When a cavity wall is built, quite a few steps should be followed.
A cavity wall should contain a DPC (Damp Proof Course). Which is basically a type of barrier in a wall that keeps moisture out of the inside skin of the wall. Usually this is a plastic type of sheet that is build into the wall. Weep holes should be installed in the outer skin of the the cavity wall construction above the PDC. The combination of weep holes and a well laid DPC in a ‘cavity wall’, ensures that your inner skin of brickwork and plaster and paint finishes remain dry.
Another example of what you do not want to see/ find in the cavity of a cavity wall construction.
If your builder has built this DPC into your cavity wall, the next step of construction happens, and the rest of the wall is built. In this process, obviously some mortar will drop into the cavity and settle at the base and on tie wires. At the end of the days work of building the wall, the cavity should to be cleaned.
Sometimes this part of the process is dismissed, or forgotten. (We all know how easy it is to just drop and run at the end of the day).
Now you might have a builder or contractor that has implemented the DPC and religiously cleaned it after every day’s work, but he might have been a little aggressive in this, and in the building and cleaning process, the DPC was torn or punctured. As you can imagine, having holes in something made to keep water out is not the best idea.
This is not what you want to find in the cavity of your cavity walls!
Of course, one thing that can never be counted out is just plain human error/ negligence. We have found things like cigarette packets, bottles and even lunch boxes inside of cavity walls. Clearly lunch was not worth trying to dig that out of the wall.
The last thing that can be wrong with your cavity wall is that your builder just didn’t use the right products. Omitting correct plaster placement as support for the DPC at the bottom of the cavity, or using cheap plastic as the DPC are two examples.
In our experience we have come across all of these problems in one wall.