Humidity Sensor for Mould-Prone Rooms
What It Reads, Where It Goes and the Alerts to Set

Why damp air, not only leaks, grows mould

Mould needs moisture, a surface to feed on and time. In a Malaysian home the moisture often does not come from a burst pipe at all. It sits in the air: the outdoor air is humid most of the year, a shower or a rack of drying clothes adds more, and a room that is shut all day lets it build up. When that moist air touches a cool wall, the back of a wardrobe or a ceiling under an aircon pipe, water condenses on the surface and the spores already in every home have what they need.

The trouble is that you cannot feel the difference between a room that is damp for an hour after a shower and one that stays damp all night, every night. The second room is the one that grows mould. A sensor turns that invisible pattern into a number with a history, which is why it is the cheapest first step before spending on treatment or repainting.

What a temperature and humidity sensor actually reads

Most home sensors report two values: air temperature and relative humidity. Relative humidity is how much water the air is holding compared with the most it could hold at that temperature, shown as a percentage. Warm air can hold more water than cool air, so the same air can read moderate in the afternoon and high after the room cools at night, without a drop of water being added.

That link with temperature is why the pair matters. A bedroom that reads damp only when the aircon is off, or a store room that reads damp every evening as the house cools, is telling you something about timing. Many apps also show a dew point or a comfort label; the useful part for mould is the humidity trend over days, not any single reading.

What reading to aim for

The US EPA’s mould guide says to keep indoor humidity below 60% if possible, and notes that relative humidity can be measured with a simple humidity meter. That is a useful line to draw in your app. It is not a pass-or-fail exam: a bathroom will jump well above it during a shower, and that is normal. What matters is how fast the reading comes back down.

Which rooms need a sensor first

You do not need one in every room. Start where mould has appeared before, or where it is most likely to start in a typical condo, terrace or semi-D:

• Bathrooms without a window, especially where the fan is weak or switched off with the light.
• Master bedrooms with built-in wardrobes against an outside wall; the mouldy wardrobe guide explains why the back panel goes first.
• Store rooms, understairs cupboards and the maid’s room, which are shut most of the time.
• Rooms that stay empty, such as a guest room or a unit left vacant between tenants.
• The yard or laundry area where clothes dry indoors in the wet months.

One sensor in each of those spaces is usually enough to show a pattern within a few weeks. Add more only where the first readings raise a question.

Where to put the sensor in the room

A sensor reads the air right next to it, so a poor spot gives a misleading number. Keep it away from the aircon outlet, which blows dry, cold air straight at it, and away from a window in direct sun. In a bathroom, put it on a wall or shelf outside the shower splash zone, not on the ceiling above the hot water. In a bedroom, mid-wall height on the side where mould has shown is the usual choice.

For a wardrobe or a cupboard, place a second small sensor inside it. The air behind closed doors can behave very differently from the room, and that difference is exactly what a mouldy wardrobe looks like before you see spots. Most sensors are light enough to sit on a shelf or stick on with an adhesive pad, so nothing is drilled.

Types of humidity sensor compared

If the house already runs a hub, choose a sensor that joins it; the guide to Zigbee, WiFi, Matter and Thread explains how those networks differ.

Setting alerts people will not ignore

The quickest way to make a sensor useless is an alert that fires every time someone showers. Set the alert on a sustained condition instead: humidity above your line for a long stretch, not for a moment. Most apps allow a delay or a “for how long” setting; where yours does not, a routine in the hub app can do it.

• Send alerts to the person who can act, such as whoever manages the rental unit or the household member at home.
• Use quiet hours so a night-time reading becomes a morning summary, not a 3am buzz.
• Give each sensor a clear name by room, such as “Store room” or “Master wardrobe”.
• Add a low-battery alert, because a flat sensor looks the same as a dry room in many apps.

These are alerts the family watches in its own app. A sensor does not call anyone and nobody else is watching it unless you share the app.

Automations a humidity reading can trigger

Once readings are reliable, the sensor can do more than warn. Common routines in Malaysian homes:

• Turn a dehumidifier on through a smart plug when the store room stays damp, and off when it recovers. Check that the dehumidifier restarts by itself when power returns; some models stay off until a button is pressed.
• Switch the bedroom aircon to dry mode through an IR blaster for an hour in the late afternoon.
• Run the bathroom fan until the reading falls, which is the fan-control job covered on the exhaust fan page.
• Send one message when an empty unit has been damp for several days, so someone airs it.

Keep each routine simple, with an off condition as well as an on condition, and test it once by breathing on the sensor or carrying it into a steamy bathroom.

Reading the history to find the cause

The history graph is where a sensor earns its keep. Compare the damp room with a dry room in the same house: if both rise together after rain, the whole house is taking in humid air and ventilation is the lever. If only one room climbs, and only at night, the cause is local, often a cool wall or an aircon that cycles on and off.

Readings that stay high while the walls show fresh stains point away from air humidity and towards water getting into the wall itself. That is a job for the damp wall and mould treatment guide, not for more sensors.

What a humidity sensor cannot tell you

Battery, app account and the MCMC label

Most sensors run on a coin cell or small batteries and report every few minutes or on a change. Damp rooms are hard on batteries and contacts, so check the battery level in the app now and then. Make sure the app account is in your own name, not a set-up account that belongs to someone else.

A wireless sensor is communications equipment in Malaysia. The mark is called the MCMC label: a mandatory certification mark showing that communications equipment is certified to the standards required under the Communications and Multimedia (Technical Standards) Regulations 2000. It consists of the MCMC logo with the manufacturer's or supplier's identification number, and may be a physical label or a digital e-label in the operating system of a device with a display. To check a specific device, do not rely on the look of the label alone: look up its serial number, IMEI, TAC or Self-Labelling Program number through MCMC's Check Your Label service at cyl.mcmc.gov.my, or through SIRIM eComM at ecomm.sirim.my.

How we set it up

We install battery humidity sensors in the rooms that matter, choose the spot in each one, pair them to your hub or app, and set the alert rules and any routines with a dehumidifier, an aircon dry mode or a fan. We do the smart device, app and low-voltage side of the job. For the rest of the smart home, see our smart home installation service.

Sources & references

• US EPA, A Brief Guide to Mold, Moisture and Your Home: indoor humidity guidance and moisture control.
• MCMC, certification label: the mark on certified communications equipment.
• MCMC Check Your Label: looking up a device’s certification.