On the prototype in this post I used 3-1/2" fiberglass batt insulation between the back of the mirror & the plastic back on the trough.

   For the final unit I may have a local contractor spray in polyisolene insulation.

   Also considering double pane glass on the front for a dead air gap & better insulation.

   I think that the parabolic style solar water heater can work well even in a cold climate if the collector tube is protected from ambient air.

Any thoughts on insulation or glass for front cover would be appreciated.