Related to improper placement of a vapour barrier within wall insulation, are there any tools or methods for measuring water/moisture content within a wall cavity?
I've used a moisture meter to do roughly this. My meter is the pinless kind. Some years ago, we had a contractor who did a less than perfect job on a bathroom remodel. (Ok, they left behind leaks in something like six separate places.) With the meter, I was able to tell where water was a problem, and also know when we had finally gotten all the leaks repaired.
I would not expect the actual meter reading to be highly accurate, since these meters are usually calibrated for wood. But the meter will indeed be able to tell the presence of moisture, and to differentiate between dry and wet. The problem is, these meters can only read to a certain depth, typically a half inch or so in wood. I'll look tomorrow to see, I might be able to test mine under similar conditions to see how deeply it can read.
Edit: I tested my moisture meter. I have a 12+ year old Wagner L609, an inexpensive pin-less meter that works nicely for wood samples. It is not tremendously accurate, but it does show moisture differences well between samples. Newer models might read more deeply than did mine.
The first test that I ran was to test a piece of 1/2 inch thick drywall, then move the flat of my hand up to it, until I saw a difference. By itself, the drywall reads at roughly 5-6%, my hand by itself reads off the top of the scale. When my hand approaches to within roughly 1/2inch, the meter sees a difference, jumping up by about 3-4%. It could also recognize the presence of my hand on the other side of a 3/4 inch thick telephone book.
Of course, this is a poor test for what you are doing. Suppose that moisture collects on the inside of a vapor barrier inside the wall, separated by over an inch from the interior wall surface? Were this to happen, the air space between the two surfaces will be maintained at a high humidity. In turn, this will significantly raise the moisture content of the interior wall surface.
I next did a test where I showed that drywall faced with a somewhat moister environment on one side would then easily register several percent high in moisture content.
This last test convinces me that you can indeed check for elevated moisture content of a wall cavity using a non-invasive and inexpensive moisture meter.
You might be able to use a moisture meter like this one, but you'd probably have to push completely through the drywall into the wall cavity. I've never used a moisture meter but the writeup states the following:
The mini-Ligno S/D gives instant, accurate moisture readings between 6-45%, indicated by a bright digital read out. Designed for inspectors, architects, contractors, painters and homeowners. This meter is a useful tool to measure moisture content in wood and soft building materials such as drywall, plaster and gypsum. Features include a green cap that serves as a handle for easy probe insertion and covers the pins during transportation. Longer pins stored inside the meter for measuring thicker material. To obtain readings the pins are pushed into the wood and the unique on-off switch between the pins turns the meter on.
I'm not certain it would work if you pushed it all the way through the drywall into the cavity. You might want to see if a local tool rental place has one you can rent before you sink the money into one for a one time reading.