Characterization of the thermal insulating properties of vacuum glazing

Methods are described for characterizing the thermal insulating properties of vacuum glazing—two flat sheets of glass, hermetically sealed together around the edges containing a highly evacuated space, and separated by small pillars. The small-area guarded hot plate apparatus gives absolute measurements of the different heat flows through the glazing due to radiation, gaseous conduction and thermal conduction through the pillars. In the transient technique, a step temperature increase is applied to one side of the glazing, and the resultant slow temperature rise of the other glass sheet is measured. This method can be used in ageing studies to characterize glazings at elevated temperatures. In the cool-down method, one glass sheet of a glazing that is initially at high temperature is insulated, the opposite glass sheet is rapidly cooled, and the rate of cooling of the thermally insulated sheet is then measured.