Solar cover vs solid thermal cover: Bubble wrap wins for heat retention and cost

The bubble wrap solar cover is best for preventing overnight heat loss because it cuts evaporation, the main source of heat loss. A solid thermal cover is heavier, more expensive, and blocks daytime solar heating. The bubble cover does double duty—heats during the day and holds heat at night—and thicker bubbles (12 mil) work better. Just put it on before sunset to avoid losing daytime heat.