Thermodynamics and Reflective Insulation

Reflective insulation inhibits heat transfer by thermal radiation. It does not necessarily protect against heat transfer by conduction or convection. Why do you need to know about this insulation? It could mean thousands of dollars saved over the years for heating/cooling, in what you choose for not only roofing materials, but more importantly, what is placed between the roofing and building framing. Bear with me here.

In today’s science lesson…

All materials emit (give off) energy by thermal radiation as a result of their temperature. The amount of energy radiated depends on the surface temperature and a property called the emissivity or the emittance. Emissivity is expressed as a number between zero and one at a given wavelength. The higher the emissivity, the greater the emitted radiation at a given wavelength. A related material property is the reflectivity or the reflectance. This is a measure of how much energy is reflected by a material at a given wavelength. The reflectivity is also expressed as a number between zero and one (or a percentage between 0 and 100%). At a given wavelength and angle of incidence the emissivity and reflectivity values add up to 1 by Kirchhoff’s law.

For those of us who never studied thermodynamics (most of us), Kirchoff did it for us over 150 years ago. While he appears to have been a pretty smart guy, most of his work will go over the heads of us average folks. The important thing to remember is – a good absorber is a good emitter (if something gains heat readily it also gives it off readily) and a good reflector is a poor absorber.

Reflective insulation materials must have low emissivity (usually 0.1 or less) at the wavelengths at which they are expected to function. For typical building materials, the wavelengths are in the mid- and long- infrared spectrum.

It may or may not exhibit high visual reflectivity. This is because while reflectivity and emissivity must add up to unity at a given wavelength, reflectivity at one set of wavelengths (visible) and emissivity at a different set of wavelengths (thermal) do not necessarily add up to unity. However, it is possible to create visibly dark colored surfaces with low thermal emissivity.

To perform properly, reflective insulation needs to face open space (like air or a vacuum) through which there would otherwise be radiation.

Moving forward in time from Kirchoff…

In the 1920’s patents were filed on reflective surfaces being used as building insulation. Recent improvements in technology had then allowed low emissivity aluminum foil to be commercially viable. Over the next 30 years, millions of square feet of reflective insulation were installed in the US alone. Notable examples include projects at MIT, Princeton, and homes such as Frank Sinatra’s house.