Answer: The short answer is that insulation slows the movement of heat into and out of your home so the furnace or air conditioning unit doesn't need to work as much and therefore uses less energy.

Heat naturally flows from warmer to cooler spaces. Insulation contains tiny pockets of air that resist that movement because they don't conduct heat as rapidly as most solid objects such metal, brick and even wood. Thermal resistance - the ability of a material to resist heat flow - is measured by R value. The higher the R-value, the more the material resists the movement of heat.

According to the Department of Energy, heating and cooling (space conditioning) account for 50 to 70 percent of the energy used in the average American home. The DOE's Insulation Fact Sheet says, "About 20 percent goes for heating water. On the other hand, lighting and appliances and everything else account for only 10 to 30 percent of the energy used in most residences. It makes good sense to turn lights and appliances off when they are not needed, and you'll save even more on your energy costs if you reduce the amount of energy needed for heating and cooling." The DOE also has a page that answers the question How Does Insulation Work for You?

There are lots of resources online to help you learn about how insulation works. In addition to the DOE Web site, I recommend:

• FAQs About Home Insulation on the Web site of the National Insulation Manufacturers Association (NAIMA)
• How Insulation Works on Simply Insulation, a website maintained by NAIMA
• How Does Insulation Work at AskTheBuilder.com
• How Your House Works on Don Vandervort's HomeTips.com
• What Does an Insulating System Do? at OwensCorning.com