There is an existing question about checking the moisture content of a wall, but I'm looking for continuous in-wall monitoring to check the integrity of the monolithic cladding throughout the house. Insteon has a water leak detector with a 10 year battery but I'm not certain this can be placed inside a wall for this purpose ( accessed by a dummy wall switch ) as it picks up water vs moisture content. I would be satisfied if I could embed a sensor into the baseplate inside the exterior walls.
You could contact the following EIMA, US Dept. of Energy, or Oak Ridge National combined these agencies conducted a test starting in 2005 on a building in Hollywood South Carolina and one in Washington State that lasted at least 4 years to include test in eight climate zones. I did the installation of the test panels on the South Carolina building. Test panels included most exterior cladding systems. Pick up sensors for moisture were placed on the back side of all materials used in the construction of each system Example Exterior stucco sensor behind finish coat, sensor behind brown coat, sensor behind scratch coat, sensor behind lath, sensor behind vapor barrier and sensor behind sheathing. Also sensor was placed in wall insulation and the back side of the drywall. I have missed place my final report from EIMA you should be able to get info from one of the above agencies.
Depending on your comfort with DIY electronics, you could rig up an Arduino with 1-wire humidity sensors. 1 Arduino could monitor several sensors, if you can get the wiring to them. The Arduino could either output info to an LCD screen, or anywhere if you hooked up a wifi/ethernet shield.
Definitely not an out-of-the-box solution, but it would do what you need.