Dew Point & Mould Guard
Know when a wall is about to get wet, in degrees rather than a vague warning.
A hygrometer says 65 %. Is that serious? It depends on the room, the season and above all the coldest wall, and the hygrometer knows nothing about walls. Mould does not grow in the air; it grows on the surface that is cold enough for water to settle on it.
This app takes the temperature and humidity sensors you already own, adds an outdoor temperature, and gives you one number that answers the question: the margin, in degrees, between your coldest wall and the moment it gets wet. A margin of one degree needs no interpretation.
It fetches nothing from outside
No network call, no outside data. Everything is computed on your Homey with the Magnus-Tetens formula, from your own sensors, and nothing leaves your home.
Requirements
- Homey Pro running firmware 12.3.0 or newer.
- Not available on Homey Cloud: the app runs locally.
- An indoor temperature sensor, an indoor humidity sensor and an outdoor temperature, from any app. A weather service device will do for the outdoor one.
- Full access to Homey. The app asks for it for one reason only: the SDK offers no other way to read sensors that belong to other apps. That access is used to read them.
One device per room
Room. At pairing you pick the indoor temperature, the indoor humidity, the outdoor temperature, and optionally a surface sensor stuck to the cold wall.
| Capability | What it tells you |
|---|---|
| Mould risk | Low, Moderate or High |
| Margin before condensation | Degrees left before water, or frost, appears on the wall |
| Dew point | Of the room’s air |
| Cold wall temperature | Measured if you paired a surface sensor, estimated otherwise |
| Indoor temperature and Indoor humidity | As read from your sensors |
Below 0 °C the margin is counted to the frost point rather than the dew point, because on a wall that cold frost forms first. The calculation holds down to −70 °C, which covers a Lapland winter with some room to spare.
Widgets
- Room shows the margin before condensation in large type with the room’s temperature and humidity, the risk level, and below them the dew point and the cold wall temperature, saying whether the wall was measured or estimated. Pick a room from the list, or leave it empty to show the room at the highest risk.
- Dew point table has one row per room with a humidity sensor, highest risk first: temperature, humidity, dew point, margin and risk, and at the foot the number of rooms at high risk. It needs no device per room; it reads the rooms of your Homey directly. Show 4, 6, 8, 12 or all of them, or only the rooms at risk. A room you do not want there can be unticked in the app settings.
Five backgrounds: Deep blue, Slate, Plum, Forest and Sand.
Flow cards
Triggers:
- [device]: The dew point rises above [value] °C
- [device]: The mould risk became [level]
Conditions:
- [device]: The dew point is above [value] °C
- [device]: The mould risk is [level]
Action:
- [device]: Recalculate now
That is enough to drive a dehumidifier, which is what most people want: on when the risk becomes Moderate, off when it becomes Low.
The weakest link, said plainly
Unless you own a surface sensor, the wall temperature is not measured. It is estimated from the indoor and outdoor temperatures and the Wall construction setting, which decides how far the wall surface drifts towards the outside temperature. That setting is an order of magnitude, not a measurement, and it is the thing that decides the risk level. Pick it honestly; a thermal bridge (lintel, reveal, corner) is where mould actually appears.
The estimate only makes sense when it is colder outside than inside. On a hot summer day the outside wall is no longer the cold point of the house; the risk moves to a cellar, a pipe or an air-conditioned room, which this model cannot see. The app then takes the wall at room temperature and says the estimate no longer applies, rather than pretending.
A reading older than two hours counts nowhere. A sensor whose battery died yesterday is not “still at 55 %”.
Settings
| Setting | Default |
|---|---|
| Wall construction | Old, uninsulated (stone, solid brick). Also: Insulated on the inside, Insulated on the outside, Thermal bridge, Manual factor |
| Manual factor | 0.35 (0 keeps the wall at room temperature, 1 puts it at the outdoor temperature) |
| High risk at or below | 0 °C of margin |
| Moderate risk at or below | 2 °C of margin |
| Hold before improving | 0.3 °C |
The last one is there for the dehumidifier. A margin hovering on a threshold would otherwise flip the level at every reading, and the dehumidifier with it. Worsening is announced at once; improvement waits until the threshold is cleared by that much. An underestimated risk costs more than a dehumidifier running five minutes too long.
Languages
English, French, German, Spanish, Italian, Dutch, Danish, Norwegian, Swedish.
Reporting a problem
Post here with your Homey firmware version, the app version, the room’s readings at the time (indoor temperature, humidity, outdoor temperature) and the Wall construction you chose. Better still, open the app’s settings page, press Generate the report and paste it: it holds the state of every sensor subscription, each room’s readings and computation, and the recent log. If you own a surface sensor, say so; it changes everything.
More about the app, every widget and Flow card included: Dew Point & Mould Guard for Homey — Rapinology
Feedback and testers welcome.
Have fun 
