I had that problem when we moved to this house 5 years ago. They added teh second floor onto the original ranch house. The piping to the upstairs bathroom went thru the attic. The first year we had a very cold day and....ooops no cold water coming out of the pipe. I went into the attic and managed to unfreeze the pipe with a hair dryer but discovered that the insulation was gone from the pipe. To be safe I put the insulation back and bought some of that pipe tape that has the thermostat built in and just plug it in every winter.