This is a classic "house as a system" problem, and if I am correct, the fix will be simple and will not cost very much. Some would wonder how adding a ridge vent on the roof could create a problem with a water line in a wall. The answer is cold air is getting brought into the wall cavity and freezing the pipe.
The "stack effect" is the cause of what you have here. In simple terms, the stack effect is when warm air rises in your house. You probably have a very leaky ceiling. Seal all the gaps in your attic -- seal around all top plates, attic access, pipe and wire penetrations, can lights, etc. All the hot air is rising in your home and getting into the attic where these penetrations occur.
Once air leaves at the top, new air must be brought in at the bottom -- hence the increased amount of air entering your wall cavity. It would also help to seal up the same holes at the bottom of your house. How to do this will differ if you have a crawl space or a slab. I would of course focus on the area where you are having the problem -- there are probably large holes where the pipes enter the floor system.
Of course if you can access the pipes, it will help to put insulation on the exterior of the pipes and you could always run some heat tape on them as well.
Hiring a BPI-certified company will ensure that you have a green contractor who understands the "house as a system" concept. Doing these few low-cost measures will also significantly improve your thermal envelope and reduce your energy consumption.
For more information:
Read Green Home Guide's Know-How article "9 Ways to Make Your Home More Energy Efficient."