Advice for long distance multi-floor wireless transmission

I would like to track the humidity of my wine cellar and be alerted in the case of high humidity. Unfortunately, I live in an apartment on the second floor while my cellar is on the ground floor. In between is someone else’s apartment. My wireless LAN is by far not strong enough to make down to the cellar.

Do I have a chance to install a humidity sensor in the cellar and transmit its status using some radio frequency? Since I am only renting, I do not see any other option for getting live data from the cellar to my OpenHAP server in my apartment on the second floor. If there is an option to get a signal up, what would you recommend? I currently own a Zigbee bridge (Philips Hue) and operate a Zwave network. Any advice appreciated.

ZWave is using the 868 MHz band and as such is the one with the potentially best range (ZigBee and WiFi use 2.4 GHz).
If that still does not work, you could add a mains powered relay node such as a power plug somewhere inbetween.

I only have a Fibaro motion detector right now, but I will try to take it downstairs and check the log to see whether it is still picked up. If not, would it help to install a Zwave power plug right in the cellar? My hope is that such a powered device has a longer range than a small portable, battery powered motion detector.

Installing anything outside of my apartment or flat unfortunately is not an option.

I will test with the motion sensor tomorrow and report back.

If you are not afraid of diy, i would go for rfm69 based devices like moteino. Rfm69 are very powerfull transceivers able to reach several hundred meters. Moteino is an rfm69 plus a microcontroller.

I am not sure whether I am that brave, but it is good to know that there are possible alternatives waiting. Thank you, @1technophile