Question by Aethias: Solar water heater Project – Is plastic wrap a good insulator?
Hi, I am doing a project for science. I have to create a basically passive solar water heater, using no panels, electricity, open flames, etc. to heat 200mL of water in 10 minutes. My idea is to take a shoebox, paint the base black, cover the sides with foil, and put in the container with the water. If needed, I will add a magnifying glass over the container. On top of the coverless shoebox, I will cover it with plastic wrap. I was hoping the plastic wrap will allow the sunrays to go in, and not to let out the accumulating heat out. Is this a plausible approach?
Edit: Thanks for the replies guys. This is really going to help me. How about if I just cover the base with foil too? That way, the only absorbing element will be the cup, which I will put a black construction paper roll on to help it absorb more heat?

Best answer:

Answer by George N
Plausible. But plastic wrap alone is not much of an insulator. It will probably still work well enough for your experiment, though.

If you really wanted to retain heat, you’d want two layers of plastic wrap, separated by a thin air gap. You want a thin gap to prevent convection currents from forming. Convention currents transfer heat. Air that isn’t moving, though, is a very good insulator.

It’s the same basic concept as a double-paned window. Or putting up “storm windows”. They just provide a gap that resists heat flow.

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