How Insulation Works, or Doesn't

It's easy to understand how insulation works if you remember what it's like to wear a goose down jacket outside on a cold winter day. Even though your jacket is light as a feather (more accurately, many thousands of feathers), your body can stay toasty warm. That's because the feathers create millions of tiny air pockets, and air has excellent insulating value - about R7 per in.

Insulation works by slowing the transfer of heat, which can move in three ways:
conduction, convection and radiation.

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. Whatever the reason, 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 is 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. Learn more about the different types of insulation that can be used to improve home energy savings:

• Information about spray foam insulation
• Information about rigid insulation board

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