Choosing a humidity sensor for the bathroom is a little different than choosing it for the rest of the home. I will share my findings here.

I control my bathroom ventilation fan using a shelly relay that is triggered by a humidity sensor, so I need a humidity sensor that updates quickly and operates reliable. I have tried these models:
– Shelly Plus H&T (wifi)
– Nedis Smart Climate Sensor (zigbee)
– SONOFF SNZB-02 (zigbee)
Let me go through each of them, so you get to know the pros and cons of them.

Shelly Plus H&T (wifi)
The Shelly Plus H&T was the most disappointing device to use in the bathroom. It operates on wifi, and wifi uses a lot of power compared to zigbee, therefore the update interval is slow to keep the temperature sensor stable.
The reaction on the display is also slow, and the range seems completely off and not suited for a bathroom at all. I even ran it with a power cord connected all the time to make sure it had the best possible conditions.

I took this picture a day after a shower.
As you can see, the sensor is standing on top of the mirror that is covered by steam after a 20 min bath with no ventilation. So the humidity was definitely above 95 percent, but Shelly Plus H&T only reports 68 percent.
This makes it very unsuited for use in the bathroom.
I use mine in my living room now, running on batteries only, and I am actually quite happy with it there, so this is not a bad temperature and humidity sensor at all, it just doesn’t work in the bathroom.

Nedis Smart Climate Sensor (zigbee)
Next up is the Nedis Smart Climate Sensor. I bought this because it was cheap and running zigbee, so I though it would be a good fit for a bathroom humidity sensor. It have a nice design, and blends in on a white wall.
This sensor will report humidity close to 100% so that´s an improvement over the Shelly device.
But the reporting interval was still locked at min 5 minutes, and while that might seem low, it´s not low enough for real-time control over the ventilation fan. What usably happened, was that 2 minutes after the shower was turned on, the humidity was a few percent up according to the sensor, so it reported that. But that might not bee enough to start the ventilation fan, so I had to wait another 5 minutes for a new reporting that would start the ventilation fan. 7 minutes is too much in my book, and I really wish the minimum reporting interval could be changed.
It made me search the internet for something with an adjustable minimum reporting interval.

SONOFF SNZB-02 (zigbee)
This takes us to the SONOFF SNZB-02 Temperature & Humidity Sensor. In Z2M this sensors reporting interval can be configured to fit your exact needs.
The sensor is small, and have the classic SONOFF design, so while it´s not pretty, it still blends in just fine in my bathroom.
This sensor is very cheep, so I had some concerns, but after 9 months of use, it still reports battery level at 100%, and the reliability and accuracy is perfectly fine for my use.
Here you can see the reporting intervals I have been using for the 9 month.

So it reports every time the humidity changes by 1 percent, but no more than every 10 seconds. It reports every hour even if there is no change.
It looks like this on a normal morning where I shower:

As you can see the reporting is very aggressive when changes are happening, and that means my ventilation fan will always start within 1 minute of the shower.
The SONOFF sensor is also very quick to go from low humidity to very high humidity, which makes this sensor perfect for use in the bathroom.

To share a little more about my setup. I use this blueprint to compare my humidity sensor readings in the bathroom with a reference humidity sensor in my living room. This have provided me with pretty good results, and is quite simple to setup and adjust.

Conclusion:
And the winner is… SONOFF SNZB-02.
This is the perfect humidity sensor for a bathroom where the humidity can quickly change from low to high. The other sensors in this article are working just fine for other rooms, but I wouldn’t recommend them for use in the bathroom.
I hope your learned something, so you don´t have to do trial and error like me.