The energy budget of single and double-pane windows subject to wintertime environmental radiation

This work applies a new model that treats solar and longwave radiative transfer, thermal conduction, and sensible heat transport to quantify the flow of energy across single- and double-pane windows. A set of radiometers based in Chicago, Illinois, observed solar and longwave irradiances incident on and emerging from a window on a low-rise building. The incoming radiation fields, averaged over a 10-day period in January, drive the energy budget calculations, while a comparison of measured and computed outgoing irradiances validates the results. The indoor–outdoor temperature contrast creates a gradient in longwave radiative heating across the glass. This drives a heat flux that increases sensible heat flow to the exterior atmosphere by approximately a factor of 3 over what would exist without thermal radiation. Absorption of sunlight by the glass also increases energy lost to the atmosphere. For the conditions analyzed, approximately 40% of the solar energy absorbed by the window-plus-interior system is returned to the exterior as sensible heat and longwave radiation. The remaining absorbed solar energy offsets the demand for heating from the climate-control system.