Heat Transfer and Radiant Barriers Role

To understand what a radiant barrier is and how reflective insulation can help lower home energy bills, one should understand the concepts of heat transfer by conduction, convection and radiation.

Conductive heat transfer takes place through a solid substance by transfer of the kinetic energy between individual atoms. This kinetic or heat energy can be transferred directly between substances only through physical contact. People are most aware of this form of heat transfer by touching a hot pan.

Convection is the movement of heat through liquids and gases and involves the actual movement of atoms. Heat transfer can occur through “natural” convection or can be “forced” by a fan or pump. Both conductive and convective energy transfer occurs from a high temperature region to a lower temperature region or from hot to cold.

Radiant heat is electromagnetic waves similar to visible light that can pass through fluid, gas or a vacuum. However, to differentiate it from the sun’s radiant energy, radiant heat is associated with the invisible infrared frequencies that radiate from any substance.

Following the same physical laws as light, radiant energy is unaffected by thermal currents or convection. Additionally, thermal radiation may pass through an infrared transparent substance. Consequently, any medium that is less than 100% transparent is a radiant barrier. However, a radiant energy barrier is not a conduction energy barrier. For example, a reflective metal is an excellent heat conductor.

Houses form a thermal island that, for comfort, is most often maintained out of equilibrium with its surroundings by heating and/or cooling systems. To lower energy costs, insulating materials help maintain disequilibrium by keeping heat in or out.

Thermal insulators are designed to limit conduction, convection or both to reduce the flow of heat from a home’s interior space. Most people are aware of the use of R-values as a measure of insulating properties for thermal insulation. R-values increase with layer thickness and decrease with a lower convection coefficient.

Radiant barriers are materials used to reduce the flow of heat from radiant heat sources. Radiant barrier comparisons are based on reflectivity. Total or 100% refection of radiant energy equals a reflectivity of 1. Emissivity is a measure of the radiation energy passed through, and is 1 - reflectivity. Values are wavelength dependent. Bright materials have high reflectivity, and dark materials have high emissivity.

In warm climates, reflective insulation is commonly installed in a homes attic to reduce cooling costs by blocking radiant heat gain. For air-conditioned homes, particularly when cooling ducts are present in the attic, reflective insulation can reduce cooling costs substantially. Because most reflective insulation serves as an excellent heat conductor, an air space (3” - 5”) must exist between the radiant barrier and the radiant heat source - the roof’s interior surface.

Most reflective insulation consists of “thick” aluminum foil applied to one or both sides of a rigid substrate that provides stability, durability and ease of installation. However, a radiant barrier does serve to reflect emitted radiation from objects in the home. Consequently, in cooler climates, a radiant barrier facing on thermal insulation provides both thermal and reflective insulation.