This is also for my solar collector.
I was thinking to ask this too
I often end up submitting the same question on other sites
Aluminum foil is a good conductor, so heat can easily flow from one side to the otherIt is also an excellent reflector, so that radiation will be reflected rather than absorbedThe latter property is of more interest for solar collectorsYou could build a solar trap out of sheet goods (plywood, cardboard, etc), and affix aluminum foil with contact cement to get a reflector that would be much less costly than using sheet aluminum, and would also work better because foil is shinier than sheet aluminum.
