Why doesn't my shower work when it's below below 0?
Since it's unlikely that the plumbing is in the attic, checking the insulation there is rather like checking the tire pressure when the oil light goes on in your car - unrelated, but you don't have to open the hood. Appears from afar to be a hack response by a contractor who doesn't care to get into the expense of actually correcting a problem that was almost certainly caused by poor construction practices (possibly exacerbated by poor design), especially for a 2012 build.

The problem will be where the pipes serving the area that stops working are. It will probably be something simple that was done wrong, such as running them on the outside of an insulated cavity (though arguably, in the experience of people from cold regions, running any plumbing in the outside walls is poor design, no matter where in the wall it is.) Fixing it properly will involve tearing walls open and putting them back together again, unless you are OK with plumbing that does not work when it's cold.