How Insulation Works
Insulation works by slowing the transfer of heat, which can move in three ways: conduction, convection and radiation. For heat to travel from your body through your down jacket, it has to move by conduction through the tiny feather fibers that are in contact with each other. Heat transfer by convection happens through the air, and there are millions of miniscule air spaces between the fibers. Heat transfer by radiation is also slow, since one fiber must radiate its heat to another.
Understanding insulation helps you identify poor home energy performance. If you know how your home's insulation works, you can also understand what makes it malfunction and contribute to poor energy performance in a house.
Here's what can go wrong:
Voids are areas where insulation is absent. They can occur by accident, because of faulty insulation. Sometimes voids are necessary because of safety requirements. For example, an insulation void is required around a recessed "can" light that is not rated for insulation contact. Insulation voids allow heat transfer by convection and radiation. Research has shown that just a 4% void in fiberglass batt insulation can result in a 50% reduction in insulation effectiveness.
Compression compromises insulation's effectiveness because it eliminates many of the tiny trapped air pockets that provide insulation value.
Air movement through insulation diminishes insulation effectiveness because it increases convective heat loss. To effectively slow heat transfer, the air pockets in insulation must be still. This explains how important it is to air-seal a section of your house (such as the attic, basement or main living area) prior to adding more insulation.
Moisture decreases R-value by making the insulation more conductive and by causing settling, compression and voids. Slowing the transfer of heat is just as important in hot weather as it is during cold weather. So even though the down jacket example focuses on insulation's role in keeping a house warm in winter, the same principles apply when it's hot outside and you want to save money on air conditioning.
Duct Work Insulation
Protection from temperature extremes. Duct insulation helps to preserve the temperature of heated or cooled air that has to travel through ductwork located in unconditioned spaces.
The best way to understand the importance of duct insulation is to imagine the inefficiency of blowing heated air through ductwork that's really cold.
If your uninsulated ductwork is located in an attic, basement or crawl space, the duct temperature will be the same as the temperature of these spaces. This means that your heating system has to heat up the ductwork before it can actually heat your living space.
On hot summer days, uninsulated ducts cause similar comfort and energy problems. The air that has been cooled by your central air conditioning system ends up being heated by the ductwork. You won't feel the relief of cooler air until your HVAC system has cooled off the hot ducts.
Water Line Insulation
If your water heater is less than five years old and sits indoors, you won’t recoup the investment cost of a water heater insulating blanket through potential energy savings—since the tank itself is already well insulated. Instead, increase efficiency by covering the hot water pipe leading out from the tank with 1/2-inch pipe insulation.
Insulating water lines in unconditioned spaces will pay back the cost in as little as 3 years.