If the pipe is in a location where it can freeze, NOTHING, other than applied heat will prevent it. Insulation will slow the process, but NOT prevent freezing. I doubt that ANY plumber would "guarantee" it will not freeze, unless it runs inside a heated area. And I doubt that the garage wall or ceiling are insulated or heated. The riser outside the wall would be the most likely place for a pipe to freeze and break. But using just your description, I am not sure but that there might be a better way to run the piping.