Smart sensors in rented rooms

01What can a sensor in a room measure?

A 'smart sensor' is a small device that measures the same value over and over and sends it on. In rented rooms you will mainly come across these:

Temperature. Shows whether a room is warm enough, whether the heating works, and whether a plug-in heater is being used.
Relative humidity. The key value for mould. Peaks after showering or cooking are normal; damp that lingers for days is not.
Dew point and surface moisture. From temperature and humidity together you can estimate how damp it gets against a cold wall. That is usually where mould starts.
CO2. A measure of how well a room is ventilated, especially at night with the door closed.
Carbon monoxide (CO). A poisonous gas from a badly working combustion appliance, such as a boiler or water heater.
Smoke. The classic smoke alarm, possibly with a link that forwards an alert.
Water leaks. A small sensor on the floor by a boiler, washing machine or drain.
Energy. A smart meter or a smart plug shows how much electricity or gas a house or appliance uses.

Not every value makes sense in every room. So start with the goal, not with the device.

02Which sensor fits which goal?

Goal	Sensor	Where
Preventing mould	Temperature + humidity	Every bedroom, bathroom
Spotting a cold wall	Temperature + humidity, plus outdoor temperature	Rooms with an outside wall
Knowing the heating works	Temperature per room	Every living space
Detecting plug-in heaters	Temperature per room	Every bedroom
Assessing ventilation	CO2 (possibly temporarily)	Bedrooms, shared spaces
Preventing CO poisoning	CO alarm to EN 50291-1	Room with a combustion appliance, bedrooms
Spotting fire early	Smoke alarm	Every floor, often every room
Limiting water damage	Leak sensor	By boiler, washing machine, drain
Tracking energy costs	Smart meter, smart plug	Per house, or per appliance

For mould and heating, temperature plus humidity per room is by far the most useful. For background, read preventing mould in worker housing and what a plug-in heater costs.

03Why damp at the wall says more than damp in the air

Warm air can hold more moisture. Against a cold wall the air cools down, and that is where relative humidity rises. So a room can seem 'normally' humid, while the corner behind the wardrobe is much wetter.

In building physics a common rule of thumb is that surface humidity should stay below 80 % on average to prevent mould. The American standard ASHRAE 160 used that limit as a 30-day running average, and has since moved to a finer mould model. In practice it means: do not look at a single peak, look at how long it stays damp.

The Dutch public health institute RIVM points out that damp and mould in homes raise the risk of asthma and respiratory complaints, and that the cause can lie either in the building or in how it is used. Per-room measurements help to tell the two apart; see also mould damage: building defect or use?

Every room warm and dry

One sensor per room for temperature and humidity, servers in the EU, and every morning one overview of what needs attention.

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04Smart thermostat or a sensor per room?

A smart thermostat measures in one place, usually the living room, and controls the heating. That is fine for comfort and energy in a family home. In a house with five or six bedrooms, that single reading says little:

the living room is warm, but a north-facing bedroom stays cold;
a resident switches on a plug-in heater and the thermostat notices nothing;
damp in a bedroom with the door shut stays invisible.

A sensor per room controls nothing, but shows what happens in each room. The two do not clash: a thermostat regulates, per-room sensors check. For a housing provider's questions (is every room warm and dry enough, is anyone using extra heaters) you need per-room measurements.

05Is it allowed to measure in someone's room?

Under the GDPR, personal data is any information about a person who can be identified directly or indirectly. Temperature and humidity in room 3 at a given address can be linked to whoever lives there. So assume that climate data from an occupied room can be personal data, and set things up properly.

Choosing a legal basis

The GDPR has six legal bases. For sensors, legitimate interest is often the most obvious: preventing mould, damage and danger. The Dutch Data Protection Authority sets three conditions for it: a real and clearly stated interest, necessity (could it be done less intrusively?) and a balancing test against the resident's interests. Record that assessment in writing.

Measure as little as possible

The GDPR requires data to be limited to what is necessary for the purpose, and not kept longer than needed. In practice:

No cameras, no microphones and no motion sensors for a climate purpose. They reveal a lot about private life and are not needed for mould or heating.
Only measure what serves your goal. CO2 or a per-room power meter can show when someone is at home; use that only if you have a clear reason.
Set a retention period and have old readings deleted or summarised after it.
Do not let everyone see everything. A coordinator needs an alert, not the full data series of every room.

