There are three ways of heat transfer: conduction, convection and radiation.
Conduction is the transfer of energy by physical contact. Convection is the transfer of energy between an object and its environment due to fluid motion. Radiation is the transfer of energy to or from a body by means of the emission or absorption of electromagnetic radiation.
Accordingly, thermal insulation is decreasing heat transfer from conduction, convection and convection.
Thermal insulation materials that reduce conduction, convection or both are called thermal insulator. The effectiveness of an insulator is indicated by its R-value. The R-value of a material is the inverse of the conduction coefficient (k) multiplied by the thickness (d) of the insulator.
Thermal insulation materials that reduce radiation is called radiant barrier. The effectiveness of a radiant barrier is indicated by its reflectivity, which is the fraction of radiation reflected. A material with a high reflectivity has a low emissivity (at the same wavelength). At any specific wavelength, reflectivity = 1 – emissivity. An ideal radiant barrier would have a reflectivity of 1, and would therefore reflect 100 percent of incoming radiation.
Industrially, there are two ways of thermal insulation according to thermal insulator and radiant barrier.
In the case of my thesis, one way for thermal insulation is to fill the honeycomb cells with thermal insulator such as spherical EPS foam or put anther layer of thermal insulator sheet on top of the panel.
The other way is to put a radiant barrier on top of the panel such as aluminum foil.
Here is the reference link: http://en.wikipedia.org/wiki/Thermal_insulation