Hi everyone,
I want to share one of my favorite automations built on Aqara temperature/humidity sensors: dew point based ventilation advice.
The problem: In summer, many people open basement or bathroom windows on hot days to “dry out” the room. But warm outdoor air carries a lot of moisture — when it hits cool walls, it condenses. You’re actually making the room wetter, which leads to mold.
The physics: Relative humidity alone is misleading, because it depends on temperature. The dew point tells you the absolute moisture content of the air. Simple rule: only ventilate when the outdoor dew point is lower than the indoor dew point. Then, and only then, airing out actually removes moisture.
My setup:
- Aqara temperature/humidity sensors in the rooms I monitor (basement, bathroom) + one outdoor reference on the terrace
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- Aqara Hub M3, everything integrated into Home Assistant
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- Home Assistant calculates the dew point from temperature + relative humidity for each sensor (template sensor with the Magnus formula)
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- An automation compares indoor vs. outdoor dew point and notifies me when it’s a good time to ventilate — and warns me when opening the window would make things worse
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- On my dashboard I use color coding: green = ventilate now, red = keep windows closed
The Aqara T/H sensors are perfect for this — cheap enough to put one in every critical room, and accurate enough for dew point math.
- On my dashboard I use color coding: green = ventilate now, red = keep windows closed
Happy to share the template sensor code if anyone is interested. How do you manage humidity in your homes?