Large-scale or intrusive?

Processing likely to result in a high risk to people requires a data protection impact assessment (DPIA). Whether that applies to you depends on what you measure and how much. If in doubt, ask a privacy specialist.

Data Act

According to the Dutch competition and consumer authority ACM, the rules of the EU Data Act apply to smart devices placed on the market after 12 September 2025. Users are entitled to clear information about what data a device collects, and to access that data.

This article is not legal advice. Have your assessment, privacy notice and any DPIA reviewed by a lawyer or privacy specialist.

06How do you inform residents?

The GDPR requires you to inform residents at the moment you start collecting data, concisely, intelligibly and in plain language. Under Article 13 you tell them at least: who you are, what you measure and why, on which legal basis, who receives the data, how long you keep it, and which rights the resident has, including the right to complain to the regulator.

In practice this works well:

One A4 page per house, in the residents' languages. For labour migrants that is often Dutch, English, Polish and Romanian.
Start with what it is not: no camera, no microphone, nobody is watching. That is usually the first question.
Explain the benefit for the resident: a warm, dry room, and faster help when the heating fails.
Give a contact point for questions and access requests. A resident may ask which data you hold; you reply within one month at the latest.
Include it in the intake at check-in, next to the house rules.

Residents who understand why the sensor is there tend to leave it in place.

07Battery, connectivity and installation

Most room sensors run on batteries and send data wirelessly. A few practical points:

Battery life depends on how often the sensor measures and transmits, and on the distance to the receiver. Choose a system that warns you itself when a battery is running low, so you do not have to visit every house.
Connectivity. Low-power sensors usually send to a small receiver box (gateway) in the house, which forwards over the internet. Thick walls and concrete floors shorten the range. Test the signal per room during installation.
Placement. Mount a climate sensor at about chest height on an inside wall, not above a radiator, not in the sun and not right next to a window. According to RIVM, a CO alarm in a room with a combustion appliance goes on the ceiling, 1 to 3 metres from the appliance, and in a bedroom at bed height.
Labels and records. Record which sensor hangs in which room. After a sensor is moved, you do not want readings attributed to the wrong room.
Smoke alarms are not an extra: since 1 July 2022 they have been required on every floor with a living space in the Netherlands. If you rent out more than three rooms, the municipality may require a smoke alarm in each room. Ask your municipality which rules apply.

A CO alarm does not replace maintenance. RIVM advises having combustion appliances checked at least once a year, and ventilating.

08What does a good alert look like?

Too many alerts are as bad as none: after a week nobody reads them. A good system follows a few rules:

Alert on patterns, not peaks. A shower causes a humidity spike. Only when it stays too damp for days is something wrong.
One alert per problem. Not again every ten minutes, but one alert that stays open until it is handled.
Say what is wrong and what to do. 'Room 2: above 70 % humidity for five days, check ventilation and laundry' works better than just a number.
Bundle. One overview per day for the coordinator, with the most important items at the top. Only what is truly urgent, such as CO or smoke, deserves an immediate alert.
Keep your own faults separate. A sensor with a flat battery or a box without internet is a task for whoever runs the system, not an alarm about the room.
Handle and record. Who did what, and did it help? That builds a record that also helps later in disputes about damage.

eyesonrooms works this way: one sensor per room for temperature and humidity, servers in the EU, and every morning at 07:30 a single summary with mould risk, likely plug-in heaters, rooms that are too cold and houses where the heating seems to be off.

09Frequently asked questions

Is climate data from a room personal data?

It can be, because it can be linked to whoever lives in that room. So assume the GDPR applies: choose a legal basis, inform residents and do not measure more than necessary.

May I use a camera or microphone to detect mould or heaters?

They are not needed for that: temperature and humidity are enough. Under the GDPR you should choose the least intrusive means, so a camera or microphone in a private room is hard to justify for this purpose. Consult a lawyer if in doubt.

Is a CO alarm mandatory?

In the Netherlands smoke alarms have been required on every floor with a living space since 1 July 2022. That does not apply to CO alarms everywhere; RIVM does recommend them alongside yearly maintenance and ventilation. Ask your municipality about local requirements for room rental.

How do I stop my team from ignoring alerts?

Alert on lasting patterns rather than short peaks, bundle everything into one daily summary, and keep technical sensor faults separate from alerts about the room.

Smart sensors in rented rooms: what to measure, and what is allowed