Sir James Dewar (1842-1923), a Scottish chemist and physicist, invented the thermos flask in 1892, (also known as the Dewar Flask or Vacuum Flask). The purpose of the flask was to store liquid gases at very low temperatures. The basic function was to thermally insulate the contents in the flask, preventing heat from flowing either in or out.
In our illustration below, radiation is reduced to a minimum by silvering the glass, generally on the two internal faces, so that the radiant heat waves are reflected. Different types of insulation products can reduce the heat transferred by conduction, convection and radiation to varying degrees. As a result, each provides a different thermal performance and corresponding “R” values. The primary function of reflective insulation is to reduce radiant heat transfer across open spaces. The low emittance metal surface of reflective insulation blocks up to 94%+ of the radiation, and therefore, a significant part of the heat transfer.
Reflective Insulation vs. Mass Insulation
A foam cup “insulates” a cup of coffee as mass insulation does and attains an actual R-value. The reflective surface of a thermos jug lining keeps liquids hot or cold. For example, if you take hot coffee, the heat within a thermos is reflected back inside while the cold outside is reflected away. Like the thermos jug in the illustration, Reflectix® Insulation reflects and separates “heat” from -60 degrees to 180 degrees F without an R-value. Hours later, would you rather be drinking coffee from a thermos jug or a foam cup?