The natural assumption on how to insulate pipes in cold spaces is to wrap insulation around them completely. That, however, is a mistake. If you wrap your stack pipe in insulation, it will keep the cold air away from the stack, but it will also prevent exposure to heat from the interior of the house, which is what it needs. If the stack is running through the unprotected garage, that is likely where it is freezing. What you want to do is box in the stack system so that it is being protected from cold on one side, but exposed to the warmth of the house on the other side. Build a wall on the side of the stack that is facing the garage. It does not have to be an elaborate wall, but just a barrier of some kind. Then you should put insulation between that wall and the stack, but leave the house side of the stack uninsulated. By doing this, the cold air from the garage will be stopped by the wall and the insulation, and the exposed side of the stack will receive heat from the house. Even if there is a solid wall blocking the way, the heat will get through. If the stack is insulated on both sides, that heat will not penetrate the insulation.