sentences:  The performance of a layer stack can be quantified in terms of its overall heat transfer coefficient "U", which defines the radiative heat flow rate "Q" between two parallel surfaces with a temperature difference formula_1 and area "A" as
  Theoretically, the heat transfer coefficient between two layers with emissivities formula_3 and formula_4, under vacuum, is
  where "T" is the mean of the temperatures (in K) of the two layers and formula_6 Wm−2K−4 is the Stefan-Boltzmann Constant.If each layer has the same emissivity formula_7 on both sides, then a stack of "N" layers placed between two high-emissivity surfaces will have an overall heat transfer coefficient
  Clearly, increasing the number of layers and decreasing the emissivity both lower the heat transfer coefficient, which is equivalent to a higher insulation value.In this equation, it is assumed that the temperature difference is small compared to the absolute temperature.