OPTICAL AND THERMAL CHARACTERIZATION OF MULTIPLE GLAZED WINDOWS WITH LOW U-VALUES

The optical reflectance and transmittance spectra of complete windows for near normal and oblique angles of incidence are calculated from sp ectra of the individual panes taking multiple reflections into account . Calculated and experimental spectra are compared for triple and quad ruple glazed windows with different combinations of low-e coatings. Fo r the annual energy balance of a window the total solar transmission a t oblique incidence is more relevant than the near normal performance. It is shown that, owing to the experimental difficulties involved in optical measurements at oblique incidence, great care must be taken wh en evaluating the annual performance. A simple equation for the annual energy balance of the window taking solar radiation and thermal heat losses into consideration is presented. Annual meteorological data for the insolation and outside temperature are used together with the opt ical performance to evaluate the net energy heat flow through a window . This can be performed for the complete heating season to evaluate th e heat load needed for the building as well as for the warm season to evaluate the cooling load needed owing to solar overheating. This mode l provides a simple way of comparing the thermal performance of window s with different combinations of advanced glazings in both cold and ho t climates, and makes it possible to estimate the cost efficiency of s uch windows.