"This isn't your mom's aluminum foil! Okay, wait it might be. Literally!"

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Cellulose is a natural polymer that is the basic component of plant fibers, including wood fibers. In homes, cellulose usually refers to a type of insulation that is made from wood fibers-most often, from recycled newspaper. It's a terrific development from an environmental standpoint, since it diverts a huge amount of newspaper from the waste stream, and because it's a highly effective insulator. Cellulose insulation is typically blown or packed into walls and other spaces, filling crevices and blocking heat transfer better than fiberglass and other batt insulation can. It is often treated with borate, a natural, safe mineral that repels pests and is fire retardant.

Wrapping a water heater in insulation can save 4-9 percent in water heating costs. If the tank...