Radiative exchange across a window and links to indoor energy demand

This work examines energy exchange between a window and the external environment with emphasis on the role of radiation in determining indoor energy demand. A set of radiometers monitored the flow of solar and longwave radiation to and from a window located in Chicago, IL from December to July. A model of energy exchange between the atmosphere, window and interior space allows estimates of the energy required to maintain indoor temperatures. Energy is lost to the atmosphere by sensible heat transport, longwave emission and reflection of sunlight, where the net effect of radiation is the difference between incoming and outgoing components. Sensible heat transport is the most important contributor to cooling, particularly in winter, while the direct effects of radiation are small, except in June and July. However, longwave radiation has a significant influence on the window's temperature. Radiative processes increase the temperature contrast between the window and the atmosphere, and thereby influence sensible heat transport. For the circumstance analyzed here, the action of solar and longwave radiation during December and January increases the demand for interior heating by a factor of four over what would exist in the absence radiant energy.

► We measure solar and longwave radiation flowing into and away from a window.
► A numerical model links indoor energy demand to the measurements.
► Solar radiation, longwave radiation and heat transport influence energy demand.
► Sensible heat transport dominates the demand for heating in winter.
► Longwave heating of window glass controls cooling by sensible heat transport.