You're right Bruce, the water leaving the hose will have to drop in temp before freezing & should be an indicator.
Hardware stores, Lowes or refrigeration & plumming supply houses have foam insulation for pipes. I would want at least 1/2" wall insul. for the hose. It's important that there be no air gaps between joints of insulation. The ID (inside diameter) of the insulation should match the OD of the hose.
I would feel safer insulating against freezing in case the heater fails.
Also, IMO the heater isn't necessary to prevent freezing if insulation is applied properly.
I like your project!
