How Thermal Insulators Work

Conduction

• Conduction is the transfer of heat between objects that are touching. During conduction, energy is transferred from the object with the most thermal energy to adjacent objects with less energy. This transfer is the result of collisions between the molecules, atoms or electrons of each object. Thermal insulators that limit conduction are made from materials with a low thermal conductivity. These materials are installed between hot and cold objects to prevent the flow of heat between them. Most metals have a high thermal conductivity and are considered poor thermal insulators. Strong thermal insulators made from materials, such as cellulose or fiberglass, are more resistant to changes in temperature and are often used to insulate homes.

Convection

• Convection is the transfer of heat through the movement of a gas or liquid. Thermal insulators that prevent convection are used to prevent heated gas or liquid from dispersing heat into other areas. In the home, this type of thermal insulator is often a simple sheet of plastic or a bead of caulk or spray foam applied to a gap in the wall to prevent the movement of air.

Radiation

• Warm objects expel heat in the form of infrared radiation. Thermal insulators that protect against radiated heat are designed to reflect heat away from one area and toward another. This type of thermal barrier is typically a sheathe of metal that has a high reflectance and a low emittance. Reflectance measures the amount of radiated energy that the radiant barrier can reflect while emittance is the amount that it absorbs and releases. In homes, radiant barriers are often installed in the roof to reflect infrared energy away from the interior of the home in the summer.

Considerations

• The effectiveness of a thermal insulator is dependent on the thermal conductivity of the insulator and its size relative to what it is insulating. Thermal insulators cannot completely prevent the transfer of heat but are intended to slow down the amount of heat transferred over time. Thermal insulators are only effective at preventing heat transfer if they cover the entire surface of the object they insulate and are thick enough to significantly reduce the amount of energy transferred through the insulator over time. Comprehensive thermal insulation systems include an air barrier, insulation and a radiant barrier designed to prevent all three types of heat loss.