One measurement, a full overview — moisture excess as the key indicator
You do not need a sensor in every room. In fact, one correctly placed measurement can give a surprisingly precise picture of the whole home's moisture profile. Here is why — and what moisture excess tells you that no other indicator can.
Introduction
When we talk to operations staff about moisture measurement, one of the most frequent questions is: how many sensors does it take? Should we measure in every room? What about the bathroom? What about the bedrooms?
The answer often comes as a surprise: in most cases one sensor is enough. Across hundreds of measurements we have seen the same pattern: the moisture in the air is very similar from room to room when you look at data over several days.
Why the bathroom does not necessarily tell you more than the living room
To understand why one measurement is enough, we need to distinguish between absolute humidity and relative humidity.
Absolute humidity is the amount of water in the air measured in grams per cubic metre — a physical quantity that does not depend on temperature. Relative humidity is the ratio between the current amount of water and the maximum amount the air can hold at the given temperature.
Absolute humidity is almost identical in every room of a home over time, because the air circulates freely between the rooms. Differences in relative humidity are mainly due to differences in temperature. In cold rooms relative humidity is higher, even though the amount of water is the same. That is simple physics.
Where should the sensor be placed?
The sensor should be placed in a central room, typically the living room or another main living area, at a height of about 1.6 metres — not up under the ceiling, where warm air collects. The sensor should represent the home's air as a whole, not an extreme.
- not in direct sunlight
- not near heat sources such as radiators
- not near windows, where the temperature is unstable
- not behind furniture, where air circulation is poor
A good position is on an internal wall, away from windows and radiators, at a height where the air is representative of the room in general.
Moisture excess: the most important indicator
When the data is combined with outdoor weather data, the home's moisture excess can be calculated, and it is the most valuable indicator we have. Moisture excess is the difference between absolute humidity indoors and outdoors, and it shows how much moisture is added to the home beyond what the weather explains.
A typical moisture excess in a well-functioning home is 2-4 grams per cubic metre. If it stays persistently higher, something is wrong — it could be high moisture production or insufficient ventilation. By following moisture excess over time you can see the effect of the measures put in place.
Portfolio analysis
The method becomes stronger still when every home in a block is measured at the same time.
- If one home is high while the others are normal, that points to a local problem — behaviour or a local technical fault.
- If several homes in the same stairwell are high, that points to a shared problem — typically the ventilation system.
- If all the homes are high, that points to a general problem — sizing or construction.
Instead of guessing where the problems are, you can see it directly in the data. That is data-driven operations in practice.
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