I have a bathroom where the water pipes pass through an outside wall. On nights where the temperature goes below 0 degrees F, I open the cabinet door under the sink so heat can get in there. My thermostat is programmed to drop to 60 degrees F at night but because the thermostat is across the house, our downstairs bathroom gets very cold. It's on a slab too. Pipes always freeze if we forget to prop the door open and the temps get down below or in the single digits.
I had been thinking of getting one of those wires you put on the pipe to keep it from freezing but it's behind a wall and very hard to get too...
