Room climate sensors: prevent mould in heating season

The physics is quickly told. Warmer air can hold more moisture than colder air. When room air cools down at a wall, a window frame or in a corner, relative humidity rises there even though nothing has changed about the absolute water content. The German Environment Agency describes exactly this conclusion: colder room air increases the danger of higher relative humidity and of moisture condensation along cool surfaces (German Environment Agency 2022). Cold external walls, cool surfaces in the room and niches where accumulated moisture is hard to carry away by airing are affected in particular.

In summer this goes unnoticed, because the temperature differences between inside and outside are small and the windows are often open anyway. From October the picture reverses. The home is heated, people cook, shower, do laundry and breathe as before, but the moisture no longer has a way out. Anyone who additionally wants to save during this time and lowers the temperature shifts the balance further in the wrong direction. That is why the room climate question belongs at the start of every saving plan and not at the end - the link between heating costs and smart home has a reverse side that is rarely counted in.

The most important mistake in thinking about room climate is the assumption that a value in the middle of the room applies to the whole room. It does not even apply to the opposite wall. The UBA mould guideline works through an example for a well-insulated building: at 20 degrees room air temperature and 50 percent relative room air humidity, even the external corner behind a wardrobe only shows a surface humidity of 73 percent (UBA mould guideline). If room air humidity rises to 60 percent, at the same spot this already leads to a surface humidity of 88 percent, which makes mould growth possible (UBA mould guideline). In an insufficiently insulated older building the figures come out considerably less favourably: there the same guideline reports 89 percent surface humidity behind a wardrobe on the external wall and condensation in the external corner (UBA mould guideline). The guideline presents these figures as theoretically calculated values under steady-state conditions that do not always apply exactly in practice.

Why this counts is shown by the second figure from the same guideline: at 80 percent relative humidity at the surface and a sufficiently high surface temperature, the growth conditions for many indoor-relevant mould species are reached (UBA mould guideline). Between 73 and 88 percent at the wall surface there are, in this example, therefore ten percentage points of room air humidity - and the threshold from which a surface works as a breeding ground. That is exactly why a single sensor in the hallway is worth so little. Anyone who has ever tried to keep a flat cool knows the theme: what matters is where you measure.

For self-monitoring, the UBA mould guideline explicitly names simple devices: to check for yourself whether the room temperature is high enough and enough airing has taken place, simple electronic thermo-hygrometers that are cheaply available are suitable (UBA mould guideline). The limitation stands in the same paragraph: the values measured with them only allow a rough assessment (UBA mould guideline). For the question of whether to air right now, that is enough. For the question of why a particular room repeatedly becomes too damp, it is not.

The difference between reading off and measuring lies on the time axis. The guideline is clear at this point: since the results of single measurements are subject to strong fluctuations, long-term measurements over several weeks, for example using a data logger, make sense for meaningful results, which also captures ventilation behaviour (UBA mould guideline). Such climate recordings are as a rule sensible in the cold season and should take place in the important areas, that is at suspected cool surfaces, in the room air and where applicable in the outside air (UBA mould guideline).

Networked room climate sensors do exactly this as a side effect today. They measure every minute, record the course and store it on the phone. What matters is that the sensors talk reliably to each other and to the hub; a foreign radio protocol or a weak network in the cellar turns the measurement series into a chain of gaps. How cross-manufacturer connections work by now is described in our article on the Matter standard; where the radio network is the limit, optimising the Wi-Fi helps more than another sensor.

Sensors alone change nothing. They provide the basis for three interventions that make sense in this order. First the temperature: a general lowering of indoor air temperatures in regularly used living rooms increases the mould risk (German Environment Agency 2022), which is why the recommended 19 to 20 degrees during the day remain the starting point and the night setback ends at 18 degrees. Second the furnishing: in poorly insulated flats, heavy furniture such as wardrobes and beds should not be placed directly against external walls, and at least 3 to 5 centimetres of distance should be kept (German Environment Agency 2022). Third the automation.

In this case, automating first of all means reminding. A note on the phone when humidity in the bedroom rises above 60 percent is more effective than any rule kept in your head. A voice assistant can read out the values on request, which helps especially where nobody wants to open yet another app. Anyone who wants to go further couples radiator thermostats to the readings, so that a room reaches temperature before the usual shower time and not after it. In our experience the benefit of this coupling is greater than that of another measuring point.

One limitation applies to everything that runs automatically: an automation is only as good as its connection. If the line drops out, rules that live in the home carry on while rules in a remote service stand still. That is why we set up locally whatever can run locally. What to do when the connection itself is the problem is described in the article on tracing an internet fault.