor dew point between them answer it.
What this site can tell you
Outdoor conditions in full: the dew point, the water content in grams per cubic metre, and the condensation margin. Those are measured figures from a weather service, and they are what decides whether ventilating will help or hurt.
For indoors, the app now keeps a list of rooms. With a HomeKit humidity sensor it reads the room directly. Without one it estimates the likely range using exactly the method above — which is useful, and is not a measurement.