Firstly, let's talk about normal ambient (outdoor) humidity for a moment, 100% means the air is saturated with the maximum moisture as it can hold.
In a normal hose, the humidity will typically be the same in every room, unless there is some special factor, e.g. the bathroom humidity will increase dramatically when someone takes a hot shower.
Next, let's think about a humidity sensor, it can only read the humidity in it's immediate (close) proximity.
Now, let's look at the wall in question, is it a normal wall in a normal house? This would be helpful to know, i.e. if it is a wall in an underground cellar (basement) it helps to explain why this particular wall is getting wet.
The fact that the wall is getting wet tells me the humidity in the immediate (close) proximity will almost certainly be 100% when wet and probably ambient 'room' humidity when not. This is where I believe your solution may be, and here is what I would suggest, assuming you want an alarm when the wall is in danger.
My Suggested Solution
I'd suggest you get two small humidity sensors and connect them both to an Arduino and make adjustments as needed in software so they read the same value (at your desk (workbench).
Note: One sensor is to be mounted in the wall and the other to be located in the room, 500mm to 1 meter distance away from the wall, so make your wires the necessary length before you calibrate the sensor readings.
Drill the smallest diameter hole in the wall to fit one sensor in. If the hole needs to be say 20mm diameter, make the hole 40mm deep (100% deeper). I would suggest you angle the hole 3 to 5 degrees so any excess moisture can drain out (and not damage sensor).
Insert one sensor all the way inside the hole, then cover the hole, but NOT airtight. Some examples: Find a plastic bottle cap the same (similar) diameter as the hole in the wall and drill lots of small holes in it. Another option would be to go to your hardware store and buy a small amount of "fiberglass fly screen' (mesh). Use two layers to make the mesh finer (smaller).
The Arduino Code
There are many ways to do this, but based on previous work with humidity sensors, here is what I would do.
a Lora radio will not give you the distance you need, so I would use an ESP8266 or ESP32 chip (with WiFi). The ESP8266 Node 1 would be ideal for this project and is very well priced. This will allow the device to upload the data to a cloud server, of which there are many free ones to select from (check out AdaFruit's I/O Cloud Service, from memory it offers nice graphic display of data etc. I support / recommend AdaFruit because the provide lots of free code and drivers to the hobby community. It's a nice way of saying thanks!
The WiFi device will also allow you to remotely connect, read the data, modify sensor calibration values etc and most importantly, you will be able to perform OTA (over the air) software updates.
*Create a 3 by 24 array of bytes for data storage.
*Every 60 minutes, take 5 readings from in-wall sensor, discard any obviously bad readings, and average the good values. Repeat the same for the in-room sensor.
*Record the hourly average for each sensor as well as the difference between to two (just for convenience), into the array of bytes, using one set of 3 bytes for each hour of the day.
*Upload the three hourly values to a cloud server, along with the date/time as required.
*At midnight (hour zero), just overwrite previous array data values.
Reasoning Behind Design
By placing the sensor deep in the wall and covering the hole a bit, this sensor will read the humidity in its immediate environment, but the holes are important to make sure water does not accumulate in the hole and damage the sensor or cause faulty readings.
By having a second sensor away from the wall, it will read the general room temperature. Be comparing the two values, you can raise an alarm when the in-wall sensor is greater than the in-room sensor by x%. This way, on cold damp winter nights, when the in-room sensor and in-wall sensor read say 93% and drop the next morning to say 70%, you know that 'normal. But I am sure over time, you will see the the difference between sensors will steadily increase as the wall accumulates more and more moisture.
Once you 'prove the system', you could have the Arduino turn on a heater, blowin(g) on the wall etc etc.
- If during the first winter testing you find moisture collecting on the sensor PCB, increase the ventilation holes a bit at a time.
- If you find the value of the in-hole sensor is almost always the same as the in-room values (even when the wall is damp / wet), Roll up a narrow (10mm) strip of newspaper, but NOT too tight and put a pin through it to hold it together as a roll (i.e. decrease the ventilation flow, but do not block it completely). You just want a small 'roll' of newspaper to put in the hole close to the vent. This paper will act as a 'buffer' to isolate the in-hole sensor from the outside, maybe a small piece of sponge will do, but just make sure nothing touches the sensor.
I can't guarantee you this design will work, but it is how I would tackle the problem and think you will find it produce fairly useful data to make a decision with.
Good luck, and please post your progress as I'm sure others would like to know also.