Our house is heavily insulated. I have been in the attic when it is below freezing and the attic is just as cold as the outside air. Two or three days of subzero weather could freeze the water supply to the water heater. I am thinking of adding valves to the supply side of the water heater that would shunt the incoming water through the pex in the attic when the weather is warm to act as a pre heater but could be shut off and drained when the weather turns really cold.