Well doing the insulation and air sealing in the attic are good things. We really don't want to "heat" our ventilated attics with the heat lost from our homes, now do we? Very costly and inefficient. I have to assume that the pipes in question are not buried in the new blown in insulation since if they were they would not be freezing. I suggest that you insulate the pipes that are exposed to the outside air with a foam pipe wrap of about R-5. This should be relatively low cost measure. Too bad the contractor didn't do it before they added the insulation. Now they will have to move it compress it to do this work.