A couple of years ago, my kitchen sink copper pipes broke (no damage, fortunately; 2-level house is over 60 years old). I hired plumbers to fix the problem. They disconnected the copper pipe in the basement and under the kitchen sink, and ran new PVC from the basement ceiling to the kitchen sink, then threaded the PVC over the first floor ceiling (n.b., the old copper pipe went under the kitchen's concrete subfloor, but, that would meant tearing up the floor rather than the ceiling and wall drywalls, which were much cheaper and easier to fix), then back down to the first floor bathroom's sink, which also feeds one of the two upstairs bathrooms. Now, when the outside temp falls below 30 F, the downstairs and upstairs bathroom sinks' hot water doesn't run (i.e., it's frozen at some point); oddly, the cold water faucets are fine! Obviously, the cold ceiling - - which has no insulation or heat, and has nothing above it (i.e., just the first floor roof) - - is causing the freeze. We tried the slow drip faucet thing, but, that didn't work. Until recently, I ran an electric heater under the kitchen sink for about a half-hour straight, which did the trick, but, today, it was 10 F, and it didn't work at all. Shortly after the PVC installation and the first frozen pipe episode, I had put heater cable wrap around the exposed kitchen pipes (inlet from the basement and outlet to the bathrooms), which may or may not have worked until now (i.e., no way of telling if it was cold enough that having the wrap did anything at all).

Two questions: 1) Why isn't the cold water pipe(s) freezing, rather than the hot water pipe(s)? and 2) Any ideas on how to solve this problem?

Thank you.