Get a heat wire. Plug it in and make sure the sensor is facing the pipe coming into the house. Then strap it tight with zip ties down the length of the pipe.
-SB
Leaving the thermostat set at 50 and wrapping all the crawlspace pipes with heating tape is the solution I adopted AFTER a burst pipe.
The heating tapes consist of resistive wire powered through an AC plug on one end of the tape. The rest of the tape is wrapped around the pipe, protecting perhaps 10 to twenty feet of pipe. The heating tape is thermostatically controlled and has a light showing when power is being applied; not many watts as I recall.
After installing the tape, I never worried about burst pipes. Only if the power was off for several days in a row would I have to think about the problem.